The Chemistry of the Radio-Elements Been Cleared up in the Most Wonderful Way by This General I I N Sat O N

MONOG RAP H S 0N INORGANIC A ND P H YS I CAL CH E MI S TR Y

D . so .

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R A D I O - E L E M E N T S

PA RT I M ONOG RA PH S ON INORG ANIC AND PH YSICAL CH EM IST RY

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T H E E M R T H - CH IST Y O F E RA DIO E LE ME NTS . By F REDER CK S ODD M . A F R. S I Y , . . . . P art I . - Par II . T H E RAD O ELEMENT S AN D ER OD C L A . 25 . t I T H E P I I W. net Two P in o V l arts ne o ume.

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C H EM IST RY OF T H E

RA D IO-ELEM EN T S

IC . F D K S Y F R S . OD D . RE ER ,

LATELY LECTU RER I N P H Y S I CAIJ UH E MI S TR Y A ND RAD I OACTIV ITY I N T H E U N I V ERSITY OF G LAS G OW : P RO F ESSOR OF CH EM I STR N ERS T OF A'B ERDEE N Y, U IV I Y

PA RT I

S E COND EDITION

[RE V ISED AN D LARG ELY REWRI TT E N!

L N G M S G N A N D C O O A N . , R E E P A T E R T E R R O W L O D O N 3 9 N O S , N

F RT H AV EN E 30T H ST REET NEW RK O U U , YO B M B Y LC U T AND M D R S O A , CA T A , A A I 917

All righ ts reserved

P REFAC E T O NEW E DI T ION

OF PA RT I

T HE Ch emistry of the Radio -E lements was ﬁrst published

E arl i as a single volume in 1 91 1 . y in 1 91 3 the mp o rtant generalisation was made which connects the chemical nature of the radio -elements with the sequence of changes in which they result, and which has given rise to the theory of isotopic

Law elements, and thrown a fresh light on the Periodic .

These discoveries formed the subj ect of a new volume, pub

s 1 1 li h ed at the end of 9 3, as Part I I of the Chemistry of

‘ R -E the adio lements, the original volume thus becoming

Part I .

In 1 1 n 9 4 , a new edition of Part I bei g called for, the present volume was written . The whole subject can now be presented much more clearly and completely than in

1 1 1 9 , and in consequence Part I has had to be largely rewritten from the new point of view . But whilst the general mode of presentation has been much modiﬁed, the ground already covered in detail in Part I I has been , as far n as possible , avoided in the prese t volume , which concerns itself rather with the practical consequences than with the theoretical signiﬁcance of the new generalisations .

I n addition , much new matter has been added , especially in connection with the a rays and recent electro and co llo ido S m chemical researches on the radio elements . o e of the topics b ut brieﬂy dealt with before have been considerably new ampliﬁed , and many members of the disintegration series, vi TH E CHE MISTR Y OF TH E RA DIO-EL E ME NTS

n n n 1 1 1 . unk ow i 9 , have had to be included There is reason to hope that something approaching completeness has now been attained with regard to these numerous and involved successive products , and though much remains to be done , it will is unlikely that many more be discovered .

FRE DERICK SODDY .

October 1914 . CONT ENT S

P R EFACE

G ENER AL DESCR I PTION OF RADIOACTI V ITY

R V C P R F AV R G L F ADIOACTI E ONSTANTS , E IODS O E A E I E AND RADIOACTIV E EQUI LI BR IUM E NUMER ATION AND NOMENCLATUR E OF THE R ADIO -E LEMENTS

— P R ELIMINA R Y DESCR IPTION OF THE DISINTEG R ATION SER IES PHYSICAL M ETHODS OF SEPAR ATION T H E CHEMICAL CHAR ACTER OF THE RADIO -ELEMENTS

A R P EL R COLLOI D o -C E R Y F R DSO TION , ECT O AND H MIST O ADIO ELEMENTS SYSTEMATIC DESC R IPTION OF THE RADIO -ELEMENTS U RANIUM (U )

R X R X U ANIUM 1 AND U ANIUM 2 U R ANIUM Y I ONIUM (I o ) RADIUM (RA ) RADIUM EMANATION “ ” RADIUM ACTI V E DEPOSIT RADIO-LEAD OR RADIUM D R ADIUM E PO LONIUM (RADIUM F)

T HOR IUM (T H ) MESOTHO R IUM 1 M ESOTHO R I U M z RADIOTHO R IUM ' SYSTEMATI C DESCR IPTION OF THE RADIO-ELEMENTS (zontznued)

T HO R IUM X T HO R IUM EMANATION “ T HO R IUM ACTI V E DEPOSIT ACTINIUM RADIO-ACTINIUM ACTINIUM X ACTINIUM EMANATION “ ACTINIUM ACTI V E DEPOSIT

T H E U LTIMATE PRODUCTS — ATOMIC WEIG HT o r LEAD 1 40

POTASSIUM AND R UBIDIUM

REFE RENCES

R F D R R A t and a wal CHA TS O ISINTEG ATION SE IES f . T H E C H E M I S T R Y O F T H E

R A D I O - E L E M E N T S

P A R T I

G ENER A L D E S CR I P TI ON OF R A D I O A CTI V I TY

RAD IOACT IV ITY has introduced into the science of chemistry r a new conception . The adioactive elements are radio active because they are in progress of spontaneous change . The chemistry of the radio - elements is concerned largely with the nature of the products of these changes , their

' isolation and separate identiﬁcation . The property of radioactivity was discovered by M 1 . Henri Becquerel in 1 8 96 for compounds of the element n uranium , which he found to be spontaneously emitti g new X- kinds of radiation , very much allied to the rays in their Th u s general nature . the new radiations pass to varying extent through all matter, quite independently of whether it is opaque or transparent to light . The new radiations , in addition to the properties possessed by light of acting on a a photographic plate and of causing cert in substances , like ﬂ u o resce X- the platinocyanides , to , resemble the rays also in “ e ionising the air and other gases , r ndering them , for the time being , partial conductors of electricity , and causing them , for example , to discharge a gold leaf electroscope . Mme The pioneer on the chemical side was . Curie, who , n n M working in co j unction with her husba d, the late . Pierre i u n Cur e , made the following f ndame tal generalisation . R adioactivity is a property of the atom . It is not affected at all by the nature of the chemical combination in which the atom exists , nor by the physical conditions . Up A 2 TH E CHE MIS TR Y OF TH E RA DIO-E L E ME NTS

to the present day , in spite of numerous attempts , it has neither been found possible to destroy or diminish the radioactivity of a radio-element by any artificial process nor to cause an element which is not radioactive to become so . The radioactive process goes on at a definite rate and i n a definite way which it is at present quite impossible to Th is influence . , of course , applies to the whole process . In many chemical and physical operations the radioactivity appears to be diminished or removed j ust as a candle Thi s b urning in air appears to be destroyed . is often simply accounted for by the escape during the process of “ ” radioactive gases or emanations, as such have come to no t f be styled . The sum total of radioactivity is a fected by n any Operation yet tried . I n spite of the existe ce at one no n time of a vague belief (a belief which has foundatio ) , t that all matter may be to a certain extent radioac ive , j ust all as matter is believed to be to a certain extent magnetic , it is recognised to -day that radioactivity is an exceedingly Of rare property of matter . the eighty or more elements i n 1 8 known 96, the year of the first discovery of the property , two only , uranium and thorium , are certainly radioactive in the now accepted full sense of that term . These two elements are at the end of the Periodic Table with the heaviest atoms known and and

before them in the Table was a great gap , the next heaviest atoms being those of bismuth and lead (O : The study of radioactivity has peopled this gap t with more than thirty unstable elements . Two o her elements , s pota sium and rubidium , possess a feeble indication of a

radioactivity of a peculiar kind , but it is far from proved that this radioactivity is of the same n ature as in the case of

uranium and thorium , and for the present it need not be - - further considered . To day , however , some thirty four dis

tinct types of radioactive matter are known with certainty ,

each with a deﬁnite and characteristic kind of radioactivity .

Radioactivity is an atomic phenomenon , and each of these

new types is fully entitled to be considered a new atom , and

therefore a new chemical element . These are all derived n n from natural minerals co taini g uranium or thorium , and their isolation and identiﬁcation as separate individuals n are due e tirely to the delicacy of radioactive methods , GE NE RAL DE S CRIPTION OF RADIOA CTI VITY 3 which far surpass in this respect the utmost that the spectroscope can accomplish .

w R i -E n - Ne ad o l eme ts . The ﬁrst discovered of these new 2 3 - o l o ni u i n radio elements were radium , p , and actinium . They were derived from the working up of large quantities of - - U O pitch blende (impure uranoso uranic oxide , S S) from the j oachimsthal mine in Bohemia , Latterly a similar study of the thorium minerals has resulted in the isolation of similar

- new radio elements of technical value , mesothorium and radiothorium . I n all of these the radioactivity of the material is relatively enormous compared with that of ; uranium , and is more or less permanent Of these new types of chemical elements only radium and its ﬁrst product, the radium emanation , have been thoroughly examined in the same way as the ordinary elements . For these the atomic weight has been directly determined and the spectrum n mapped . All the others either exist in too small qua tity e n for this yet to have be n possible , or are incapable of bei g separated from the inactive elements with which they are always associated in the mineral . I n radium no diminution of the radioactivity with time after separation has yet been observed , and the same is true of the common radio elements , uranium and thorium , while in the others a a Slow decay over a period of ye rs takes place . Besides these , numerous other types of radioactive matter are known which are more transitory, lasting in the various s cases only month , days , hours , minutes , or even seconds . S ome of the earliest of these types of transitorily radio 4 active matter to be studied were the emanations . The - radio elements, thorium , radium , and actinium , but not n uranium or polonium , produce , i addition to radiations , n what are known as radioactive emanatio s , and the two phenomena must on no account be confused . A current ﬁrst- of air passed through the named substances , preferably in solution , carries away radioactive gases or emanations which give out rays of similar kind to those from the other radioactive elements . The radioactivity of the emanations the is, however, transient , lasting a few days in case of m n radium , a few i utes in the case of thorium , and a few seconds in the case of actinium . The emanations can all be condensed out from the air at the temperature of liquid 4 TH E CHE MIS TR Y OF TH E RA DIO -E LE ME NTS

air . Chemically they resemble the rare gases of the argon group , in that it is impossible to absorb them by chemical n n reage ts or cause them to enter into chemical combi ations . The emanations all have the power of imparting radio activity to solid objects coming in contact with them . This “ active deposit (also called the “ induced ” or “ ex ” 5 cited activity ) again is transient, decaying according to regular laws at characteristic rates in the three cases . That from thorium lasts a few days, whilst that from actinium and from radium lasts a few hours before the decay is complete , the imparted activity from actinium decaying slightly more slowly than that from radium .

i n — Radi oact v e Cha ge. A chemical examination of the radioactivity of thorium revealed the following remarkable S facts . imple precipitation of the thorium from its solution gives thorium hydroxide , from which the major part of the

a - radioactivity is found to have been removed . The radiation i s n much e feebled , and the substance has completely lost n the power of produci g the radioactive emanation . The solution from which the thorium had been precipitated possesses the whole of the emanati ng power of the original n am solution , and on evaporation and ig ition , to expel m ni u m o salts , gives a minute residue containing the whole new of the radioactivity which the thorium has lost . This radioactive substance separated chemically from thorium was termed thorium X . The activity of the thorium X ns n is , however, only tra ie t . I n the course of a month it s n completely di appears , decaying to half its i itial value in four days . The emanating power disappears at the same rate . The thorium hydroxide , however , at first but slightly recov ers and n radioactive , its radioactivity emanati g power th e X In n just as fast as that of thorium decays . a mo th n its activity regai s its old value . Now a fresh treatment with ammonia separates anew a fresh amount of “ thorium X X of the same activity as the first . Thorium is a - short lived radioactive product of the thorium , which itself n produces the gaseous ema ation , and the emanation in its turn produces the active deposit responsible for the “ excited ” “ ” n or induced activity . The case has bee treated at length because it was the first “ disintegration series to be ‘l h an es c . elucidated , and is typical of all radioactive g GE NERAL DE S CRIPTION OF RA DIOACTI VITY 5

As a result of these and similar observations , a complete theory of radioactivity was put forward which is now uni 7 v ers ll - a y accepted . The radio elements are unstable and n n n change sponta eously, the cha ge bei g accompanied by the expulsion from the original atom of rays and the pro n new ductio of a type of radioactive atom . The latter is n ofte very much more unstable than the parent element , and changes again with expulsion of rays and formation of n a other new atom . This process of successive changes pro ed ce s often through a great number of steps . Thus an a - n ray is expelled in the change produci g thorium X , the thorium X atom expels another a- ray and changes into the

n a- gaseous emanation , the emanation atom expels a other ray and changes into the non - volatile matter constituting the i n n n active deposit , which turn u dergoes further cha ges of V precisely similar type . Th e extension of this iew has n n n gradually i cluded all the k own phenome a of radioactivity . The thirty new types of radioactive matter before mentioned which occur in the uranium and thorium mi nerals are all almost certainly produced from either ur anium or thorium n a n by this process of successive cha ges . Lacun e still remai to be ﬁlled . The production of radium and of actinium O from uranium has not yet been directly bserved , but there n and is strong indirect evide ce that it occurs , that time alone is required for the experiments to yield positive results . Very deﬁnite conclusions have been established as to the n n In precise ature of the atomic disintegratio . spite of very f i n i - great di ferences the stability of the var ous radio elements , the periods duri ng which they exist vary ing from thousandths of a second up to geological epochs of time, all the changes Ch n are of the same type . The rate of a ge varies enormously , but th e changes all obey the same law as that o f a mono

n. molecular reactio As expressed by Ostwald in words , this is : As time i ncreases in arithmetical progression , the quan ” tity of substance decreases in geometrical progression .

This is the simplest law possible . The rate of change at every instant is proportional to the quantity remaining nu a changed t that instant . It has been established that radio s o an activity , far as the individual atom is concerned , is s instantaneou phenomenon . Before and up to the actual moment of disintegration the atom of the radio -element is in 6 TH E CHE MIS TR Y OF TH E RADIO-E LE ME NTS

n no way distinguished from the atom of an i active element . n n After the disi tegratio has occurred , the new atom produced is in no way different from an ordinary atom until it in turn n - again disi tegrates . But in any mass of a radio element the no t atoms do all disintegrate at the same instant , so that the emission of rays appears at ﬁrst sight to be a continuous phenomenon . The simple resolution by a lens of the glow produced by a -rays on a phos phorescent screen of zinc 8 S ir i sulphide , as in the spinthariscope of Will am Crookes , is su fﬁcient to show that the emission of the radiation is n perfectly disconti uous .

Anal si s of th e Radi ati ons — y . The rays from the radio active substances have been proved by physical methods to be , a or to rise from , electrically charged particles proj ected from n 9 the substance radially in all directio s with great velocity . This forms an ess ential part of the theory of atomic dis

n. o f integratio Three types radiation are distinguished .

a They are , in order of penetrating power, the B and

- n th e a- n n n y radiatio , rays bei g very feebly pe etrati g , but n by far the most energetic and importa t type . Three main effects produced by all the rays more or less in common are chieﬂy used in their detection and measurement . The rays act on the photographic plate , cause n certain ﬂuorescent substa ces to glow , and ionise the air

a - s and other gases . Photographically the ray have slight n action , while their io ising action is enormous compared with that produced by the other two types . The B and

- a - y rays possess , relatively to the rays , rather feeble ionising O action , but are chieﬂy perative in the photographic actions, the action of the 7 -rays being always very feeble compared - with that of the B rays which accompany them . Different ﬂuorescent substances behave differently to the

a- three types of rays . For the rays a preparation of zinc

- t n . 8 sulphide is mos se sitive For the and 7 rays , barium and other platinocyanide s and willemite (zinc silicate) are flu r r M the o esce s usually employed . agnesium platino n X- cya ide , which responds brilliantly to rays , is little afl ected 3 - by , and y rays . The scintillations shown by a lens in the fluorescence of zinc sulphide under the a-rays are

a - due to the individual impacts of the particles , each a impact causing separate ﬂash of light . This is one of GE NE RAL DES CRI PTI ON OF RADI OA CTI VI TY 7 the methods by which the actual number of a -particles expelled from a radioactive substance in a given time has 10 been directly counted . The three types of rays are distinguished in the ﬁrst pl ace

a- by their power of penetrating matter . The rays are most o easily absorbed . None are able t penetrate a piece of ordinary paper or more than centimetres of air at

- atmospheric pressure . The Brays go through thin metal

foils with ease , but are for the most part absorbed by a

- millimetre of lead . The y rays are able to pass through

great thickness of metal without complete absorption . The more penetrating types are those of the thorium and radium

series , and for these every cm . of lead cuts down the

radiation to about half its initial value . The three types are

distinguished also by their behaviour in the magnetic ﬁeld .

a - no t — The and B types are deviated , the 7 are . The B rays , - like the cathode rays or radiant m atter of the Crookes tube , and - being negatively charged almost mass less , are readily S coiled into circles or spirals by a magnetic ﬁeld . ome of

- the Brays travel with a velocity very nearly that of light ,

which is many times greater than that of the cathode rays,

8 — f so that the , rays are considerably more di ﬁcult to deviate

a - than the latter . The rays, being positively charged , are - deviated in the opposite sense to the B rays , but owing to - the mass being much greater than that of the B rays , the

deviation is extremely slight, and reﬁned experiments with very powerful magnetic ﬁelds are necessary to detect the

deviation. -Ra - a y s . In all cases where the disintegration is aecom ani ed a - p by the type of radiation known as the rays , the f t atom su fers a deﬁni e loss of its mass , part of it being

projected as a separate atom with great velocity . It was predicted from the association o f helium in the minerals containing the radio -elements that one of the ultimate pro 0 ducts of the disintegration was the element helium . In 1 9 3 the continuous production of helium from radium was 1 1 a - directly observed by spectroscopic methods . The rays were shown ﬁrst to consist of positively charged particles of atomic size expelled with a velocity about one -tenth that

of light . The existence of this charge renders th e particle

liable to deviation by electric and magnetic forces . By the 8 TH E CH E MI S TR Y OF TH E RADI O -ELEMENTS

application of these and similar physical methods of attack, data were gradually accumulated from which the atomic weight of the radiated particle constituting the a — rays could

be determined . This proved to have the value 4 in terms of n a - hydrogen as u ity . The particle carries two atomic charges n io n of positive electricity ; that is to say , it is a divale t . The identity of the a -particles with the helium produced in radioactivity has been cleverly established by experiments in

- - thin walled glass tubes of capillary bore , perfectly gas tight, but so excessively thin that the a -rays are able to pene trate

. S the walls uch a tube , ﬁlled with the gaseous radium

emanation , generates helium in the surrounding space outs ide

the tube , though when ﬁlled with helium none escapes . If the tube is wrapped round with lead -foil the helium is driven

into the lead , and can be so stored . On melting the lead

in a perfect vacuum the helium is liberated , and can be 12 detected by its spectrum .

a - s The rays have some very remarkable propertie , which n n are unique to this type of radiatio . They travel a deﬁ ite an distance , known as the range in y homogeneous

s a - medium , before being ab orbed . The various types of rays expelled i n the different disintegrations differ from one onl n another y in the i itial velocity of expulsion , which varies from of the velocity of light for the a -particles of thorium

C a - n to about 5 Z for the particles of uranium . It has bee proved that all the a -particles expelled in any one type of n n 13 disi tegratio travel with exactly the same velocity . This velocity is gradually diminished to very nearly the same extent for each particle in passage th ro ugh a homogeneous

absorbing medium , until the critical velocity , Z of that n of light , is reached . The io isation produced , and the same is true generally of the photographic and ﬂuorescent effects n increases in any given le gth of its path , as the velocity of the n particle diminishes down to the critical velocity , whe all ff s a- e ects cea e with great abruptness, and the particle is

absorbed or passes beyond all means of detection . “ ” n a - He ce the range of the particle , which is proportional n n to the cube of its velocity, is an important co sta t, char “ acteri sti c n in of the type of disi tegration which it is expelled . The ranges given for the a -rays are taken for the most part from the annual tables of radioactive constants published

I O TH E CHEMIS TR Y OF TH E RA DIO-E LE ME NTS

O th e farther it travels . Nevertheless , if atoms were in reality the hard solid spheres occupying space to the total exclusion s of everything el e , which the older phenomena of physics and chemistry made appear probable , the almost undeviated

a- ﬂight of particles right through them would be impossible .

a- ~ But this is not the whole truth . Thousands of particles so pass , each through thousands of atoms in their paths, but an occasional one is violently swung out of its path by a single exceptionally close encounter , and may even emerge from the side it entered . This occasional single R f scattering, as it is termed by utherford , is very dif erent from the slight hither and thither deﬂections according to the laws of probability which also take place . I t gives us our ﬁrst experimental evidence of what the interior of the atom really is . It shows that practically the whole mass of the atom must be concentrated in an excessively minute R volume or nucleus , which utherford estimates as of the order of a thousand million million times less than the “ apparent and usually accepted atomic volume . R This nuclear theory of atomic structure of utherford ,

a - s founded initially on the scattering of particle , though still in s co embryo , has already proved itself of great as istance in t ordina ing various phenomena, and a very brief description of its central idea may be included here . The further experi o f mental basis it rests on the generalisation , later to be

n a — considered , connecti g the expulsion of and B particles and n the positio of the product in the Periodic Law , the researches - X- on the high frequency or ray spectra of the elements , and

— the nature of the B rays dealt with in the next section . It is supposed that the successive places in the Periodic Table b b 2 o from hydrogen to uranium , pro a ly 9 in all , corresp nd with successive unit increments in the nett positive charge n I 2. of the atomic nucleus , from to 9 Arou d this nucleus, in some unknown way , and occupying the atomic volume i as usually understood , c rculate a number of negative electrons equal to the nett positive nuclear charge; or “ atomic number , as it is called . Whereas all the older phenomena encountered in the study of matter are con cerned solely with the negative electronic outer system , and n usually o ly with the outermost ring or shell of it, the newer phenomena of X- rays and radioactivity are concerned with GE NE RAL DE S CRIPTION OF RA DIOA CTI VITY I I

the innermost region and the positive nucleus respectively .

— The atomic number that is, the magnitude of the central positive charge on the nucleus — rather than the mass controls the chemical and physical properties of the atom , so that atoms of different masses but of identical atomic number are identical in chemical properties . Numerous such chemically identical elements, thanks to the systematic n no w study of the products of radioactive cha ge , are known , in and will be considered the sequel . I t appears , therefore , that the fundamental idea of the nature of the chemical elements may be false . Chemical analysis may not be an analysis of matter into atoms homogeneous as regards t e mass , but rather homogeneous as regards h nett nuclear positive charge . I n the great majority of cases , no doubt , only one atomic structure of a given atomic number may be f n su ﬁciently stable to exist perma ently, and hence the maj ority of elements may be homogeneous as regards e atomic mass . But it app ars likely that in some cases more th an one atom of certain atomic numbers may be stable , and such atoms would not necessarily have the same mass , though they would appear as homogeneous elements to every test chemical analysis could impose . - and r — - B 7 ay s . The B rays also consist of expelled par ti c les m , but in this case the particles are not atoms of atter, “ ” but free atoms of negative electricity or electrons . The velocity with which the B- particles move varies in different cases up to a maximum almost indistinguishable from that

' of light . They have considerable penetrating power, and are able to traverse thin plates of metals and other sub stances before being completely absorbed . It a - is probable that, as in the case of the rays, each - n disintegrating atom expels one Bparticle o ly , but the velocities with which the individual B— particles escape vary - widely from atom to atom of the same radio element . In a magnetic ﬁeld the B-rays are coiled up into circles of smaller or greater radius according as their velocity is small or great, and it is possible to analyse a beam of B-rays by this means a - a ccording to their velocity . As in the case of the rays, the rays lose velocity in their passage through matter , and, in order to obtain the initial velocities , it is necessary to work with preparations , such as the active deposits, that can be TH E CHE MIS TR Y OF TH E RADIO-E LE ME NTS obtained free from other matter as invisible and u nweigh n able ﬁlms deposited on surfaces , in which the absorptio of - in n - the B rays the preparation itself is egligible . The B rays starting, for example , from the surface of a layer of uranium n oxide , possess a higher velocity tha those that come beneath n n the surface, and , eve if such rays were homogeneous i itially , they would be heterogeneous as they emerged from such a preparation . The results of such analysis h ave been to show that in no

- case are the Brays expelled initially all with the same velocity .

— In some cases for example , those of uranium X , meso

2 E — n n thorium , and radium resolution by a mag etic field i to rays of different velocity has shown that there is a more or less s continuous di tribution of velocity among the individual rays . s o h as The magnetic spectrum is , far as it yet been - s n resolved , continuous . For other fi ray disti ct groups of n rays of defi ite velocity have been observed . The magnetic spectrum shows “ lines corresponding with rays of definite n n velocity . But such lines are freque tly very umerous . I n - B n 1 6 s the B rays of radium and C , no less tha line for 8 radium B and 4 lines for radium C have been tabulated , the first between and and the second with a range up to of the velocity of light . R i s According to utherford , there evidence that the energies of the successive rays in the magnetic spectrum n 2 n differ by a constant qua tity . Thus for 9 li es of radium C the energies were integral multiples of the same fixed n quantity, the integers bei g the alternate odd ones from 59 t n n to 47, and , wi h the exception of 4 5, every i teger betwee

46 and 24 . It is possible to suppose that the initial energy ' - n - of each B ray from a si gle radio element is the same . The B-rays originate in the central positively charged nucleus of and the atom , in escaping therefrom have to pass through the successive rings or shells of surrounding negative elec in trons , which process energy is successively abstracted i n - uanta . q which , possibly, reappears as 7 ray energy The subject, it will be seen , is full of interest , and promises to o n shed fresh light the nature of radiant energy , in the same direction as the quantum theory of Planck and the essential

- identity of y rays and light, about to be discussed , have f n 19 already e fected so many revolutionary cha ges . GENE RAL DE S CRIPTION OF RADIOA CTI VITY I 3

' o - acco m A third type of radiation , the y radiation , usually - n panics the ﬁ radiatio . For long it was supposed to be a - secondary electro magnetic disturbance in the ether, due to - the motion of the B rays , and the same explanation em n X- braced also the relatio between rays and cathode rays . n 20 After havi g been challenged by a rival hypothesis , which ascribed to the vy-rays a discrete nature like the

- f B rays , with the di ference that the discrete particle was supposed to be a neutral pair or doublet, consisting of n one negative and one , still unk own and probably non existent , positive electron , the original theory has triumphed . X- - - rays and 7 rays are light waves, the wave length being of the order of a thousand times less than that of visible light . We owe this advance to the fact that , though they are of too short wave - length to be resolved by any n mechanically ruled grating, in crystalli e solids the ordered disposition of the atoms in space serves the same function n as the ordered ruling of a grati g to ordinary light , the spaces between the atoms being j ust of the right dimensions ” - o - f to diffract the X rays and y rays . By this di fraction of h - the rays , in their passage throug crystals of rock salt,

- diamond , and so on , their wave lengths have been determined . I It ranges from about cm . , or Angstrom unit , for - - d the soft or non penetrating y rays of ra ium B, to x - 22 C . cm . , for the very penetrating 7 rays of radium More exhaustive study has recently shown that the n n f ﬁrst rough divisio by penetrati g power is not su ﬁcient .

8 — Now , both , and y rays are recognised of all degrees a- of penetrating power , from that possessed by the rays up to the most penetrating types , to which at first our 23 c o nfined knowledge was . The first analysis having been made by successively weeding out the less penetrating types by causing the rays to pass through thicker and - thicker screens , naturally the B rays of the same order a - - of penetrating power as the rays , and the y rays , similar

- 8 a . in these respects to , or even rays , were overlooked - A B ray , whatever its penetrating power, is always a single

— ﬂ in n t n f n free y g electro , its veloci y , o ly, being di fere t , those n S o moving most rapidly bei g the most penetrating .

- - also with the 7 rays , the wave length or its reciprocal , the

— frequency , fixes the penetrating power the shorter the I 4 TH E CH E MIS TRY OF TH E RA DIO-E LE ME NTS w - ave length , or greater the frequency , the greater is the n t n pe etra ing power . But it is a co venience to distinguish th e les s penetrating varieties of B and 7 -rays from the n and lo ger known , therefore still regarded as typical , n penetrati g varieties . For this reason the feebly penetrating i : varieties are throughout character sed by brackets, thus But it must be understood that the distinction no is made for practical convenience only , and has theoretical significance . - Lastly , when the velocity of the fi ray falls below a certain critical value it fails to ionise gases and can no longer be detected by this test , but solely by the negative n ch arge which the rays carry . To such rays the ame d- h as n rays been given , but they are not further co sidered here . They come into evidence as a secondary radiation ,

a - when rays bombard matter , but from their nature and absence of penetrating and ionising power , they would n n o ly make their presence felt in experime ts in a vacuum .

h mi A i n — C e cal ct o of th e Ray s . l n addition to the photo f n graphic ef ect , the rays produce many other chemical actio s .

The most notable is the decomposition of water, which renders it dangerous to sea l up radium or other highly n i m radioactive preparatio s , either in solution , or in an n n n perfectly dried co dition for i deﬁnite periods , owi g to the possibility of the vessel bursting and the contents being scattered . For pure radium salts dissolved in water , under

a - which condition the rays are operative , the amount of hydrogen and oxygen evolved is some 1 5 per day per M n gram of radium . uch the same rate would be i itially produced from the emanation dissolved in water . When the - n B and y rays o ly are operative , as when the radium or the emanation is contai ned i n a sealed glass tube immersed in water , the evolution of gas is per day per gram of radium , and it consists of pure hydrogen , hydrogen peroxide being also formed , in accordance with the equa tion H [1 H 2 0 0 . 2 2 2 2 calories

This may account for the fact that in the mix ed gas es evolved from a solution of radium , the hydrogen is always th e i in slight excess . If rays from the emanat on act on water GE NERAL DE S CRIPTION OF RADIOA CTI VITY I 5

0 vapour this excess of hydrogen attains 5 by volume . ° 1 8 0 Whereas for ice at there is no excess of hydrogen , but the amount of decomposition is only 5 z of that which would be produced in liquid water . Conversely, when n hydrogen and oxygen , in prese ce of phosphorus pentoxide , 24 are exposed to the rays, the gases combine to form water . The same type of reversible reactions occur with most ases other g c Ammonia is rapidly decomposed , whereas a mixture of hydrogen and nitrogen , in presence of a reagent that absorbs ammonia , contracts in volume . Carbon dioxide ri is decomposed into carbon monoxide , carbon , a d oxygen , whereas carbon monoxide forms carbon dioxide , carbon , n and oxyge . N itrogen , in presence of oxygen , is oxidised . n If air , contai ing emanation , is stored over mercury , nitrous oxide will be found to be formed . No doubt the formation of combined nitrogen from the air , hitherto attributed to silent electric discharges , is largely due to the radium emanation and other radioactive products invariably present in minute amount in the air . The general chem ical action of the rays seems to be of the nature of a shattering of the molecule and the n s o - transient formatio of free atoms , which , as in the called nascent state , readily recombine and enter into new com b inatio ns , which the same elements in the molecular con f dition are unable to e fect . It would appear also , at least n n ’ as a ﬁrst approximatio , th at an exte sion of Faraday s law of liquid electrolysis controls the action of these rays o n the quantitative side . The rays in the gas produce a certain amount of ionisation . If this amount be expressed in faradays , it will be found to be of the same order as the

' h mi c al o f c e . amount action expressed in moles Thus , in ' the decomposition of liquid water , for example , the number of molecules decomposed is approximately the same as the number of pairs of ions formed when the rays traverse a gas . This holds even more accurately when the gas in question is water vapour or electrolytic gas rather than air . The numbers of ions produced in various gases by the same f n n n rays are not very di fere t , varyi g over a ra ge of from 1 to Other chemical actions of the rays which have been studied are the decomposition of hydrogen peroxide, the 16 TH E CH E MI S TR Y OF TH E RA DIO -E LE ME NTS

b - li eration of iodine from iodides of the alkali metals, i n n especially acid solution , the reductio of silver nitrate n solution to metallic silver, the oxidatio of organic acids , such as acetic acid , and of ethyl alcohol to acetaldehyde and acetic acid , the inversion of sterilised solutions of cane M sugar and many others . any of these changes are pro

- du ced by ultra violet light , and it must be remembered that the quan tities of material decomposed by the radium rays 25 is always relatively very small .

Lastly , the colouring action of the rays on common n glass and o many gems may be referred to . Ordinary soft laboratory glass is coloured usually a purple tint , but sometimes brown according to its composition . H ard glasses are coloured brown. Diamonds of the purest water B eilb are probably not coloured ( y ) , but ordinary stones frequently assume more or less brilliant colours, in one S i r i remarkable case , observed by W lliam Crookes , a very

ﬁne green being developed . Pure silica is rapidly di sinte grated under the rays , and for this reason should not be n used to contain radium preparations permane tly . Probably n th e owi g to the formation of nitric acid from air, paper and fabrics in which radioactive preparatio ns are wrapped are v reduced to powder in the course of time, e en when the s radium is in sealed glas tubes . Quanti ti es of Materi al capab le of b ei ng worked with i n

- — Radi o Ch emi stry . Radioactivity being the manifestation of a spontaneous process of atomic change or transformation, the least amount of any type of radioactive matter that can be detected is the least amount in which this change c an be S o detected . that if the type changes rapidly , proportion ately less of it will give a detectable effect than if it changes E n n slowly . ven for the most slowly cha gi g types, thorium and uranium , the sensitiveness of the radioactive tests are considerably greater than , though of the same general order as , the sensitiveness of the usual chemical reactions of these n elements . For the most rapidly cha ging types , the presence of a few hundred individual atoms is suffi cient to ff n give a clear radioactive e ect . The signiﬁca ce of this will be the better understood when it is remembered that the most delicate spectroscope test of matter known requires not less than many millions of millions of individual atoms .

AD I OAC T IV C ON TANT P R I OD S OF R E S S , E A V E RA G E L I F E A ND RAD I OAC TI V E EQ U I L I B RIU M

Th e Radi oacti ve Constant — The most important property of a radio-eleme nt is its period of average life or its rate of n n cha ge . This is not only a atural fundamental time n nt all constant , servi g to ide ify the particular element in cir c u mstances n ff n , and co versely a ordi g an absolute standard of a time independently of the sequence of stronomical events , but it ﬁxes also the whole character of the knowledge it is - possible to obtain as to the nature of the radio element . I n n n the case of a rapidly cha gi g substance , the period is determined by direct obs ervation of the rate of decay of the

- radiation with time . For a single radio element free from its parent , and therefore not being reproduced , the quantity n in existence dimi ishes , owing to its disintegration , in an ex po nenti al manner with the time accordi ng to the equation

7\ . or 0 . 4 343 t logl o If the product of the disintegration is inactive or does not n interfere , as is the case , for example , with polo ium , the n In radioactivity decays also accordi g to the above law . l the above equation , therefore , o is the initial activity or the t t an t X initial quan i y , I , the activity or quantity after y time , n n n n n e a co sta t k ow as the radioactive consta t , and is the number The physical meaning of the radioactive constant A is the fraction of the total amount of radioactive subs tance disintegrating in the unit of time (provided that the time u nit con s idered is so small that the quantity at the end of the time unit is not sensibly diff ere nt from that at the n - begin ing) . The time required for one half the radioactive substance to change is known as the period of half-change and A n T , is calculated from by means of the equatio XT 0 . log 434 3 . Hence T is or 1 8 RADI OA CTI VE CONS TANTS 19

No w it can be shown mathematically that in a system law changing according to the of radioactive change , the period of average life of the atom is The term period

of average life relates to the future life of the atom . It has nothing to do with and is not affected by the period the i n atom has already been existence . Any time may be ' life i s chosen as the starting point . The period of average the sum of the separate periods of future existence of all the individual atoms divided by the number in existence at the starting point . There is thus a simple connection between the radio o f active constant , the period average life , and the period - E of half change . ach of the latter periods has its con v eni enc es n S in rapid mental calculatio . uppose it were desired to know the fraction of a mass of radium changing in a time so short that the total mass is not much diminished , say a few years . The period of average life of radium is 2 0 44 years . The radioactive constant or the amount changing per year is (which is usually expressed

' 4 x 1 0 ) , and the fraction ch anging in any time , provided it is not too long, can be found from this by simple proportion . But if it were desired to ﬁnd the 1 6 d amount of polonium (period of average life 9 ays , period - 1 6 n of half change 3 days) remaining u changed after , say , n three years , the followi g application of the period of half

- change would be useful . I n 11 times the period of half change the amount of a radioactive substance is reduced to - n n Three years is times the period of half cha ge . He ce m zl e the amount of poloniu remaining is nearly . 1 2 6th n / 5 , and by the use of logarithms can be fou d at once exactly . I t is worth remembering that in ten times the period of half-change the quantity remaining is less than and in twenty times the period of half- change less than a millionth . The periods of average life of the thirty -four radio n eleme ts range from some thousands of millions of years , - n in the case of the primary radio elements, ura ium and t in horium , to less than a millionth of a second the case of n the shortest , the existe ce of which , naturally , is inferred rather than directly demonstrable . The primary elements change with such excessive slowness that they preserve the 20 THE CHEMIS TR Y OF THE RA DIO-E LE ME NTS b strain , as it were , of their more unsta le successors through ° In 1 0 out the ages . a ton of the element uranium ( milli

- grams) about one eighth of a milligram disintegrates annually , and it is the disintegration of this inﬁnitesimal quantity which , for a year , endows the whole of the ton of the uranium with its radioactivity . Compared with radium the radioactivity is , naturally, very feeble, but the radioactivity of one milligram n of uranium , or even less , could , evertheless, be detected - with ease , by a suitable gold leaf electroscope , taking n - perhaps an hour over the observation . A thousa d millionth

- of one eighth of a milligram , reduced in the proportion of n a , b hour to a year , is therefore , the measure of the a solute quantity of matter actually disintegra ting which is detectable no t by the electroscope . It does much matter whether this quantity is a small or large fractio n of the total radioactive

n . matter prese t , so far as radioactive methods are concerned n As the illustration will serve to show , to any one ponderi g over it , it is the quantity divided by the period of average

n . life , not the total quantity prese t , which is of importance

- — Ab sor ti on of 8 and Ra s . It p , y y happens that some of the foregoing co nsiderations are applicable in a totally f di ferent connection in radioactivity , and it will save repeti tion to refer to them incidentally here . The law of absorption n 8 - of a homoge eous , or 7 radiation is mathematically the n t same as that of radioactive cha ge , on the assump ion that the absorption is due to the abrupt stoppage of individual rays by each layer of the absorbing matter , and not to the gradual sapping of the energy of the whole beam . The n absorption , or dimi ution in the number of these radiations, is proportional al ways to the amount of radiation remaining unabsorbed , and therefore follows the exponential law denoted by the equation

l d where I , and , are respectively the intensities of ra iation

t t initially and after passage through any thickness , and ﬁ is “ f the coe ﬁcient of absorption . The radiation is half

n u recr ro cal absorbed in a thick ess T given by 3/, . The p “ of the absorption coeffi cient r/p is the average path of - the individual rays, on the afore mentioned hypothesis . RADI OA CTI VE CONS TANTS 21 From the absorption coefficients the thickness of matter required to reduce the radiation to any required fraction of its initial value can be readily calculated , though it must be understood that these laws are not always exactly followed . f u d I n the sequel , the absorption coe ficients , , , are expresse n in centimetre u its of length , for the metal aluminium for the

f - - o B and y rays and for the metal lead for the hard 7 rays . Fr m f these absorption coe ﬁcients the average path of the rays,

- and the thickness in which one half is absorbed , are readily K deduced . nowing the latter thickness , T cm . , it follows T - 2 . that in cm the radiation is reduced to one fourth , in - T . nT 3 cm to one eighth , and in cm . to Thus, as an f AI 1 example , from the absorption coe ﬁcient a 4,

in 1 1 . the average path of the ray aluminium is 4th cm , the - thickness required for half absorption is about mm . , and b the thickness for all but some 3 z to be a sorbed , about

mm . n a The underlyi g hypothesis , th t absorption is due to sudden stoppage of a proportion of the rays, the remainder no t proceeding with undiminished energy, is certainly true

- in the case of the B rays . But then neither are th ey homo geneo us . These departures from the assumptions made in the ’

f . mathematical theory , in p art neutralise one another s e fects

A homogeneous beam , suffering a continuous drain on all rather than the sudden stoppage of a part of the rays , would be absorbed more rapidly than the exponential law expresses , whereas a heterogeneous beam , in which the slower rays are being gradually weeded out by absorption , will be absorbed c o nse less rapidly than the exponential law expresses . I n u ence q , over moderate thicknesses , to the point at which the rays have been reduced in intensity to a few per cent . of

- their initial value , the exponential law usually holds for B rays with very tolerable accuracy . - In the case of y rays , the exponential law holds nearly absolutely, probably , so long as the rays are homogeneous . n I n the most common cases of the h ard T rays of radium a d

1 . thorium , the law holds after the rays h ave traversed cm l f of ead or its equivalent , which is su ficient to weed out all - the softer types of 7 rays present in the beam . f As regards di ferent absorbing materials , in the case of all these rays as a rough first approximation , the absorption 22 TH E CHE MIS TR Y OF THE RADIO-ELE ME NTS may be regarded as proportional to the density of the sub n In - sta ce . the case of the B rays the atomic weight as well n as the de sity is of influence , metals of high atomic weight absorbing more strongly in comparison with those of lighter atomic weight than the density law would indicate . The f density law , that the absorption coe ficient multiplied by the density of the material is the same for all materials , holds

- very nearly for y rays , very light as well as very heavy sub stances absorbing slightly more than this law would indi cate in comparison with substances of intermediate density . The numerical value of the absorption coeffi cients depends o n also , sometimes to a considerable extent , the particular n experimental dispositio employed . - r h r Re n r i n f R i m — G owt o ge e at o o ad o El e ents . Resuming the subject of radioactive change , we have considered the exponential diminution of quantity of a single radio -element removed from its parent . The parent continues to dis in te rate n g and produce the product that has j ust bee separated , so that the latter commences to reaccumulate in the parent, whilst that separated commences to diminish and its activity R to decay . emembering that the mere act of chemical separation in no way affects the rates at which these radio n active cha ges proceed , it follows that j ust as fast as the s separated product diminishe in quantity , will the quantity n of it , reaccumulating in the pare t , increase . This is ex 00 1 pressed by the relation I ,/I where I , is again the quantity or ac tivity after any time t and l oo is the ﬁnal or

equilibrium quantity , that is the quantity which ﬁnally n will accumulate after an i ﬁnite time .

A changing substance is being regenerated . This is analogous to a leaky vessel being ﬁlled . When the rate of n r cha ge equals the rate of production , or when the ate of leakage equals the rate of ﬁlling , no more accumulates . An equilibrium is produced , in either case , which the further passage of time no longer alters . I f, in this state , the pro duct is separated by a chemical operation completely, the

l oo equilibrium quantity so separated starts to diminish , just

n . as the i itial quantity I , of the ﬁrst equation did The

f 00 0 00 su ﬁxes o and refer to the time , which is either or in accordance as the diminution of the sep arated product or its reaccumulation is being considered . RA DI OA CTI VE CONS TANTS 23

Radi oacti v e E ui li ri um - q b . In the three disintegration n series there is a long succession of cha ges , A producing B ,

B producing C , C producing D , and so on . A , B, C, D , are n ff f all changi g at di erent , usually vastly di ferent, rates but all n according to the simple exponential , or mo omolecular law . The principle of radioactive equilibrium applies not only to a single product and its parent , but to the whole series from one end to the other . The radioactive disintegration series has f been likened to a number of reservoirs of di ferent heights , but u f of the same cross section , with capillary t bes of di ferent sizes in the bottoms , supplied at one end with a steady flow of water which runs through the whole series , from the bottom of each reservoir into the top of the next . The steady flow of water in at the top represents the steady production of n the first disi tegration product from the parent . The period th e of the latter is of order of thousands of millions of years , so that a small fraction of a thousand -millionth part changes n annually . The dimi ution of the quantity of the parent is quite inappreciable over short periods, and consequently the n supply of the first product is constan t . The water flowi g out from one reservoir to the next represents the proportion of one member of the series continuously being transformed into the next . The reservoirs with small outlets represent - - the radio elements of long life , the size of the outlet being inversely proportional to the life , or directly proportional to accu mu the rate of change . In these reservoirs the water lates till a certain head or pressure is produced at which the outflow and intake are equal . The law of radioactive n change , that the amount transformed in u it time is pro portional to the amount present , is the same as the rate of th nc flow of water under e circumsta es of the analogy , the amount flowing out being proportional to the amount of 1 water in the reservoir , that is to its height or head .

A system of the kind described will , after the water has f m and been running a su ﬁcient time, co e into equilibrium , then the outﬂow and intake are for each reservoir throughout In the series the same . a radioactive mineral in which the products of transformation accumulate and do not escape , the amount of each product formed is equal to the amount S transformed per unit of time throughout the series . uch is

1 Wi a ill r ri ﬁces The law o nly h o lds stri ctly fo r reservo i rs th c p a y o . 24 TH E CHE MIS TR Y OF TH E RA DIO-E L EME NTS known as radioactive equilibrium . But by deﬁnition of the n A radioactive consta t , if the amounts of the successive mem bers are OI , Q2, Q3 Q”, in radioactive equilibrium A = A = A = 1Q 1 2Q2 8Q3 MQ .

A A A A co n where 1, 2, 8 ” are the respective radioactive s ants t . Therefore the relative amounts of the successive members of the disintegration series in equilibrium are n inversely proportional to their radioactive co stants, or directly proportional to their periods of average life . Thus in uranium minerals there is a constant proportion between the quantities of the successive members . There is always about times as much uranium as radium , that is , n there are about to s of uranium per gram of radium , so that if the period of average life of radium is known to be 2 0 8 44 years , that of uranium is about , ooo , ooo , ooo years . Again the period of average life of polonium is about 20 0 n days . Consequently per ton of ura ium there is present - about one fourteenth of a milligram of polonium . In this way have been determined the periods of the very slowly changing elements on the one hand , and , on the

- other, the amounts of the very short lived products in minerals . I n the ﬁrst place the period cannot be directly determined , but the relative quantities can . I n the second case the periods can be determined , but the quantities are too small for direct measurement . For the radium emana n tion , which is a gas, the quantity , though i ﬁnitesimal , is n capable of exact measureme t . The volume of emanation in equilibrium with 1 gram of radium (element) is about

cub . mm . measured at normal temperature and pressure . The period of average life is known very exactly by direct n observatio of the rate of decay of its radioactivity . H ence n for this one member both data are k own , with the result that the same information can be obtained by calculation for most of the other members of the uranium series and for uranium itself . I n the case of the thorium and actinium series the periods of the primary elements are less perfectly “ known . It should be stated that if branch series occur , n these calculations are to a certain exte t vitiated . The order of the results is, however , probably correct . S n ome very important conseque ces , most easily illustrated

26 TH E CHE MIS TR Y OF TH E RADIO-ELE ME NTS

The effect of the a -activity of radium A deposited from the radium emanation on the walls of the containing vessel during a few minutes is comparable with that given by the emanation itself, whereas the emanation of thorium has to be maintained in a continuous stream for many hours before an eff ect of the thorium active deposit at all comparable with that of the emanation is produced on the walls of the vessel . This illustration is complicated , but the main reason is that in the ﬁrst case the period of the parent 0 is about 20 0 greater than that of the product , whereas in the second example the period of the product is about 70 0 times that of the parent . All of these illustrations are merely several examples of the fact already alluded to , that the quantity of radioactive s ubstance divided by its period of average life ﬁxes the n number of atoms disintegrating per u it of time , and hence S the degree of radioactivity . ince in the maj ority of cases the quantity of the matter concerned in the phenomenon is below the quantity detectable by any test except radio activity , the quantity itself is of no importance . What is of importance is the radioactivity or quantity divided by the period .

N o product of uranium X has yet been detected .

I n the theoretical scheme the product is an element ,

a - giving rays , which , from the magnitude of their range , by the relation about to be discussed , has probably a period of some millions of years . Hence , though an infinitesimal n absolute qua tity of uranium X , the period of which is an f about one month , will give enormous radioactive e fect , - when this changes into its much longer lived product, it passes altogether beyond the limit of experimental detection . Phy si cal Meani ng of the Peri od of Average Li ta — We may well inquire what it is upon which the period of a - n w radio element depends . This ca not be ans ered . The fundamental difficulty is to frame even a reasonable hypothesis which can account for the precise law of change , and this must , necessarily, precede the explanation of the meaning of the numerical constants which fix , in each case, the rate of change . We can say definitely that there is no gradual change of the disintegrating atom with lapse of time leading i up to its ultimate destruct on , because the period of average RADI OA CTI VE CONS TANTS 27

life is constant . We may compare , for example , the period of a collection of atoms of radium emanation j ust freshly born , and none of which were in existence a minute before , n with the period of a other collection , say the millionth part of an initial quantity , all of which have survived at least t ﬁfteen weeks from their formation , or wenty times the ff average period of life . No di erence in the rate of dis integration will be found between the two sets . There is no gradual ageing . Out of any collection of N radio A atoms N are selected and change per second , but - the remainder are left without perceptible alteration . - 14 Th ei r Nu tal l Rel ati n — e G ge t o . N evertheless there is one remarkable relation between the period of life and th e character of the disintegration , as it is revealed by the velocity of the projected particle , in that the shorter the

a - period the swifter is the particle that is expelled . I n

- the case of the B particle , it is probable that something

s a- of the same kind applie . But in the case of the particle a mathematical relation has been found to hold by Geiger

a— and N uttall . The range of the particle is proportional to the cube of its velocity . If the logarithms of the periods of the various radio - elements are plotted against the logarithms

a - of the ranges of the particles they expel , straight lines

. rath er th an result The plotting of logarithms of quantities , ' the quantities themselves , is a common graphical device to disclose a relationship existing between any unknown powers of the quantities involved . In the present instance , for example , n obviously , o ly the slope but not the straightness of the f lines would be a fected , if, instead of the range , the velocities - of the a rays were empl oyed .

n F i . 1 2 8 This relatio ship is shown in g , p . , in which the logarithms of the ranges are plotted on the horizontal axis and logarithms of the radio -active constants are plotted on the vertical axis .

This, still empirical , relationship may be destined to be of great importance in the problem of atomic structure . n I n each of the three disi tegration series , of uranium , thorium , n and actinium respectively, straight li es of similar slope , but not exactly superimposed , are obtained . The only

- a - clear exception is radio actinium , the range of the particle being longer than that from actinium X , though its period 28 TH E CHE MIS TR Y OF TH E RADIO-ELE ME NTS

is longer . The divergence of thorium may be due to n o ne experimental error , this being, of all the poi ts , the most diffi cult to obtain with accuracy . The relationship enables us to calculate four unknown periods from the range of the - a t . f par icle Thus, if the law holds good , the period o

F IG . 1 .

2 average life of uranium should be about 3 years , n and that of ionium years , periods too lo g for m in direct conﬁr ation , as yet , though the latter case the value agrees very well with what had been p reviousl y

’

d n S ee I ama m. pre icted from direct experime ts . ( ) On the

a - s other hand , the extremely long range of the particle RA DI OA CTI VE CONS TANTS 29 from radium C ' and thorium C ' show that the periods corresponding are so short that the separate existence of the products producing the rays can only be inferred . Thus the period in the ﬁrst case works out between and and in the seco nd case between and second . It might be thought hopeless to expect to know anything at all about the chemistry of such meteoric indications of elements . Nevertheless , their complete ! chemistry , no less than their existence , can be inferred As there will be no other opportunity of discussing the

- Geiger N uttall relationship , attention may here be directed to

S o a- its most interesting feature . long as any particle has n the same ra ge or velocity as any other , an equality which it is easy practically to secure by passing the swifter particle through a suitable thickness of absorbing m aterial , they n are i distinguishable in every respect . I t would not be

a - possible to distinguish , say , an particle of the thorium series from that of the uranium series . But the three straight lines in the above diagram are not exactly super n imposable . For a given ra ge the products of the uranium series are shorter lived than those of the thorium series , and these again than those of the actinium series , Hence , whatever be the meaning of this connection between range n and period , it involves somethi g that is the same for all the members of one series and is different in the different i n series . In other words , the atoms their successive dis n integratio s preserve a feature distinctive of their origin . n n If this is conﬁrmed it is highly sig iﬁca t, especially as it is contrary to what one might expect from the other evidence n no t take as a whole , and from the suspected , but yet proved eries genetic interconnection of all three s . It would appear that some of these newer processes of radioactivity involve in the conception of physical time new ways , and may lead in turn to a deeper and more fundamental understanding of its nature . ENU MERAT I ON A ND NO ME NC LATU RE OF T H E RA DI O E L EM ENT S P RE L I M IN A R Y D ES C RIP T I ON OF TH E D IS INTE G R AT ION SE RI ES

T HE thirty or more radio -elements known may be classi ﬁed in various ways . They are all almost certainly derived from one of two primary parent elements — uranium and thorium . One of the commonest and most useful methods n n w is to arrange them in two disintegratio series, starti g ith two these elements in order of their production . The Table n 2 s at the e d of the book ( Fig . ) comprise all the radio n n elements know , arra ged in this way . I t includes the period of average life of each member and the nature of the radiation expelled when it disintegrates and changes into the next member of the series . Inside the circles representing the elements are placed th e atomic weights to n the nearest u it, estimated , when not directly known , from the atomic weight of the parent and the number of a - n particles expelled . There is an uncertai ty as regards actinium , as the atomic weights throughout may well be R n r re four units less than is shown . The oma numerals ep n sent the family the eleme t occupies in the Periodic Table . In in n the uranium series , additio to the main series , n comprising io ium , radium , and polonium , there is a sub s idiary series consisting of actinium and its products . The manner in which actinium comes to be produced is still not completely elucidated . Only a very small proportion of the uranium atoms disintegrating appear to go to produce t actinium , by far the grea er proportion probably going by the main series ultimately into polonium and probably lead .

At some point in the series, which has not yet been deter th e mined , atom disintegrating most probably has two modes n and f of disintegration wi th differe t periods dif erent products , some of the atoms giving one and the others the other 30 EN UMERATI ON AND NOME NCLA TURE 31

S ee . . 2 2 product . ( Part I I , pp and It is practically certain that the product of polonium is lead , although this has still to be established directly . Th e ultimate products of the thorium and actinium series also remain ex perimen n tally unknow , though their nature may now be inferred . n The nomenclature adopted has gradually bee evolved , and is temporarily accepted until a complete scientific system can be agreed upon it h as been frequently altered as new discoveries are made . At present the names themselves mean little or nothing . But there are good grounds for n thi king that the three series are now nearly , if not quite , n completely elucidated . The ames chosen for the new radio -elements which were discovered before their position and in the scheme was known have been usually retained , sometimes new products in one series have been named by In analogy to similar products in one of the others . the case of the end products of disintegration of radium , which were worked out comparatively late , the system has been adopted of calling them in sequence radium A , radium B , & S 0 . radium C , ometimes it has been found that what has M been considered a single change consists of two . eso thorium 1 and mesothorium 2 constitute such an instance . In some cases the two separate products are not i ndividually n in known , their separate existence bei g only indirectly ferred . This is provided for without disturbance to the rest in of the names in the series , as the case of the hopelessly ’ ’ - n short lived radium C and thorium C , already co sidered , to disti nguish them when necessary from their parent sub stances radium C and thorium C . Another possible method of classification of the radio elements is based upon their periods o f average life or rates n of change . The parent eleme ts , uranium and thorium , a have the longest periods and ch nge the most slowly , so that their chemistry does not differ at all from that of the inactive elements . If their products are then considered n without reference to the series to which they belo g , in f descending order of their periods of li e , we pass gradually th rough a series of substances in which the chemical methods of investigation become more and more different from those usually employed , and more and more peculiar to the study of radioactivity . The practical methods of preparation and 32 TH E CHE MIS TR Y OF TH E RADIO-E LE ME NTS purificatio n of the radio -elements are entirely controlled by o f the period of average life the substance . I n a mineral the various members of each disintegration series exi s t in quantities directly proportional to the period - of average life , so that a radio element like , for example , polonium , with a period of life of less than a year, must be present in the mineral in far smaller quantity than radium, n which has a period some thousa ds of times longer . With

- the short lived substances , with periods of only a few hours or minutes , the actual quantities dealt with are always quite inﬁ nitesimal and quite incapable of being detected except by n their radioactivity . A complete k owledge of the chemical behaviour of a radio -element in presence of any possible n mixture of other elements , such as is i volved , for example , i n n n n its extractio from the origi al mineral , is o ly required and is only possible for those radio— elements of period of s the order of a year or more . This clas comprises some n n - s n n i e radio element , which , arra ged in desce ding order of R period , are Thorium , Uranium , Ionium , adium , Actinium , R - R M 1 R and adio lead ( adium D) , esothorium , adiothorium , s Po lonium . The e are usually derived directly from the n n mineral , and i deed for most this is the only way of obtai and - ing them , though radiothorium cannot radio lead and no t polonium need be obtained in this way . Radio-elements of shorter life than these cannot well be n n n n separated from the origi al mi erals , if o ly because duri g the progress of the necessary chemical operations they tend to change as fast as they are separated from their parent n n n substance . They are i variably obtai ed by separati g in the parent as pure a state as possible , in the radioactive

— t sense hat is, as free from other radioactive matter as

— - possible and allowi ng it to g row its short lived products . For the separation of the latter it suffi ces to use means of separation capable of removi ng them from their parent o n

the one hand and from their own products on the other . S uch operations are often relatively simple and rapid , and are often effected by adding a sensible quantity of an inactive substance which has a chemical resemblance to the - n radio element required , which is only present in inﬁ itesimal

amount, and so using the former as a vehicle in the separa

tion of the latter . Thus uranium X is prepared from a

34 TH E CHE MIS TR Y OF THE RA DIO-E LE ME NTS last members of the other series, is not entirely stable , and in the completion of this series there are two moderately long - lived members , radio lead and polonium , not represented at all in the other two series .

But neglecting these , which seem to arise in a ﬁnal recrudescence of activity after stability has been very nearly reached , we may say broadly that the whole of the elements

— of period greater than a year thorium , uranium , ionium ,

e 1 — radium , actinium , m sothorium and radiothorium are at the beginning of the series . We have already seen that , excepting the last , these are the only ones it is necessary to separate directly from the minerals . All the products subsequent to these , and also radiothorium and four others in the early part of the series , are prepared by regeneration R - and separation from the pure parent . adio lead and polonium , as already discussed , are capable of separation by either method . But for another reason it is natural to so divide the series into two , for the purposes of general classiﬁcation .

After six changes in the uranium , ﬁve in the thorium , and two in the actinium series, the products after passing from the character of acidic metals to basic metals , as represented n n by the seque ce ura ium , thorium , actinium , radium , now and assume the gaseous form , these ganseous products are known as the emanations . The precedi g steps have been analogous to the passage from vanadium to calcium , through n scandi u m o r tita ium and , from niobium to strontium , through nd s n zirconium a yttrium . But thi ext step is analogous to n n that from calcium to argo , strontium to krypto , or barium n to xeno . Thus the division of the disintegration series into - t n and - pre emana io post emanation members, dictated by n i n co venience , corresponds with the division the Periodic n n Table , at the end of one long period and the begin i g of the next .

The point of practical importance is that these products , being gaseous at ordinary temperature , actually separate themselves under suitable conditions without any chemical all treatment at . The whole of the succeeding products, l with the two exceptions in the radium series a ready noted , " - “ being quite short lived , they can all be grown in a a form already separ ted from all inactive matter, without E NUME RATI ON AND NOME NCLA T U RE 3S

chemical operations, and can be collected by purely physical methods . Hence the problem of the separation of radioactive minerals resolves itself into the separation of some seven parent elements . From these all the others can be con v enientl y grown as required , the larger number of them very n conve iently via the vol atile emanation . It will be con v i n en e t to consider the latter process ﬁrst, which is the ﬁrst step in the separation of some sixteen , out of the total of - - thirty four individual radio elements that exist . P H Y S I C AL METH OD S OF SEP ARAT I ON

m — n The E anati ons . The ﬁrst ema ation to be discovered 4 was that of the thorium series . Thorium compounds are sharply distinguished from uranium compounds in giving

a - out a radioactive emanation , a gas emitting rays , which f di fuses away from the compound and , when carried by s current or draughts of air , produces radioactive effects away from and out of the direct line of ﬁre of the preparation itself . The gradual accumulation of this emanation inside the electroscope or measuring instrument causes the leak due to the preparation to increase for the ﬁrst few minutes after a thorium preparation is inserted . The emanation escapes freely from the thorium compound under some fi circumstances and with dif culty under others , and the general conditions which favour or retard the escape may

ﬁrst be dealt with . The actinium and the thorium emanations are short- lived and are usually worked with by passing a continuous stream t of air or o her gas through the preparation , which at once S effects the desired separation . olid preparations vary enormously in the ease or diffi culty with which they part with their emanation . As regards thorium , the best emanators in the solid condition are the hydroxide and

. w carbonate After ignition to oxide , the emanating po er is greatly reduced , the more strongly the compound is ignited , until , after ignition at a white heat, it may be reduced to a few per cent . of that of the unignited compound . S olid nitrate of thorium retains its emanation as tenaciously as the fully ignited oxide , and the oxalate and sulphate are both 5 poor emanators . As regards radium , the bromide allows more emanation to escape than the chloride . From the sulphate , and, to a less extent, the carbonate , but little escapes from the solid . Apparently identical compounds may , however , in this respect exhibit a marked variation in behaviour . PH YS I CAL ME THODS OF S EPARA TI ON 37

From what has already been said , it will be understood that the rate of formation of the emanation from any given quantity of radioactive material is totally independent of all physical or chemical considerations . I t is the rate of escape from the solid substance which var i es so widely with the chemical nature and physical conditions . These differences disappear when the substance is dis solved . Bubbling a rapid stream of air through a thorium solution at ordinary temperatures sufﬁces to remove about one half of the emanation formed before it disintegrates , whereas from a nearly boiling solution almost all may be so S - removed . ince the period of half change of the thorium emanation is approximately one minute , this means that , on the average , it takes about a minute for an atom of emana tion formed in the cold liquid to leave it and enter the gaseous phase . The half period of actinium emanation f being only seconds, clearly it would be a very di ﬁcult matter to remove the emanation from an actinium prepara tion without leaving by far the greater part of it to decay

' th e in s ztu . On the other hand , in the case of radium

- emanation , the half period of which is some four days , it is easy , by passing a stream of gas through , or by exhausting the solution , to remove practically the whole . I n the case of thorium the best solid emanators are about equal to a solution at ordinary temperature . That is , some 50 z of the emanation produced may be removed from the solid unignited hydroxide or carbonate . Another method of removing the emanation , usually applicable only to radium n preparations , is by fusion or stro g ignition . I n the case ° n and 20 of radium chloride, the melti g point is at 9 there is a point of minimum ema'nating power which is associated with a point of polymorphic transformation . No doubt at sufﬁciently high temperature even infusible salts would be deprived of their emanation . In a recent m method for the rapid estimation of radiu , thorium , and n actinium in mi erals by the aid of their emanations , a frag ment of the mineral is ignited in a cavity in a carbon rod maintained at dazzling temperature for a few seconds by an 26 electric current . The practical points to be noted are that if the emana w tion is anted, the material is best kept as a solution , but if 38 THE CHE MIS TRY OF TH E RADIO-E LE MENTS the radioactive substance is required to retain its emanation in the open , then , in the case of radium , it is best to keep it - as sulphate , and in the case of the other two radio elements in the form of strongly ignited oxides . It is interesting to note that radium salts , precipitated along with the rare earths as hydroxides or carbonates , have a very strong mana in t g power in the cold dry state . The foregoing deals with the removal of the emanation from the radioactive preparation , for example by a current of air . From the air they can be readily and completely removed by condensation , by passing the gas through a ” - U tube at liquid air temperature . To a very large extent they c an be condensed also by cocoanut charcoal at ordinary “ temperature , a method that might be useful if liquid air was

no t . available , though it is not to be recommended otherwise can The radium emanation , with its relatively long life , be obtained by condensation in a fairly pure condition . The usual plan is to attach a mercury pump to a bulb to o ff containing the radium solution , and pump from time t to time he emanation generated , along with the hydrogen and oxygen from the decomposition of the water . On explosion the emanation is left with the slight excess of hydrogen in very small volume . I t may be handled by ’ the methods described in detail in Travers Exper imental S tudy of Gases exactly as any other gas of the argon family . For complete puriﬁcation it may be sparked with oxygen over caustic potash , or submitted to the action of vapour of calcium . Owing to its energetic chemical action it readily becomes impure again after puriﬁcation , no doubt through its attacking traces of organic matter in the vessels fi employed . I t is dif cult to remove the last traces of carbon dioxide from its Spectrum on this account . The equilibrium quantity of emanation in one gram of radium (element) possesses a volume at normal temperature and pressure of only cubic millimetre . I n view of this extremely minute quantity the amount of information that has been accumulated on the physical constants of this new gas must be regarded as truly amazing .

A ti i — od n c ve Depos ts . The pr ucts of the emanatio s are - in each case non volatile at ordinary temperature , and h ave a relatively short period of life . They are collectively known P HYS I CAL ME THODS OF S EPARA TI ON 39 as the Active Deposits . The separation of the emanations mixed with air or other gases is , naturally, the ﬁrst step in the separation of the active deposits, which , though in inﬁnitesimal quantity, are capable of separation in the pure state , with the greatest ease . The successive products merely have to be separated from one another, not from , as is usually the case , an overwhelming preponderance of inactive - matter . However short lived , they can be simply separated in the pure state by physical methods alone . Their collection and concentration on a metal surface or point is uniformly and simply effected by charging the surface in question negatively with respect to the surface of the containing vessel . I t is probable that all products of disintegration , so formed and free to move in a gaseous atmosphere , possess initially , or acquire instantly , a positive E charge . ven when the radiant particle expelled in the same

a- and disintegration is an particle , is therefore positively charged , the charge of the product , or residue of the dis o f integrating atom , is always positive . The explanation this apparent anomaly is , probably, that in the collision of the recoiling residue of the atom , after disintegration , with the gas molecules in their path a positive charge is acquired , that is , a negative electron is shaken out of the recoiling atom during the collision . If it is desired to collect the active deposit from the thorium or actinium emanations , the latter may be passed by a current of air into a metal cylinder provided with a central electrode of wire along the axis held in place and insulated from the cylinder by rubber corks . The central electrode is attached to the negative pole and the outer cylinder to the positive pole of any source of potential , such , for example , as the electric light mains , a lamp being inserted in the circuit in case of an accidental short circuit . I n most

0 . cases 5 volts , or even less , are ample for the purpose To collect the radium active deposit, the emanation may be stored in a silvered glass ﬂask , the internal coating of which is made positive , and a wire or knitting needle , sheathed in a glass tube to expose only the length required to be made active , and held in the ﬂask by a rubber cork , is made negative .

The potential gradient , or volts per centimetre of dis 40 TH E CHE MIS TRY OF THE RADI O-ELEMENTS

tance , between the electrodes must be increased if the

intensity of the radioactivity is very great . As a general rule it may be stated that the potential gradient required to concentrate the whole of the active deposit on the negative electrode is the same as would be necessary to produce “ saturation of the ionisation current through the gas under

the same circumstances , that is , to collect all the ions in the ” 0 gas at the electrodes . Higher potential gradients than 5

volts per cm . are rarely necessary except in the case of i n

tensely radioactive preparations, or where the concentration f is to be ef ected on a very small area of surface . U nder these circumstances the active depo s it is formed n e tirely on the negatively charged surface , which may be removed at any time from the emanation and studied by n o ff itself . Ofte the active deposit is then dissolved in hydro

chloric or nitric acid , or volatilised in whole or part by

exposure to a high temperature, so th at in such cases the n n egative electrode is preferably made of plati um .

l atﬂ i i nMet — S Yo sat o hods , imple volatilisation at suitable temperatures often effects the removal of a radio-element - n from inactive material , or the separation of one radio eleme t M - from another . any of the radio elements , even when they - exist only in inﬁnitesimal quantity , are completely non volatile ,

when heated in air, even at the temperature of the electric S arc . uch , for example , are uranium X and radiothorium ,

both of which have the same chemical properties as thorium .

But, like the latter , they may be distilled as chlorides from a n carbonaceous mixture in a curre t of chlorine . As a general n rule, the products in the series preceding the emanatio s are - non volatile when heated in air, whereas those following the all emanations may be volatilised at suitable temperature . Under ordinary circumstances polonium is one of the most Mme volatile , and was separated by . Curie from bismuth ,

which it resembles in chemical nature , by fractional volatili S sation . imilarly the C members , which are chemically s identical with bismuth , may be eparated from the B

members , which are chemically identical with lead , as they

are much less volatile , when heated in air .

Latterly , the nature of the atmosphere in which volatili sation occurs has been found to exert a considerable in l f uence . Thus in hydrogen , radium A, B , and C are all

42 THE CHE MIS TR Y OF TH E RADI O-ELEMENTS itself is concentrated , after its formation from the permanently gaseous emanation . The recoiling particles have a certain very feeble pene tratin o n -ﬁv e- a g power, about e hundredth of that of the particle which accompanies their production . Thus the “ recoiling radium D atom from radium C has a range of 1 mm . of air at atmospheric pressure , or roughly mm .

10 0 . in a vacuum of cms of mercury . They exhibit over

a - this range many of the properties of particles , and strongly ionise the air in their path . I n the case of the ultimate

a- products , for example , that produced in the ray change of n polo ium , this ionisation of the recoiling particles of radium " n n G is the sole experimental i dicatio , as yet available , of the existence of this ultimate product , which , not disintegrating further , can no longer be detected once the change in which it is produced is over .

From what has been said , it will be obvious that , as the recoil atoms have o nly penetrati ng power suffi cient to carry them through an excessively thin superimposed layer of material , the successful separation by recoil of the product of an a - ray change depends on obtaining the material as a uniform unweighable , or scarcely weighable , deposit on a t very carefully polished surface . This is suppor ed in a good vacuum opposite to the surface on which it is required to collect the recoiled product , the latter being connected to the negative and the polished plate , bearing the active ﬁlm , with the positive pole of the electric mains . As in all such cases , it is advisable to operate in metal vessels or to cover insu l at - ing glass surfaces internally with tin foil , and to connect all such surfaces, on which the deposit is not wanted , with the S n positive pole . i ce the atoms recoil in every direction , only n those recoili g in a direction away from the plate, obviously,

can be separated , but under favourable conditions nearly the whole of the 50 so recoiling from a plane surface may be - separated . There is no perceptible recoil in B ray changes ,

contrary to what was at one time supposed . TH E C H EM I CAL C HARACTER OF TH E ' RAD I O -E LE M ENT S

Th Rel ati on b etween Radi o -C emi str and C emi — It e h y h stry . has always been a matter for surprise, not to say scepticism , that it should be possible to investigate the chemical character of the evanescent products of radioactive changes , few of which are , or ever can be , studied in weighable or s pectre s c o icall p y detectable quantities , and only one of which , f radium , has been obtained pure in quantities su ficient for an ordinary atomic weight determination . Nevertheless the complete chemical character of the successive products has been steadily unravelled , and very simple and surprising general principles have been found to underlie the whole subject , which have notably enlarged our conceptions of elements in general . Uppermost in the mind of any one approaching the subject from the point of view of chemistry i s the question as to how far such knowledge as can be acquired of the chemical properties of these short - lived radio-elements can be accepted literally , and used as a basis of classification of the radio elements in the same way as the common elements have been n i . classifi ed the Periodic Law , for example It may be urged n n that although an i fi itesimal quantity of ionium , to take ’ one case , behaves , when mixed with a relatively enormous excess of thorium , like the element thorium itself in all respects , the chemical nature of ionium , if it were possible to f prepare it free from thorium , might be quite di ferent . The vague idea that infinitesimal amounts of radioactive - n n matter may possess a chameleon like ature , reflecti g the properties of the substances with which they are mixed

r no t . ather than their own , is j ustified The properties of - a the radio elements , and with certain reserv tions with regard n to adsorption , their behaviour under any defi ed set of conditions, is at least as definite as those of the common n elements . The statement that ionium can ot be separated 43 44 TH E CH EMIS TR Y OF THE RADI O-ELEMENTS

from thorium , and many similar ones to be detailed in the - an sequel , is tantamount to saying that these radio elements c be separated completely from all the other elements differing in chemical reactions from the one specified . Admittedly many have not, and , probably , never will be separated from the element they most resemble . But it is inconceivable that a radio -element when pure might possess chemical pro erties p , allied most closely to those of, say, lanthanum , and be n capable of being separated from lantha um but not from , say , thorium out of a mixture containing both of these elements . In n the first place , we must defi e more clearly the term “ in n - s pure con ection with the radio element . Fir t , there “ is the usual sense conveyed by the word s chemically an n n n pure , denoting ideal rather tha an attai able co dition ,

- be it with radio elements or any other, a sense that may be rightly used , possibly, for radium preparations , such as those on which atomic weight estimations have been made . S is n n econdly, there the sense of radioactively pure , mea i g that the substance is free from all other radioactive sub a stances , though mixed with possibly minute , but never th eles s overwhelming absolute preponderance of inactive matter . Lastly , there is another and very important sense , “ - which may conveniently be termed radio chemically pure ,

- in which the radio element , though mixed, possibly, with any quantity of inactive matter of totally different chemical n nature to act as a vehicle for its tra sport , is yet free from

b . su stances chemically analogous to , or identical with itself S uch is frequently the condition of short- lived products repeatedly separated from their pure parents , and of those prepared as active deposits or by recoil and analogous methods . As the methods of detection and measurement are based wholly on radioactive phenomena, the presence of inactive matter does no t necessarily interfere apart from secondary considerations involved in the absorption of the rays therein and the weakening of the activity on that account . A simple example will perhaps make this clearer . The pro pet ties of the radium emanation by which it is studied are i n not shown by any common gas , and it is a matter of f dif erence with what gas it may be mixed . By passing it n in through various absorbe ts , each case in presence of f su ﬁcient of another gas, not absorbed by the reagent, to serve as a vehicle to transport it , it is a simple matter to CH EMI CAL CH ARA CTER 45 prove that the emanation is not absorbed by any known reagent , and th at it is , therefore, a new member of the argon family of gases occupying the zero group of the Periodic

Table . To thus characterise the chemistry of the radium emanation , a quantity millions of times less than would be visible as a bubble , or could be spectroscopically detected ,

- or weighed on the micro balance , did actually , as a matter f of historical fact , su ﬁce in Though , since then , enormously larger quantities have been available , and it has been worked with in a state of approximate p urity, nothing has been added in consequence to our knowledge of the - chemical properties of this radio element . The example is, of course , a negative one , as it were, because it concerns the absence of all chemical properties in this particular case , but it will perhaps serve as a step towards the understanding of how it is that the absolute m inuteness of the quantities involved are not in themselves a barrier to the study of the

- chemistry of the radio elements .

I n the next step , let us neglect any inactive matter present and concentrate our attention on the case of separating two chemically different radio -elements from S f one another . uch separations are e fected as completely and simply as ordinary chemical analysis, always provided that, if such a mechanical operation as filtration is involved , there must always be sufficient quantity of matter to form n n a filterab le precipitate . U der such co ditions it is always found , for example , that polonium is completely precipitated n in acid solution by sulphuretted hydroge . Ionium is not so precipitated, but is by ammonia , whilst radium is precipitated S by neither reagent , but is completely by sulphuric acid . o a that , in presence of precipitable qu ntities of any of the

members of these three analytical groups , the complete - separation of these three radio elements , chosen as ex u m amples , from one another is as easy with absolutely

weighable quantities as ordinary analysis . Going further, it is equally possible to show that polonium resembles bismuth more nearly than any other of the metals of Group

I I , but does not resemble it absolutely , and is in fact a new

type of chemical element , though it has never yet been i n f obtained quantity su ﬁcient to be worked with , apart

from inactive material to serve as a vehicle for its transport . S imilarly , radium resembles barium more nearly than it 46 TH E CH EMIS TR Y OF TH E RADI O -ELEMENTS

does any other of the metals of Group IV , but does not resemble it absolutely , and is, in fact , another new type of

. n chemical element I t is fortunate , but not esse tial to this o ne ffi conclusion , that in this case su cient radium can be b b b gotmto e weigha le and visi le by itself as a new chemical ele ent , and almost as full information can be obtained for it as for the inactive elements . n Lastly, ionium not o ly resembles thorium more nearly n I II n tha any other member of Group , but is fou d to be n in chemically ide tical in properties with it . Ionium , pre f i n sence of su ﬁcient thorium to act as a vehicle, may quantities , however inﬁnitesimal , be completely and simply separated from any mixture containing any or every known element, by the simple device of separating the thorium ne therefrom by known methods . I onium is a w radi o chemical element , but is not a new type of element . This is a new conception . In every detail of its radioactive pro erties a- s p , the velocity or range of its ray , its period of n n change , the character of its product and of its pare t , io ium f - is distinct and dif erent from every other radio element .

But in its chemical nature it is not new . On account of its t identity in this respect wi h the element thorium , every detail of its chemical nature is known “ by proxy as com l l ete . p y as though it were obtainable , like thorium , by the ton Ionium cannot be obtained “ radio -chemically pure on account of its long period and the presence of thorium in all minerals from which it may be obtained . This is in - s contrast to many of the radio element , similar to ionium in ” new “ n not being chemically , for they can usually be grow , n often v ia the gaseous emanatio , in a condition of complete

- radio chemical purity .

Before pursuing this remarkable new line of development, let us push the question of chemical separations a little further to such operations as do not involve mechanic al s processes like ﬁltration . Volatili ation is such a case, already considered . The extraction of one liquid by another E n is one not yet considered . xtract a uranyl itrate solution

m , b with ether, for exa ple and it will e found that the uranium can be extracted from the aqueous solution completely by a f n n su ﬁcie t umber of extractions , but this uranium , so ex

a- - tracted , gives only rays, whereas the B rays of the original t uranium come from a constituent , really a produc , called CHEMI CAL CHARACTER 47 m uraniu X, which remains in the aqueous layer completely . Now if such a thing as pure uranium nitrate or pure ether were practical , rather than ideal existences , then it would be easy to evaporate away the ether and obtain this B-ray - product of uranium , which is quite non volatile , in the pure “ state . But , unfortunately , however pure the materials used might be , the impurities so extracted would probably outweigh a millionfold the uranium X so obtained , of which , in a ton of uranium , it may be calculated these never can - exist much more than one hundredth of a milligram . A very insigniﬁcant part of such a quantity would make an X- ray screen in the dark visible to a thousand people in a large hall , even though it were mixed with millions of times its own weight of foreign impurities . radioa tive n But if , relying on such c evidence and eglecting such unavoidable impurities , we study the chemical nature of uranium X , again we reach a surprising conclusion . I n chemical nature it also is identical with thorium and with

. nd nothing else In radioactive nature it is unique , a as f 8 di ferent from ionium , on the one hand , giving , and not a - - - o n rays , being short lived instead of long lived , and so , as it is from thorium on the other . But in chemical character f it is not unique . Di fering recognisably from every other known element , it yet resembles one with such undeviating ﬁdelity that processes which are capable of removing from it every other known element always leave the concentration of the subst ance relatively to this one element unchanged . n f th Radi o -El em nt in P i Th e Posi ti o o e e s th e er odi c Table.

— Thus in the broadest way the results of the purely chemical study of the radio — elements lead to the distin

u i shin — a w g g of two classes ﬁrst class, hich are new only on the radioactive side , but which on the chemical side are already completely known and familiar ; and a second class d which , in ad ition to possessing a characteristic and dis tin tiv e c radioactivity, are also new in chemical properties In that is to say , they are new types of chemical elements . other words, we have those which are new only in the fact n a d manner of their demise , but during their existence are - indistinguishable from well known elements , and we have those which , in their normal life as well as sudden death , are unique in behaviour and distinct from every kind of element previously known . 8 4 TH E CH EMI S TR Y OF TH E RA DI O-ELEMENTS

n The ﬁrst class , naturally , are the less i teresti ng but the easier to work chemically with . The second class combine with all the charm of the unknown in chemical behaviour

, b n the added elusiveness of rarely if ever , eing prese t in weighable quantity . Hence the complete elucidation of their chemistry presents a beautiful new problem , which it need hardly be said is very far from worked out . But we must now combine with the chemical knowledge of the products the study of the actual processes of disintegration ,

a- accompanied as it is with the expulsion of particles , or - helium atoms carrying two positive charges, and ﬁ particles or negative electrons . The disintegration series affords a most remarkable picture of the actual process of the production of the n n L eleme ts from one a other , of which the Periodic aw is , as it were , the consequence . J ust as from an instantaneous photograph of a waterfall the movement of the apparently motionless water can be inferred , so from the Periodic Law the continuous transformation of the apparently R unchangeable elements has been suspected . adioactivity has, as it were , cinematographed these transformations , with - the result to day , which none ten years ago could have dared to imagine possible , that in three separate instances we are tracing the successive transit of matter from group to group of the Periodic Table . As was originally pointed out i n the fi rst edition of this b a - ook , the expulsion of the particle causes a change in the position of the radio - element in the Periodic Law by two places, in the direction of diminishing mass, that is , from as S right to left in the Table usually represented . ince then , the increase in our knowledge of the successive products in n the disintegration series , of their order of seque ce, and of M ch aracter their chemical , has resulted in a complete gene ralisati o n being established connecting the position of the

- radio element in the Periodic Table , and the nature of the change in which it is produced . This generalisation , more or

S R K . F n . . a a s less independently arrived at by A ussell , j , and 35 b the author, has been dealt with in Part I I of this ook , and its theoretical signiﬁcance detailed . I n this Part it will only n be necessary brieﬂy to state the nature of the ge eralisation , and to point out its v ital importance i n the practical problems ’ - that arise in the chemical separation of the radio elements .

SO THE CHEMI S TR Y OF TH E RADI O -ELEMENTS

- r element and the other rare ea ths very closely , though there f fﬁ are probable di ferences . The di culty is that actinium , n n bei g a minor bra ch product , is a very scarce and difﬁcult - n K substance to obtain , even for a radio eleme t . nowledge of its chemistry is rather backward on that account . The sole known representative of the homologue of tantalum , which X b rev i u m is known also by the names of uranium 2 or , has a period of average life of only some two minutes, and hence not much detailed knowledge of its properties can be ex ected S p unless a more stable representative is found . uch Z 2 an one (called uranium in Fig . ) in all probability does n exist , but has not yet been obtai ed . There remains polonium , and , as the homologue of tellurium , standing in the Periodic

Table next to bismuth , its chemistry promises to be extremely interesting . But the relatively short life and consequent minuteness of the quantities available mitigate against any complete chemical knowledge in the ordinary sense being obtained . Not that a great deal is not known , but much of t it is not direc ly comparable with other chemical knowledge , and belongs to an order of concentration not directly inv es i ab l e t g except by radioactive methods .

The result , therefore , of this brief survey of the subject is to show that , apart from the three last examples considered , the chemistry of the whole of the radio -elements may now be considered to be known with singular completeness , and the somewhat gloomy prognostications of the hopelessness of attempti ng to investigate such inﬁnitesimal quantities of evanescent materials have been strangely falsiﬁed . Bu t not only has the chemistry of the radio-elements been cleared up in the most wonderful way by this general i i n sat o n. We are face to face with somethi g that is new and vital in the nature of the elements in general . This aspect is discussed more closely in Part I I . The new conception that different radio-elements with ff f di erent atomic weights, derived from di ferent parents and ff yielding di erent products , may yet be absolutely identical in chemical properties , receives a ready explanation from

Fig . 3. Chemical tests would only distinguish between ten - kinds of radio elements , but the radioactive evidence shows

- that there are thirty four . The separate places in the Periodic t o Table hus do not correspond with single elements , nec s saril i . y , but with s ngle chemical types of elements All the CHEMI CAL CH ARACTE R 51 elements in the same place exhibit identical chemical proper “ ” ties . For this reason these are called isotopes . Ionium , thorium , radiothorium , and uranium X are isotopes, meso 1 - thorium is isotopic with radium , radio lead with lead , and so on . I t is probable that it is the nett internal positive charge of the nucleus of the atom , and not its mass , which o ne conditions its chemical properties . The expulsion of a and two B-particles from the nucleus leaves this charge and the chemical properties the same as originally , though the mass is reduced by four units by these changes . The successive “ places in the Periodic Table correspond with f unit dif erences in the nett charge of the atomic nucleus, and the identity of this charge is all that is implied by chemical n n proofs of homogeneity . This charge cha ging u it by unit gives us the Periodic Table of elements . I n Fig . 3 the magnitude of this positive nuclear charge , which is called the

a . atomic number , is indicated t the bottom of the diagram Uranium is the last and heaviest of ninety -four possible elements . The real complexity of matter, now that isotopes

- have been shown to exist among the radio elements , may be ’ far greater than the C hemist s table of the elements indicates . The possibility is not excluded that some common elements may be isotopic mixtures .

I n contradistinction to the unit negative charge , or electron , which is so nearly massless , the positive charge is invariably associated with a mass of atomic magnitude and may be such a mass . It is not impossible even that its mass may be to the ﬁrst approximation constant , and the same as

a- that in the case of the particle , namely , two units of mass n per one u it of positive charge , though hydrogen with mass

1 and atomic number 1 is an exception . For all the other elements, the atomic mass is either equal to or greater than twice the atomic number , and as the atomic mass increases , it increases faster than twice the atomic number . We have

, evidence that negative electrons , as well as positive charges enter into the constitution of the heaviest elements , because - they are expelled therefrom as B rays . Uranium so expels six electrons in its charges . The atomic m ass of uranium 2 38 — corresponds With between 1 1 9 and 1 20 positive charges on the simple idea being considered . Its atomic 2 number or nett nuclear positive charge being 9 , it would be necessary to suppose that the nucleus contains 27 or 2 8 52 TH E CH EMI S TR Y OF THE RADIO -ELEMENTS negative electrons in its constitution . Whether or not such simple ideas may help to solve the problem of the true significance of atomic weights it is , however , too early yet s to tate . Practi cal Consequences of th e Exi stence of Isotopes In dealing wi th very large quantities of minerals of complex i n composition , which the only safe plan is to assume that o ne- probably half , at least , and possibly all the known eleme nts are present in concentration greater than that of - the radio element it is desired to separate , it is necessary to concentrate by successive stages . At some stage it is usual an n f n to add eleme t , if not already present in su ficie t quan tity , either chemically similar to or isotopic with that which it is desired to separate . As an example of the first case, in the manufacture of radium , the addition of barium to the n reage ts before use , to remove sulphates and to ensure a n co venient quantity of concentrate , capable of being handled

. 8 0 without proportionately large loss , is a common practice n s also i analysi , the addition of a member of the same group as the radio -element is often a very useful preliminary to its complete separation , ﬁrst with and then from the added n substa ce . But when the element added is isotopic with that being separated , though the addition may enormously simplify the separation and enable a complete separation to n be carried out , it can never be removed agai . The device then entirely prevents the radio-element from bei ng prepared - H in a radio chemically pure form . Thus oltwood in his work on ionium adopted the device of adding thorium nd repeatedly a separating it , to insure the complete removal

of the ionium from the mineral , which was the object of the work rather than the preparation of the ionium in co ncen “ ra d . On in h i s t te form the other hand Welsbach , work on the rare -earth fraction of thirty tons of J oachimsthal pitch

blende , purposely added nothing analogous to the radio 37 fi elements therein . However, there was suf cient lanthanum and thorium present in the original mineral to preclude the actinium and thorium being separated in the radio - chemically n pure state . Nevertheless , the amou t of thorium present was so small that the ﬁnal ionium preparation weigh ed only

a few grams , and , there is little doubt , must have contained a considerable percentage of its more radioactive twin brother

ionium . CHE MICA L CHARA CTER 53 Considerations such as those j ust considered underlie and limit the possible extraction of the technically important - n d l . I ra io e ements in minerals the uranium minerals , radium and polonium , being new chemical elements , of which no stable representatives exist, are readily obtained in a conc on trated form . If the first only has so far been obtained in the pure state, there is at least no known theoretical reason why ultimately the latter and also actinium should no t also In be so obtained . accordance with the principles already stated , they are separated first with and then from the barium , bismuth , and lanthanum in the mineral . The attempts so to separate the predicted long-lived homologue of tantalum have so far failed , owing, doubtless , to the extreme variability and uncertainty of the chemical properties of tantalum , and possibly to the resemblance not being sufficiently close to 38 serve the purpose . But there is no possibility of obtaining radio - lead in n (radium D) from minerals co centrated form , because w it is isotopic with lead, and there is al ays a considerable amount of lead in such minerals . I n consequence , this product is technically of very little use , though if it could be concentrated it would be as valuable as the others . As lead is believed to be the end product of the disintegration , it would seem to be definitely impossible ever to extract the - n radio lead from uranium minerals in a co centrated form . The only way of preparing radio- chemically pure radio -lead is to grow it . The emanation of radium must be pumped o ff as it forms , and left to undergo its rapid changes in

- sealed vessels . The radium D or radio lead, in course of time , then accumulates on the walls . Using considerable ’ fractions of a gram of pure radium , the quantity of radio lead so accumulated may , if capillary tubes are employed to contain the emanation , come within the region of visibility as a submetallic deposit . Ordinarily, however, it is quite invisible , though its radioactivity may be very great . The

- - radio lead , separated from pitch blende with the whole of the contained lead , although but feebly radioactive , is not entirely u seless , because it is a valuable source of polonium . The polonium that accumulates i n it as time elapses may readily be separated from the lead and got in concentrated form .

But, of course , the somewhat short life of polonium militates - against its usefulness, and the newer radio element, ionium , 54 THE CHEMI S TR Y OF THE RADI O-ELEMENTS

is, on account of its permanence , to be preferred for almost all purposes for which polonium at one time was used .

The case of ionium has already been considered . Clearly the limit of concentration is set by the quantity of thorium in the mineral . It is very fortunate that many of the secon dary uraninites contain such minute amounts of thorium , not so much on account of the ionium , but for another reas on that will shortly be clear . This exhausts the list for S - the uranium minerals . hort lived elements like uranium X are , of course , always grown from the pure parent , which , after puriﬁcation from any especially objectionable impuri ties , is simply left to reaccumulate the product , which is

n . the separated As that separated decays , another quantity reforms , and the separation must be repeated as often as , fo r or as long as, a supply of the evanescent material is required .

I n the case of the thorium minerals , matters are more complicated . Though uranium minerals are known , like the

- secondary uraninites or pitch blendes , which are practically free from thorium , the converse is not true . All thorium minerals seem to contain uranium , the lowest ratio between n the eleme ts being that for Ceylon thorite , which contains 1 some 60 z of thorium and from or 2 z of uranium . Hence all thorium minerals contain all ‘ the known radio In elements to greater or less degree . Ceylon thorianite , where the thorium is present in from three to six times the quantity of uranium , the radioactivity of the products of the two series is of about the same order , owing to the rate of change of uranium being more rapid than that of thorium .

Considering ﬁrst a pure thorium mineral , to which Ceylon n thorite nearly approaches , and assumi g , what is never the

- case , that uranium is completely absent, the long lived and technically important constituents are mesothorium 1 and radiothorium . Neither is a new chemical type , the ﬁrst being M isotopic with radium and the second with thorium . eso thorium is readily separated , l ike radium , by the addition of barium , if necessary , but the separation of radiothorium is ,

- and will probably for ever be, impossible . Like radio lead in co ncen the uranium series , the only way of obtaining it in trated form is to grow it from its pure parent mesothorium, after the latter h as o ﬁrst been separated from the thorium al mineral . The whole of the radiothorium in the miner CHEMI CAL CHARA CTER 55 originally— and this is responsible for by far the greater part

— of the total activity is thus useless . The period of average f life of radiothorium is about three years . In e fect, one loses three years ’ production of radiothorium however well the chemical operation may be planned and carried out . But the further consideration of this somewhat complicated case may be reserved until the thorium products are con s id r e ed more in detail . L astly , let the mineral contain both uranium and thorium , and such minerals are very numerous and important . The period of thorium being three or four times as long as that of uranium , in a mineral with three or four times as much thorium as uranium , the thorium series of products will be of similar order of activity to the uranium series of products . M esothorium , being isotopic with radium , is necessarily separated with the latter . Hence pure radium can only be prepared from a pure uranium mineral . All radium pre pared from mixed minerals is necessarily contaminated to greater or less extent with mesothorium . As the activity of pure radium is the usual standard in which radioactive d measurements with concentrate preparations are expressed , and as mesothorium has the comparatively short period of average life of eight years and its activity is a complicated function of the time, the origin of the radium to be used as t standards of measurement is thus of vital impor ance . As already remarked , it is fortunate that so many of the tech ni cally useful uranium minerals are practically free from thorium . All the ionium and radiothorium in thorium minerals will remain with the thorium , and cannot be concentrated s therefrom whatever chemical processe be employed . Hence t all horium preparations necessarily contain some ionium , a and if prepared from common thorium miner ls, this radio active impurity may be very important . It would be possible to cite numerous instances from the literature where failure to appreciate these f undamental considerations had involved possibilities of error .

S ince ionium is the parent of radium , all thorium pre ar i o p at ns, however carefully they may be puriﬁed from this w element, will gro radium in the course of time , withal in a e e s minute, though not , in special c s s, n ce sarily unimportant , quantity . 56 TH E CH EMI S TR Y OF TH E RADI O -ELEMENTS

Radioactive analysis recognises some thirty -four radio

- elements , whereas chemical analysis, of these thirty four , only distinguishes ten . Hence by chemical analysis of a mixed uranium-thorium mineral only ten groups of radio-elements n are able to be separated . But by coupling chemical a alysis with the lapse of time , the analysis may be pushed further . As a single example — Is the radium separated from a mineral pure or does it contain also mesothorium ? S ome s I II time after the eparation a trace of a Group element, such as aluminium , added to the radium and precipitated by ammonia , will bring down the regenerated radiothorium , if no t n R mesothorium is present , and if it is abse t . adio thorium may be distinguished very sharply by its power again after lapse of time — to produce the characteristic n thorium ema ation . It may be remarked that this test is so delicate that it will detect the mesothorium in radium pre - pared from joachimsthal pitch blende . That is to say , it will reveal the thorium present in this mineral , although it is less, n probably , than one gram per ton of ura ium . I n the foregoing section the main consequences of the existence of isotopes in the separation of radioactive minerals n s have bee discu sed purely from the practical point of view . We have learned that radio -lead and radiothorium cannot be separated from the original constituents of the mineral , n but that , ﬁrst , their parents must be separated , and the , from these parents after the products have accumulated , n separatio by chemical means is possible . This , the universal

- method of obtaining short lived products , is in these cases tedious and time -consuming by reason of the comparatively long life of the products being grown . Also , that radium and thorium are very liable to contain mesothorium and in ionium respectively , in addition to their own products , which cas e no process of chemical puriﬁcation is of the la e slightes t avail . The only way is to start in the ﬁrst p c

- from suitable minerals , thorium free uranium and uranium t free thorium minerals , respec ively . But these are the only important cases . The wonder is , rather, that the chemical identity of s o many of the products with one another and with common elements interferes so little with their separation by chemical methods .

. 58 THE CHEMIS TR Y OF THE RADI O-ELEMENTS nature of the substance adsorbed . In the analytical re actions o i the common elements it is only remarked when the adsorbing surface can take up weighable quantities of

— the adsorbed materials quantities , that is , which correspond 18 to more than 1 0 molecules . A given quantity of an n adsorbe t might be able , for example , to adsorb a million 18 1 0 molecules of most substances, and more than molecules of one particular substance or molecular species . I t would an then be regarded as adsorbent of the one substance alone . In a solution containing a radio -element the adsorbent might adsorb the radioactive substance , if present in quantity below a million molecules , completely , provided only that no other chemically similar substance was present . But if the radio -element were physically and chemically indistin u ish ab le g from some common element, which was present in great excess , though , possibly , still far less than weighable

- quantity , the adsorption of the radio element might be practically completely prevented . - Owing to adsorption those radio elements, with chemical and physical nature i ndistinguishable from a common element always present with them in the natural minerals , i will behave def nitely, whereas those others which are not allied to any known common element in properties will probably behave the more indeﬁ nitely the shorter their period of life , and the more they depart in chemical character from any impurities that may be present . Thus in these respects the properties of radio-lead and polonium , separated directly from minerals, are likely to be very different from those of the radio -chemically pure substances grown from the emanation . Another example is the prevention of the adsorption of ‘ 0 uranium X by a trace of thorium . On the view put forward , provided that the added isotope is not adsorbed , its addition must completely prevent the adsorption , as it h as been shown to do in this case . Clearly this suggests a test as to whether a radio -element is the isotope of a

- common element . For if the radio element continued to be adsorbed without the isotope , a separation would thus be effected and the two substanc es could no longer be

- regarded as non separable . There is, however, no example of this, though speciﬁc cases have been examined . Not ELECTRO-CHEMI S TR Y OF RA DI O-ELEMENTS 59

as much work as is desirable , however, has yet been done

on this point .

Later work has shown , both , that other substances than thorium , such as zirconium and even benzoic acid , have a ff similar inhibiting e ect on the adsorption of uranium X , and also that thorium prevents the adsorption by charcoal of 41 other substances than uranium X , for example , copper . This is of great interest from the point of view of the nature of adsorption , but there is nothing unfavourable in it , as has been alleged , to the existence of isotopes . The important point is that thorium does inhibit the adso rp tion of uranium X, and no case is yet known of an adsorption process altering the ratio between a radio n element and a inactive isotope .

- mi tr — M - El ectro Che s y . any of the radio elements , parti cu larl y those following in the series after the emanations , are readily deposited from solution on immersed metals . Thus polonium is one of the most noble or readily deposited, and may be deposited on platinum , bismuth , or - copper , and even (in the radio chemically p ure state) on glass to a considerable extent . If a separation from other

- easily deposited radio elements is desired, bismuth is to be recommended . The C members, which are isotopes of bismuth , are more noble or readily deposited by metals than the B members, which are isotopes of lead , and nickel , n in sheet or fi ely divided form , is much used to separate these two successive products . Its specific behaviour in this respect , depositing the C member nearly completely, is rather remarkable . Thorium D and actinium D , the isotopes l of thallium , are less easi y deposited than the B members . kno wn' v o n Before the latter fact was , Lerch put forward as a general rule , which is known by his name, that the successive products in a disintegration series are pro gres s v - i ely more noble in their electro chemical behaviour . This is true of the B and C members and of radium F , which were the members first studied . A glance at Fig . 3 will show that to the left of the emanation group the successive places in the Periodic Table represent a continuous decrease - in electro chemical nobility , in order thus : ( 1 ) polonium 2 or radium F, and its isotope radium A ( ) bism uth and its E isotopes, the C members and radium ; ( 3) lead and its 60 THE CHEMI S TRY OF THE RADI O-ELEMENTS

isotopes , the B members and radium D ; ( 4) thallium and the isotopic D members . To the right of the emanation o n group , the members are never deposited immersed metals . I n electrolysis , radium and its isotopes , like barium , may be to some extent deposited on a mercury cathode , or n 42 on other electrodes in alkaline solutio . Actinium and its isotope mesothorium 2 may be electrolytically deposited “3 from a boiling, nearly alkaline , solution on a silver cathode . The same method has been recently employed to separate and obtain in the form of a good coherent deposit radio thorium from its parent mesothorium But with these isolated exceptions , electrolytic deposition is usually confined - to the post emanation members . The chemical identity of isotopes has been fully con - firmed , and extended by the study of their electro chemical “5 behaviour . Further , in this field , as the researches of von

Hevesy have recently shown , the possibility of obtaining the materials experimented upon in the radio -chemically pure condition , in absolutely infinitesimal but yet easily n determi able concentration , has resulted in advances of - “6 much interest in the science of electro chemistry itself . I n the first place , dealing with ordinary substances , before a detectable amount of a metal is deposited on the cathode , f a certain potential dif erence , known as the decomposition voltage , must be exceeded . But with radioactive methods the deposition can be detected even with such infinitesimal an quantities as , according to theory, must be deposited by y difference of potential , and the decomposition potential need not be attained before notable deposition occurs . This leads us to expect that every metal is capable of causing the ‘ n - depositio of every radio element to greater or less , but always detectable, degree , and , in neutral solutions , this has been found to be the case for a large number of the radio elements and many metals , not excepting even the noble metals .

Again , in dealing with these inﬁnitesimal concentrations , even insulators , like glass , can act as a metal and deposit - n a perceptible amount of a radio eleme t . There is, probably, a potential difference between glass and the solution in which it is immersed which can be detected and studied - t by this deposition of radio elements , hough not by the ELECTRO -CH EMIS TR Y oF RADI G-ELEMENTS 61

- ordinary electro chemical methods . This is one of the n n - da gers in worki g with radio chemically pure polonium . E xtreme precautions are necessary to avoid its loss by deposition on the walls of the containing vessels . It may e be redissolved by str ng acids , but on dilution it is again deposited . The mental picture one forms of the deposition of copper on the cathode , for example , by electrolysis of a copper solution , is very far from correctly representing - the deposition of a radio element . I n the ﬁrst case the electricity transported to the cathode is carried entirely by in the copper ions , whereas the deposition , for example, of radium B and C on the cathode by electrolysis of the solution , the quantity of radium B and C ions present is

an . insufficient to convey appreciable current It is necessary , rather, to neglect almost entirely the transport of electricity, n or the electric curre t as such , and the total voltage between n the cathode and anode , and to co fine attention solely to the single electrode potential between the depositing metal and the solution . Without current flowing, every metal immersed in a solution containing its own ions takes up f a definite potential di ference , and , according to the mag n - ni ud P . D a t e of this . , will deposit y radio element in solution to greater or less degree , without an anode or without any electric current flowing . But , when a current

P D . flows , this natural . . is more or less modified In the case of a noble metal such as is frequently used for a cathode , the natural potential the metal takes up is positive to the solution , but when a current flows from an anode f to this cathode its e fect is to diminish , and, if strong e f i n enough , to rev rse this natural potential di ference , f - creasing the amount o the radio element deposited . I t is “ by such considerations as these, rather than on ordinary c - electrolyti lines , that the behaviour of radio elements in the radio-chemically pure condition on electrolysis is regu th e lated . Polarisation of the anode , again , by passage of ff a heavy current , may by its e ects on the single potential n - d iff ere ce cause a radio element , like polonium , to be deposited on the anode as well as on the cathode . Let us now consider a solution of either of the three S active deposits in dilute acids . uch a solution after the 62 THE CHEMI S TRY OF THE RADI O-ELEMENTS disappearance of the short-lived A member is practically a mixture of the B and C members . Now immerse in such n a solutio a suitable electrode , and, as may be done t n practically in a variety of ways , pass from a s ro gly electro

P . D positive . , such as is given by a noble metal , to a strongly - P . D . electro negative , such as is given by easily soluble n metals like zinc or mag esium . At ﬁrst the deposit on the

' C memb er electrode will be practically the pure , but as b - it ecomes more and more electro negative , more and more of the B member will accompany it . At a potential of volt, measured against the calomel electrode , the two members are deposited in equilibrium proportion . “ ” Th Radi -El em t a l ndi cat rs - e o en s s o . Now the ratio of the B to C member in the deposit is readily deduced from the form of the decay curve of the a -rays from the deposit obtained . The radiation from pure C would decay n expo entially and relatively rapidly with the time . That from pure B would ﬁrst increase owing to the growth of

C and then decay more slowly . From the form of the curve the initial ratio between the two members may be

. P D deduced Hence , from a knowledge of the . corre s po nding with any given proportion between the B and D C member deposited , the R . itself may be determined if the proportion of the two members in the deposit is In - found . this way the radio elements may be used as i ndicators in determining single potential differences not otherwise experimentally determinable .

Thus , on the Nernst theory of the origin of every metal immersed in absolutely pure water should be n i negative to the water u t l some of the metal , not necessarily n an analytically detectable qua tity , has dissolved as ions . Thus copper at the ﬁrst moment of its immersion in a copper -free solution should behave as a strongly electro negative metal like zinc . After an immersion for second , of a copper electrode in a solution of the active deposit , the ratio of the B and C members deposited showed

— 2 V . that the potential of the copper to the solution was , with reference to the calomel electrode . Thus the use of the radio - elements as indicators in electro -chemistry has already produced interesting and welcome conﬁrmation of the exist ing theory, and the ﬁeld opened out is a very wide one . ELECTRO-CHEMI S TR Y OF RADI O-ELEMENTS 63 Quite in accord with the inhibition of the adsorption

- - of a radio element , is the prevention of the electro deposition 45 by an inactive isotope . Thus , if by suitable adjustment f of the potential di ference , it is desired to deposit pure thorium C from the solution of the active deposit , the addition of lead , the isotope of the B member, in appreciable de quantity is advantageous . I n presence of lead , the V i . position o thorium B is appreciable only at , whereas in the radio- chemically pure condition perceptible In deposition occurs at V . presence of bismuth , the

V . C member is deposited at a potential of , and a bismuth plate immersed in the solution showed the same potential . A very interesting detail about thorium B is l n that , exactly like ead , it is deposited on the a ode as peroxide at a potential between and V . Good use of this behaviour of lead has been made in the separation of

- E polonium from radio lead extracted from minerals . nough concentrated nitric acid is added to the lead nitrate solution to prevent the cathodic deposition of lead . U nder these conditions most of the polonium can still be deposited at ‘ E the cathode , along with radium , which , however , quickly decays . S o far as it has yet been possible to examine them , the

- radio elements isotopic with lead and bismuth show, in

- presence of these elements , an identical electro chemical behaviour whereas by studying them in the radio chemically pure condition , it has been possible to examine, - by proxy , the electro chemical properties of lead and bismuth at potentials both above and below the decomposition vol

— tage over a range, that is , which otherwise could not be studied . But it is not only in electro -chemical work that the - r radio elements may be made to give , by proxy , info mation in about common elements . The solubility of the most o i and soluble compounds lead , the chromate sulphide, have been directly determined by experiments with the isotopic 47 fi radium D as indicator . If a suf cient quantity of the latter i s mixed with the lead compound , the small part dissolved will have a detectable activity, from which its quantity may be deduced , although too small to be estimated analytically . I n this way it has been found that the solubility of lead 6 4 TH E CH EMI S TR Y OF TH E RADIO -ELEMENTS

° 2 x a chromate at 5 is grams per litre , and of le d sulphide 3 x in water, and x in presence of M excess of hydrogen sulphide . any other examples suggest themselves in which ionium or uranium X might be used as n E i dicators for thorium , radium for bismuth , and so on .

Exi stence of Coll oidal Sol uti ons of th e Radi o -E m n le e ts . fi n n Although , in suf ciently conce trated acid solutio , it is b b - t d pro a le that the radio elemen s exist as positive ions , an their transport to the cathode during the passage of a current to is analogous ordinary electrolysis , with the differences already alluded to, the researches of Godlewski and Paneth have shown that frequently the radio-elements exist as n colloidal rather tha ionic solutions , and their electro chemical transport is allied to electrophoresis rather than “ electrolysis .

To take ﬁrst a very striking and simple case . Polonium (radium F) may be separated from radium D and E by dialysis of a nearly neutral solution of the radio -lead nitrate through animal membranes or thin parchment paper . and E b u t The lead , radium D , pass through as crystalloids , n the polonium remai s behind in the cell in a pure condition . Further work showed that i n neutral solution the velocity E n of dialysis of radium , and in an ammo iacal solution of thorium B , may be much reduced . In general , according to and the degree of acidity , both hydrosols ions of the same - t radio element may exist toge her in the solution . The change from crystalloid to colloid is accompanied by a n cha ge of sign of the electric charge , and the colloid particles migrate in an electric ﬁeld to the anode instead of to the cathode . I f a parchment membrane is inserted in such a solution containing both forms , the cathode deposition is not prevented but only hindered , whereas the anodic i s deposition completely suppressed . Much work has also been done on the radium active deposit formed in pure water containing dissolved emanation .

Under these circumstances all the products, radium A, B, and C are present as hydrosols , but the A member is deposited on the anode , the B member on the cathode , In and the C member on both electrodes . other words, the A member exists as a negative hydrosol , the B member as a positive hydrosol , and the C member as

66 TH E CH EMI S TR Y OF TH E RAD I O-ELEMENTS sharp and definite chemical and phy s ical behaviour of these almost inconceivably small amounts of the radio-elements t itself raises many ques ions of the highest general interest . The great advances made have been largely due to the clear and consistent recognition of the important conclusion inherent in the theory of atomic disintegration , that radio elements are completely normal and ordi nary in their behaviour and in no way to be distinguished from inactive elements , during their whole period of existence . Their radioactivity is an instantaneous phenomenon terminati ng n ff that existe ce , but not modifying or a ecting them previously , i n n though sometimes , as recoil phenomena , givi g them at n the moment of production , certain ascent peculiarities of i s s n their own . This , of course , the antithe is of the atural an attitude of y one approaching the subject for the first time , who would be tempted vaguely to ascribe any abnormalities encountered in their behaviour to their possession of radio active properties . But as our understanding of the subject becomes deeper, it may probably be necessary to make some distinction between the chemical and physical proper o f n n ties radioactive and i active eleme ts , respectively, owing , n not directly to their disi tegration as such , but to the n n n liberatio of electric charges duri g the disintegratio . S n n uch charges may or may not, according to the co ditio s , and neutralise one another, in any case they are likely to be n in f n n more e during their e fects tha the recoil phe omena , s for example , j u t alluded to . One of the earliest examples of the extremely s harp and definite behaviour of almost inconceivably small amounts of the radio -elements was furnish ed by the condensation of the emanations at low temperature . S ince then many equally definite examples of differences in volatili ty of successive in and products a series have come to light , indeed , this n affords one of the general methods of separatio employed . Much work has been done on the condensation of the s emanation , without entirely elucidating the nature of the s n process . The vague idea that every ubsta ce has a vapour pressure at every temperature , and that , if it were possible to follow the vapour-pre s s ure temperature curve into the region of infinitesimal pressures , it would be found to approach a zero value asymptotically rather than abruptly , COLLOI DO-CHEMI S TR Y OF RAB I O-ELEMENTS 67 does not accord well with the facts disclosed in the study of the condensation of the emanation . I n a state of compara tive purity, the radium emanation exhibits a vapour pressure curve very analogous to those of the other inert gases .

When in considerable relative quantities , a small vapour pressure seems to exist even at the temperature of liquid air , for the emanation condensed at the latter temperature may be slowly and continuously pumped away by a mercury t pump . But in the smallest quantity , the completeness wi h c whi h it is removed from admixed air , for example , by passing through a tube cooled in liquid air , is very remark f o n able , and points to some e fect of the absolute quantity the vapour pressure which is foreign to our experience with d M or inary liquids . inute droplets of condensed liquid may be expected to have a higher vapour pressure than drops of a larger size , but here we have , on the one hand, the complete removal from a gas of a minute quantity of emanation at liquid air temperature , and on the other , the steady vaporisation at the same temperature of far larger quantities of the emanation from a considerable quantity n n m of condensed ema atio in vacuum . There is also so e evidence of a maximum of vapour pressure, a point , that n is , from which both by raising and loweri g the temperature s the vapour pressure is reduced . Pos ibly this is the explana n n R i n tio of the fact , first oticed by utherford , that, a tube containing radium emanation immersed in liquid air and left n for some time , the ema ation leaves the place where it first condensed, although this is kept below the level of the liquid ai r and concentrates in a ring a few millimetres abov e the th e surface of liquid air, at a place , that is, some few degrees above the minimum temperature . If the tube be then further immersed, in the course of time the ring of con densed emanation , as revealed by the fluorescence of the glass , mounts to its former position relative to the level 49 of liquid air . Making all allowances for the secondary effects of radio activity, the heat liberated during the disintegration , the

i and n disturbance produced by reco l , the liberatio of free c f charges of electri ity , and for e fects analogous to adsorption n on the walls of the co taining vessel , it would appear that the phenomena encountered in the condensation of the 68 TH E CHE MIS TR Y OF TH E RAD I O-ELEMENTS

n ema ation are not fully elucidated . Further study might lead to important new light being thrown on the proces s and n of vaporisation condensation , a alogous to that already - thrown on electro chemical phenomena . s Nor, on the chemical side , is the harpness of many chemical separations of the radio -elements entirely to be anticipated . Often , no doubt, there are causes which con tribute to make the behaviour of the radioactive substance more deﬁnite than could be foreseen , and one of these is undoubtedly the presence of other substances with similar n properties . Thus in a solutio containing barium and t radium , the radium may be precipitated quan itatively by sulphuric acid although present in quantity so minute that it is inconceivable that the radium sulphate would not be n completely soluble if the barium were not prese t . It is , s however , signiﬁcant that the radium in such a ystem will redissolve again in the course of time if alternately heated 50 and cooled and agitated .

If one took the simple theory literally , before any pre c i itate n p could be formed , the solubility product of the io s n must attain a certai value . But , in the vast majority of s - ca es dealt with , the concentration of the radio element is far below the saturation value of the most insoluble sub n n f . o stances k own Yet , freque tly , the precipitation the - n t radio eleme t wi h a precipitate of a substance , chemically similar but not identical , is extraordinarily perfect . It is clear that whatever may be the co nditions of ﬁnal equi librium , a very rough similarity between substances results in their replaci ng o ne another in the initial precipitate o ne produced , though the may be in concentration nowhere approaching its true solubility product in the radio -chemically In state pure . n Lastly , what is perhaps even more surprisi g , some of

- - these radio elements , even in the radio chemically pure state , especially in neutral solutions , are found to exist in the colloidal state . Here again , since the concentrations are a re far below the probable solubilities, it is clear that the gg gati o n into colloidal particles cannot be due to completely known causes . It has been suggested that what in fact may occur is the adsorption of the atoms of the radio -elements by existing colloidal particles in the solution , such as those COLLOI DO-CH EMIS TR Y OF RADI G-ELEMENTS 69

a of luminium hydroxide or silica, derived from the walls of the containing vessels . But , if this were so , it would mean that such existing colloids must exercise a selective action ,

- adsorbing some and not others of the radio elements present . The radio -elements offer exceptional facilities for the nearer study of adsorption phenomena, and it may be expected that along these lines the , at first sight , very inexplicable facts in connection with the definiteness of the physical and chemical behaviour of matter in infinitesimal quantity may

ﬁnd a further explanation . SY STEM AT I C DES C RIP T I ON OF T HE RAD I O -E LEME NTS

U RAN IU M (U)

U RANIU M I AND U RANIU M 2

g i i i 2 s 51 i. f e S3 2 3 S é a S 5 5 5 9 ‘ E a g 3 s ; 5 2 s s : a s m i s

Atomi c weight 234 (estimated to nearest

uni t) .

I: e Abo ut 8 ooo ooo ooo ea P eri od a er 3 , , . y rs . Abo ut yea rs (i ) - Radi ati on a rays a -rays - f Range of a ray s (cm. o ai r)

P arent U rani um X, (P) t rani um X D is i nteg rat io np roduc U l Io ni um(P)

r i um 1 an U rani um 2 R Note on U an d . ight at the commencement of the series , we are faced with the necessity n n of co sideri g a chemical element , hitherto regarded as n homoge eous , to be a mixture of two chemically identical or isotopic elements , the second the product of the ﬁrst after

- - a . one ray and two B ray changes (see Part I I , p The evidence of this is necessarily indirect , for the product uranium 2 is too long in period for it yet to have been grown from its supposed parent in detectable amount . The main evidence is that uranium gives two a-particles of f slightly di ferent range per atom of uranium disintegrating . The period of uranium 2 has been deduced from the range

- a . 28 of its rays (p ) as about years, and , if this is correct , it follows that there must be about one part of uranium 2 in every three thousand parts of uranium 1 in ” 2 all uranium . Its estimated atomic weight is 34 instead 70 URANI UM 7 1

2 8 of 3 , so that the error in the determination of the atomic weight of uranium on this account is negligible . The division of uranium into uranium 1 and 2 is of n purely theoretical importa ce at present . Practically, the sub a - two stance behaves as homogeneous radio element , giving a- s particles per atom disintegrating . By no known proce s has it yet been found possible to disturb the existing equ i f librium ratio , and though fractional di fusion of the solution , f or better of a volatile compound , would , no doubt, e fect a separation , this has not yet been done . But if such a sepa ration were possible , then one would expect to divide uranium into two parts , the one consisting of some the per cent . of whole mass retaining only one half of the a - radiation , the other only per cent . of the whole mass

a- but possessing the other half of the radiation . Thus , if 2 1 0 0 pure uranium could be isolated , it would be about 3 times as radioactive as ordinary uranium , mass for mass . The slightest alteration of the concentration of the two constituents could thus be readily detected experimentally , and the case is a very favourable one for further investiga tion . An experiment by the author in which uranyl nitrate was placed at the bottom of a long tube , which was then f carefully ﬁlled with water , did not, after di fusion had been in progress for over a year, lead to any detectable increase in the uranium 2 in the upper layers .

— . f n Occurrence The most important deposits O ura ium , - U O & c . radium , , occur as pitch blende or uraninite (chieﬂy s S ) at St. J oachimsthal , Bohemia , which , however, produces rela tiv el s S y le s than formerly at J ohann Georgenstadt , axony ; E in Cornwall , ngland in North Carolina and Connecticut,

A . U S . . I t has been found in small quantities but very high E quality in German ast Africa, and recently considerable amounts of an exceedingly rich uraninite have been found

. 0 in India , An ther important ore is carnotite (uranium n i n potassium vanadate , fou d chieﬂy

S A. 2 U . . the It comes into the market containing z, or

, ) , and the residue , left after more of uranium oxide ( U SOS extraction of the uranium and vanadium , forms the chief, if not almost the only , important source of radium in the large - quantities in which it is demanded for medical uses to day .

Another ore is autunite , found ﬁrst at Autun and now in 72 TH E CH EM IS TR Y OF TH E RADI O-ELEMENTS

Guarda, a district of Portugal , a uranium and calcium phos phate , occurring in pockets in very n pure conditio , but for the most part as a very low grade

4 M h earin 1 . U O . ore , g from } to per cent of B S ore recently , some Aus tralian minerals have assumed technical importance . s n The e are usually very complex a d refractory . The best O S known is that from lary , outh Australia , where it occurs in a lode formation of magnetic titaniferous iron , magnetite o w and quartz , in association with bi tite . As orked up at S s ydney for radium , the cru hed ore is ﬁrst concentrated

0 s o b , magnetically, and , the 3 of the whole , o tained con 1 1 U o ’ v 0 0 , tains of 8 8 of 2 5 ' of 2 z

S iO . Fe 0 of , 70 of rare earth oxides , of 2 3, and ° “2 F eO . /o of Almost all the thorium minerals con n tai a certain quantity of uranium and radium . In thoria n n b 60 T hO ite , which contai s a out 7° 2, the quantity of 1 0 20 uranium varies usually from z to z , and may reach as high as 30 Z.

— n Chemi cal Properti es . Like iron , ura ium forms two

U 0 U 0 . series of salts corresponding with the oxides 2 and 8 n U O The commo black oxide a a, formed by heating other oxides in air or oxygen , is regarded as a compound of these , O O U 2U . 2+ s The higher oxide is acidic , like chromic oxide , C rO a n a, and forms s lts with alkalies , known as the ura ates, having usually analogous composition to that of the di

s Na U 0 K 0 0 . chromate , as , for example , 2 2 7 and 2 2 7 Uranium is , however , less acidic than molybdenum and tungsten , the S U 0 other members of the family . alts of the lower oxide 2, U Cl l as, for example , v are not usual y met with in ordinary as work , , like ferrous salts , they are powerful reducing n n In age ts , and are readily oxidised eve by air . absence of n n n iro , ura ium may be co veniently estimated by reducing it i n sulphuric acid solution by zinc or magnesium , and tri t rating the solution with permanganate . The uranous salts are usually green .

The commonest compounds of uranium , however, are the uranyl compounds , which are usually yellow . These l U 0 contain the divalent radica ( 2), and are derived from the

U O , d higher oxide s j ust as sulphuryl chlori e may be S O . regarded as derived from S Thus in acting on the U O d higher oxide a with hy rochloric acid , the product is

74 TH E CH EMI S TR Y OF TH E RA D I O -ELEMENTS n from other constitue ts of the mineral , and may be pre ci i at d ff p t e as sodium uranate by acidifying , boiling o the carbon dioxide completely , and adding sodium hydrate . t The precipitate is fil ered and ignited without washing , washed free from alkali , and weighed as sodium uranate . This pouring of a solution of uranium containing a com plex mixture of other constituents into excess of an alkaline carbonate is always a valuable first step in the separation of the uranium , and when the latter element is the only one f being separated or estimated , it a fords a ready means of obtaining the whole of the uranium in the filtrate in a state of approximate purity . For analytical estimations it is almost always an advantage to add some ammonium sulphide to complete the precipitation of the iron . Any suitable method can then be employed for the separation and estimation of u s the ranium in the filtrate . One of the mo t commonly n employed is , after acidification and removal of the carbo dioxide , to add microcosmic salt (sodium ammonium hydro n gen phosphate), to neutralise as early as possible with n ammonia without causing a permane t precipitate to form , and then to add a considerable excess of sodium th io sul en phate . Boiling for t minutes brings down the whole of the uranium as phosphate with a bulky sulphur precipitate which settles and filters excellently . After washing, the precipitate is ignited at a dull red -heat and weighed as the “ em iri green phosphate , a body which has been found p n cally to correspond with of ura ium , and which is far less hygroscopic than the pyrophosphate When . pure it dissolves completely in concentrated nitric acid , but m E when i pure the solution is more diffi cult . vaporation of this solution and ignition leave the yellow pyrophosphate ° U O O n n a . ( 2)2P2 7 which co tai s /o of ur nium The con version of the “ green phosphate ” in the crucible into the f yellow pyrophosphate a fords a check on the estimation , but the latter body is excessively hygroscopic and diffic ult ’ B rearle s A nal tical Chemisti to weigh . (For further details, y y y of U rani um should be consulted . )

U ranium may also be precipitated as ammonium uranate , and U O f ignited and weighed as B B. The only di ﬁculty is the washing of the precipitate, which must be done with a 4 ammonium nitrate solution , as water causes the URA NI UM 7S precipitate to become colloidal and run through the ﬁlter . Another method is to precipitate uranium with ammonium U O hydrate and sulphide , as uranous oxide 2, and to weigh U o as such after ignition in hy drogen , or as , , after ignition in oxygen . Autunites and similar phosphates containing calcium are best estimated as phosphate , acetic acid being present to prevent the precipitation of the calcium . An u rani nu m excellent colorimetric method of estimating , applicable to very dilute solutions , is to add to the solution and to a similar volume of water in small evaporating basins the same amounts of potassium carbonate solution and half a gram or so of sodium peroxide . The intense yellow colour due to uranium is visible in extremely dil ute solutions , and is more easily matched when faint . The colour in the basin containing the uranium is then matched with that of the blank by running into the latter standard uranium solution .

Iron interferes badly with the tint . It is best removed by treating the uranium solution with the ammonium hydrate , carbonate, sulphide mixture as described . A tenth of a milligram of uranium can readily be detected , and , with care, estimated in this way . The method is of value in the & c rapid examination of precipitates, . , obtained during the separation of uranium , to test whether all the uranium has been removed .

Treatment of Radi oacti ve Mi neral s — T h e radioactive constituent of uranium minerals of greatest technical value is , of course, radium , whereas the valuable constituent in n the case of thorium mi erals is mesothorium , which has identical chemical properties . H ence they are always sepa if rated together, present , and the same treatment is applic able to both . The older methods were largely developed in consequence of the existing practice in the treatment of uranium minerals . These were fused with sodium sulphate, and the uranium lixiviated out with water and dilute sulphuric acid , the insoluble residues containing the radium and radio and d lead as sulphate , part of the ionium , polonium , an actinium . Probably these residues are now no longer b o tainable . The separation of the radioactive constituents has become at least as important as the separation of the u and startin ntio uranium , g with fresh mineral ab i i , far simpler “ methods could be devised according to the character of the 76 TH E CH EMIS TR Y OF TH E RADI O-ELEMENTS M mineral . any of the minerals , for example , carnotite, b autunite, and thorianite , are solu le in dilute acids, and n from such solutions, by pouri g into sodium carbonate , the uranium can be at once recovered from the ﬁltrate, whilst the radium and other active constituents are precipitated as carbonates in a form very easily redissolved and further concentrated . No doubt , gypsum , if present , would prevent the radium going into solution , and in such cases the older methods might have to be resorted to . I n the case of - s pitch blende , which is oluble in nitric acid , the existence of sulphides causes sulphates to be produced , and part of the radioactive materials is precipitated as sulphates . But s uch precipitation can often be prevented by use of concentrated acid until the solution has been separated n a and n from the bulk of the i soluble m terial , the amou t of material to be worked up for radium could no doubt be enormously reduced if desired . But sooner or m later the separation of the radium and esothorium , if present , which constitute the two most important of f in the radioactive constituents , is ef ected through the soluble sulphates and their separation with barium sulphate . R ad um This method will be referred to under i , but for full details of the factory p rocesses the originals must be con

” ' u lted s . When the active elements have been precipitated n with the insoluble sulphates a co siderable concentration , in f the laboratory , can often be ef ected before solution of the active material by treatment with 25 Z sodium acetate solu tion , which dissolves the lead sulphate and part of the calcium sulphate . wo I n the rking up of the Olary ore, in which the ﬁrst treatment of the magnetically concentrated material is a a fusion with salt c ke (acid sodium sulphate), i t was found that it is only necessary to decompose about half the ore to secure a practically complete yield of the radioactive contents , and on this fact the successful economical treatment of such - low grade material largely depends . The undecomposed mineral , which is free from radioactive constituents, settles out more quickly from the aqueous liquors than the part dec o m 52 posed, and may be at once rejected by this means .

— Al l Radi oacti v i ty uranium salts , and uranium itself , in

’ a - ordinary circumstances give , in addition to the radiation , URANI UM 77

- an important and fairly penetrating Bradiation , due to n uranium X . The latter is easily separated i most of the processes of purification in vogue , so that freshly prepared - - uranium salts gave at first only a rays . The B rays are rifi gradually regenerated . After about three weeks from pu n n catio they are early half the ordinary equilibrium strength , equilibrium being practically reached after from six months ‘ to a year . Standards of a -Ray Acti vi ty — Ou account of the con stanc a- y of the radiation of uranium , which , till now , has not ff been diminished or a ected by any chemical process, the most convenient standards of radioactivity to employ in the standardisation and calibration of electroscopes and other measuring instruments employed in radioactive work , are U O made of thin films of black oxide of uranium a s. The commercially pure uranyl nitrate may be further purified by

one of the methods previously alluded to, either by crystal li s ati o n n , solutio in ether, or pouring into ammonium car

bonate . I t is then cautiously ignited in a platinum crucible . n The ignition is continued on the blast in a current of oxyge , which is maintained after the heating is stopped . The pure black oxide is ground up in an agate mortar with freshly

distilled chloroform to a thick paste . By means of a clean - camel hair brush , shallow circular copper trays of deﬁnite area are coated with the smallest quantity of the paste neces and sary completely to cover the surface , the chloroform is n “ n the allowed to evaporate spontaneously . An alter ative ’ and , in the author s hands, preferable method is to grind n up with water , instead of chloroform , and spread on a grou d glass plate by means of another small ground plate cemented to a cork as handle . The ﬁlms are then dried in the water oven . The a -radiation from the trays is proportional to their

area , and , so long as the surface is completely covered , is quite independent of the amount of material used to cover n them . Provided no more of the ura ium oxide is used - th an is necessary to cover the surface , the B radiation is adv i s practically negligible . For very accurate work it is

able to use a preparation of uranium oxide , which has been

— prepared for some months , of which the B radiation is S constant . uch standards are , of course, used bare , but are 78 TH E CHE MIS TRY OF TH E RAD I O -ELEMENTS

s n kept covered to protect them from du t whe not in use . and They are easily prepared , the film adheres well to the n surface . By prepari g similar films of minerals or other materials and comparing their dis charging action on an electroscope with that produced by the standard uranium a- b U 0 film , the activity of the su stance in terms of that of 8 8 may at o nce be accurately de termined by means of a simple a- t electroscope . The total activi y of a uranium mineral is times that of the uranium it contains if there is no n n thorium prese t . He ce by dividing the activity of the n U O mi eral in terms of that of B 8 by this factor , a rough U O b evaluation of the proportion of the contained B 8 may e - . n s made If larger qua tities are available , a y ray test is alway S e d e Ra i um. preferable . ( ) Uranium and uranium X form for practical purposes n a disti ct and complete disintegration series in themselves , which is connected with the long ionium radium radio lead polonium series on the one hand and the actinium series on the other only by indirect theoretical reasoning . For in spite of the most exhaustive and delicate experiments , no t direct produc ion of ionium , radium , or actinium has so far been observed from uranium . Neither from the uranium X , s 0 eparated from 5 kilograms of pure uranyl nitrate , was 55 there observed any production of a radioactive product , n 2 though , theoretically , ura ium should be formed from n ura ium X . For all practical purposes the decay of the i s as activity of uranium X complete as that of polonium , th e end member of the radium series , or of thorium C , the last member of the active deposit and of the series of thorium .

With this may be contrasted the decay of radium C , the last member of the active deposit of radium , the decay of which R is only apparently complete . eally the activity falls to a very small value , and then steadily increases again for many years , due to the formation of later products which are radioactive . The extreme simplicity of the uranium series proper, which contains only two radiating substances, one

a- and - giving all the rays the other all the B rays , the com ar tiv l s - p a e y hort period of the B ray producing product , and the absence of all slowly changing or gaseous products of n n disi tegration , make the compou ds of uranium especially adapted for use as standards of radioactivity and for illus URA I UM x AND URA I U M X N l N , 79 trati ng the main principles of radioactive change in their

o a - least c mplex aspect . I n fact uranium expels two particles per atom disintegrating , and is not a single element , but a - f i n mixture of two , chemically non separable, di fering atomic

a - S weight by four units, and both expelling rays . uch a n mixture , however, acts as a si gle element expelling two a - no rays per atom , and this theoretical complication in way interferes with the practical simplicity of th e case referred to .

R M x AND R M x U AN I U 1 U AN I U 2.

U rani u m X r a i u m X 1. U n 2. E sti mated atomi c wezlgkt 234 a ra l i minut s P er i od of v e ge f e e . t o and -ra s R adi a i n 6 y y .

i n coe ci ents Al z Absorpt o fﬁ uﬁ ”MAI 24 “(DAI Al = O 1 uy . 4 = M Pb 0 . 2 7 7 r m o to e o f th o i um. H o o lo u o f an a C/zemi eal anal og ue I s p g e t t lum.

ranium 1 . an um U U r i X1 .

raniu m X . rani m D i si nteg r ati onp rodu ct U 2 U u 2.

L n b ike uranium , its product, ura ium X , has recently een - n n analysed into two distinct radio elements, the differe ce bei g that , in this case , the products are successive and both give

- n n 8 a . , instead of rays The exact positio , o ly , of these two products in the series is a matter of theory rather than experiment . For any experimental evidence to the contrary, they might be regarded as the products of uranium 2 rather

a - than of ur anium 1 . But the and B ray change rules , to b b e o eyed , necessitate that the series should run as shown 0 n on p . 7 , and hence this is ge erally accepted (see Part I I , X b rev i u m . U , b p ranium 2 also named , eing a very - n short lived product , is always prese t in equilibrium amount with uranium X except during the ﬁrst few minutes after n separation . It is co venient to consider ﬁrst the two “ products together as one , under the old name uranium X .

The chemical reactions refer, naturally, to those of the 80 TH E C H EMI S TR Y OF TH E RA DI O -ELEMENTS longer -lived uranium X the radioactivity to a mixture of the two in equilibrium . Uranium X is responsible for the penetrating rays of n s ordinary pure uranium compou d , and may be easily separated by many of the methods of puriﬁcation described n uranium re- u der , only , of course , to be formed with the s lapse of time . Thus, on crystalli ing uranyl nitrate by

s . allowing a hot solution , of p gr . to cool , about two thirds of the salt crystallises , and the mother liquor contains - about six sevenths of the uranium X present . On shaking crystals of the same salt with ether, two layers are formed , of which the lower aqueous layer retai ns the whole of the “ n n . n v ura ium X Other solve ts , such as aceto e , arious alco and n hols , ethyl methyl acetate , have been recomme ded , f In but it is doubtful if they of er any advantage . all such cases the solution in the orga nic solvent may be shaken with freshly precipitated ferric hydroxide and filtered , which makes the separation easier in unpractised hands . On pour ing a uranium solution into excess of a s olution of carbonate s im of odium or ammonium , the iron , alumina, and other s puritie are precipitated , and these retain the whole of the uranium X . If the uranium is too pure , it is necessary to add a little ferric salt to s erve as a nucleus for the uranium n s reci i X . It may be separated from iro by di solving the p p tate in concentrated hydrochloric acid and extracting the n n solution with ether saturated with hydroge chloride , whe n 57 In the ura ium X remains in the aqueous layer . the n n s n ammo ium carbo ate proces the ura ium X precipitate , if too great , may be redissolved in acid , and poured into very re re concentrated ammonium carbonate , and the solution p c ipitated fractionally by boiling . According to the n ature n of the other substances prese t , the uranium X comes down sharply and definitely with one of the fractions , usually in 55 the middle of the series . Uranium is readily absorbed by the action of barium S . o h sulphate formed in the solution or by carbon oot , n rec o m tamed freshly by burning aphthalene , has been I f mended . t is always di ficult or impossible to remove n ura ium X completely by these means . Perhaps owing to the common presence of thorium as an impurity , they n T are apt to be rather u certain . he addition of thorium

8 2 TH E CH EMIS TR Y or TH E RADI O -ELEMENTS

- n The y radiation is extraordi arily feeble , relatively to - n the B radiatio , in comparison with radium C , mesothorium

2 - - or thorium D , the other y ray producing radio elements . 60 It is also somewhat less penetrating . Recent work has

— disclosed in addition (y ) rays of two types , one less penetrating than the fl- rays and the other intermediate “l n i -ra s in penetrati g power between the B and y y . “ ” Anal si s of U rani um X i nto U rani um X and y 1, m - X U rani u X . , Uranium 1 is the isotope of thorium , and X n uranium 2 is a alogous in properties to tantalum . If uranium X is subj ected to any operations , capable of fi separating these two elements with suf cient speed , it will be found that the B— rays come from the portions resembling n tantalum , and , after separatio , these rays decay completely - with a half period of about minutes . Thus , filtration of . the uranium X solution through a filter containing a few f milligrams of moist tantalic acid ef ects a partial separation . - The portion resembling thorium at first gives only (3) rays , and proceeds to regenerate the B— rays at the corresponding n rate , the half value being reached mi utes after separa n tio and the equilibrium value after some 1 5 minutes . I t no t n - is vet know how the y rays originate , but it is usually assumed that they accompany the B— rays i n the disinte rati o n - X . i g of uranium 2 W th regard to the (7 ) rays nothing is n n yet k ow or can be inferred .

U RAN I U M Y

U rani um ! 9 V! 8 X to y ear s

U rani umX NA ‘ fi8 J fi fi da/ s .

U rani umX ; I 65 mmu res

i h — 0 E sti mated atomi c weg t 234 o r 23 .

— - Radi ation (ﬁ) rays . r i on oe cient- Al = abo t A bso pt c ﬁ umj u 300 . u — h Chemical analog e Iso to pe o f t o ri um.

— m o r ura i m 2 P a rent Eith er urani u 1 nu .

ro uct — n wn D i si ntegrati onp d U nk o . URA NI UM Y 83

There is an abnormality in the decay of the (B) -rays of X - uranium 1, whereas the B rays decay perfectly normally . f 1 0 - It a fects only about z of the total (B) radiation , and this portion is distinctly more penetrating (awAl z ab o u t 30 0 ) than the rest . But if the uranium X , after having been completely separated from the uranium , is allowed to reform n re- for a few hours o ly before separating, th en it will b e found that the abnormal part of the (B) -radiation is relatively very much increased, and may be many times greater than n the ormal part . Under such conditions it is very much easier to study . Instead of this part decaying like the ﬁ-rays with the

- days half period , it decays much more rapidly, to half i n value about a day , and therefore must be due to a new Y 62 product , termed uranium . The elucidation of the pro f blem presented considerable di ﬁculty , partly on account of f the minuteness of the e fects and also for another reason . h Y X It as been shown that uranium and uranium 1 are and - isotopic therefore non separable, and that they are s imultaneous ly produced from uranium . The much quicker period of the former results in the proportion between the two isotopes being the greater the shorter the time of ac ff cumulation , and so , as detailed above, the e ect of the former may be increased relatively to that of the latter . The lapse of time may be used to produce a change of con n centratio between the two isotopes, which chemical methods are quite unable to effect . The scheme shown above is the most probable . It is supposed that either uranium 1 or uranium 2 (the in ﬁrst is depicted in the diagram) undergoes a dual change ,

a- both of which rays are expelled . In the ﬁrst mode, which 2 claims some 9 of the atoms disintegrating , uranium Xl , n n 8 ° and in the second mode , which claims the remai i g / , uranium Y is th e product . Both , being formed from - a . uranium in an ray change , are isotopic with thorium - Both give (B) rays , those of uranium Y being somewhat the more penetrating , in accordance with its shorter period . But the product of uranium Y remains experimentally “ ” Z . unknown . It is called uranium in the diagram It X - must be an isotope of uranium 2, in the eka tantalum - v place in the Periodic Table , and it must be long li ed to have 84 THE CH EMI S TR Y OF TH E RADI O -ELEMENTS

I a - escaped detection . f it gave rays on disintegration it e would produce a memb r isotopic with actinium , or actinium itself . It is extremely probable that it does this, and that the actinium series is joined to the uranium series , through n - - uranium Y and such a unknown long lived eka tantalum , a- E giving rays , but this remains to be proved . ven if this is done it would still remain doubtful whether uranium 1 2 or uranium is the starting point of the series , as either u n would fit the case equally well . It is perhaps a little f fortunate that the cases , without exception the most di ficult to elucidate experimentally , should come at the very com mencement of the uranium series , and therefore fall to be dealt with first . For there is nothing in the whole of the rest of th e series so complicated or diffi cult to put to experimental test as are the precise relations between uranium X 1 2 . , uranium , uranium 1, uranium Y , ionium and actinium

Nevertheless the scheme outlined is extremely probable . I t is only the diffi culties due to the enormous life - period of some of the members which prevent it from having been as yet completely put to the proof . If any one took the trouble to put the precise signiﬁcation of the above diagram into words , it would rather be a matter for surprise , not that we still have to build to some extent on conj ecture , but rather that in the short period since radioactivity was discovered it has been found possible to learn so much .

I ON I U M ( l o )

— E sti mated atomi c weight 230.

— P er i od of auerage life Pro bab ly between and years .

i i — - R ad at on a rays . - o f ai r. R ange qf a ray s cm.

— Chemical analogue Iso to pe o f tho ri um.

P arent— U rani um 2

nt r n r uc — m D i si eg ati o p od t Radiu .

The existence of this substance , an intermediate product - of long life between uranium X and radium , was fore shadowed by the minuteness of the growth of radium in

uranium preparations puriﬁed from radium . It was dis covered and isolated from pitch -blende and other uranium I ONI UM 8 5

36 B o ltwo o d minerals by . I n the preliminary treatment of the mineral it is separated with the actinium , and indeed the ﬁrst actinium preparations separated owed their radioactivity

d . almost certainly in part to a mixture with ionium It is , 63 however , easily separated from actinium . Its chemistry can be fully and accurately described in a single sentence.

I t resembles thorium in its whole chemical nature , as far as it is known , with absolute completeness , so that not only is no separation of the two elements possible by any known method, but no concentration of the one constituent with S reference to the other has been accomplished . uch an alteration of concentration could be readily detected , if it occurred , even with the inﬁnitesimal amounts of ionium

a- d h l . use , because of the intense activity of t e atter It must be remembered that the chemistry of thorium has been closely and exhaustively studied on account of its resemblance to and association with many other rare earths of no technical value , and in consequence a large number of characteristic and effective methods for the purification of thorium from all other elements are known (see Thori um) . A certain and easy method of separating ionium from any mixture is to add thorium thereto , if not already present, and then to separate the thorium and subject it to a rigorous purification process by the ordinary methods . This is a general method adopted in radioactivity . It is doubtful if ionium free from thorium has been prepared . Cerium , carefully freed - from thorium , has been employed for the separation of ionium , for example , by precipitation with hydrofluoric acid in strongly acid solution . The cerium may then be separated from ionium by the methods used to separate it from thorium . However, there is probably some thorium in all uranium minerals . In J oachimsthal pitch blende the quantity is extremely small , but, in the working n up of thirty tons of mi eral , Auer von Welsbach accumulated a few grams of pure thorium containing the ionium of the 37 - mineral . The nitric acid solution of pitch blende , after c & . b . d removal of lead, , y sulphuric acid may be precipitate directly with hydrofluoric acid . The fluorides are dissolved by boiling with sulphuric acid and fractionally precipitated by n zinc hydroxide paste . The first precipitate contai s all the 58 thorium and ionium present . 86 TH E CH EMI S TR Y OF TH E RADI O-ELEMENTS

I n the working up of thorium minerals , the whole of the ionium associated with the uranium present remains n with the thorium . As practically all thorium mi erals n co tain some uranium , all sources of thorium contain some ionium , the preparations obtained from monazite contain ing less than those from thorianite , and those from Ceylon thorite least of all . It must be remembered that the period n In of thorium is about times that of ura ium . con

a- sequence , the activity of thorium , free from its own products , separated from a mineral like Ceylon thorianite , ( n 60 1 6 co taining , for example, Z of thorium and z of n a- ura ium , would consist of the rays of ionium and of thorium in similar amount . In all commercial thorium the a-radiation of the ionium present is a considerable fraction of that due to the thorium itself . If we reckon monazite to and b contain of uranium 4 z of thorium , a out one a- fifth of the radiation ascribed to thorium itself , in com merc ial thorium preparations , would be due to ionium . The most characteristic feature of the radioactivity of ionium is its power of producing radium steadily with the lapse of time . The radium may be easily separated from in the ionium , for example , by precipitating barium sulphate the solution , but in course of time it continuously reforms . It is usually detected by means of the characteristic radium emanation , the solutions being kept in sealed flasks and the gases accumulating boiled out from time to time and intro - du ced into an air tight electroscope . Owing to the long period of ionium the amounts of radium so formed are , of course , extremely minute . As a consequence of what has already been said , it follows that all thorium preparations n ge erate radium , to a greater or less extent, according to the amou nt of uranium i n the minerals from which they were derived . Otherwise, ionium , in the radioactive sense , is very similar to polonium , its radioactivity consisting of - - low range a rays only . It possesses the great advantage over polonium for many purposes , in that its radioactivity is constant and does not sensibly decay . It will no doubt find large application where a steady , powerful source of a- radiation is required , as in compensation methods of radioactive measurement and for a -ray standards more active n tha those of uranium . The proportion ch anging into RA DI UM 87

radium , even in a lifetime , is so small that it may in most 13 cases be neglected . I t only by reﬁned emanation tests that it can be detected .

The period of ionium remains uncertain . The period

cannot be greater than a million years, because of the intense a-activity of the ionium preparation separated by E Welsbach . ven if these were pure ionium , free from thorium , the period , as estimated from the activity , would

not exceed the maximum stated . To get the real period this maximum must be reduced by the factor representing the unknown proportion of ionium present with the thorium

in the preparation . The period estimated approximately

- from the range of the a particles is years . The ’ author s results , on the rate of production of radium from

very carefully puriﬁed uranium preparations , led , after more ’ n minimum o f . 1 0 0 o o o tha ten years work, to the estimate , 64 n years . The most rece t results with these preparations

indicate that this is probably not far from the actual period , but a few more years must elapse before this c an be

considered deﬁnitely established .

RADI U M (RA)

— A tomi c wei ght 22 5.

— ars P eri od of av erage l if e 2440 ye . ' - — ra R adi ati on a and (y d ys .

— - m o f ai r. I c s . R ange of a r ay s 3. 3

— bsor ti on coe i ci ents L AI ab o ut 200 . A p ﬁ I QB) = ﬂ A1 4 I 6 ; 0 27° (y ) 35

— C/zemzcal anal og ue H o mo l o gue o f b ari um.

— P ar ent I o ni u m.

— n ito n . D i si ntegr ati onproduct Radi um emanatio (N )

— n . Occurrence. All ores containing uranium contai radium I n the great majority of cases the ratios of the quantity of

n . x radium to that of uranium is a consta t , viz “ "65 which is known as the equilibrium ratio . That is to say , n to n 32 3 milligrams of radium (element) are prese t per of n I n uranium (element) . Certain minerals are exceptio al that

the constant of radium is less than this . The most important

of these are the autunites , for which the ratio varies from

8 0 . 20 to z of the equilibrium Conversely , the only 88 THE CH E MI S TR Y OF TH E RADI O-E LE MENTS certain case of a mineral containing radium and no uranium ’ l E v é ue is a certain deposit of pyromorphite ( Issy q , France) t found in h e neighbourhood of autunite deposits , which are superficially coated with a radiferous layer , doubtless derived from the autunite by the action of percolating water . t According to recen researches , it seems that geologically old formations contain a somewhat greater radium ratio than “ b d . recent ones, ut the difference , if any , is small an uncertain All common rocks and minerals of the earth ’ s crust contain minute amounts of radium , of the order of a few million millionths of the total mass . It is found in natural s n b , n b pri gs , oth hot and cold ma y of the spas most cele rated medicinally co ntaining the greatest quantities either of fe radium or of its emanation . There are w materials in which radium cannot be detected by the emanation test , if sufficiently large quantities are examined , but whether this has any special significance or is merely due to the almost incredible delicacy of the emanation test is not easy to decide . There is no reason to doubt that in all these common materials the three -millionfold greater equilibrium quantity of uranium is also present, although this cannot be detected chemically , owing to its minuteness . - R new Properti es . adium is the most important of the me M M . radioactive elements . It was discovered by . and Curie in the years immediately following Becquerel ’ s first observation of the property o f radioactivity of uranium com 8 6 Mme t 1 . h pounds in 9 . Curie formulated e fundamental principle that radioactivity is an atomic property , and this led her to the examination by chemical analysis of the n n ura ium mi erals , the radioactivity of which is several times greater than can be accounted fo r by the radioactivity of n s S h e the ura ium pre ent . found the radioactivity was con c entrated in the second group with bismuth (polonium) and - R in the alkaline earth group with barium (radium) . adium resembles barium in all the usual analytical reactions , the sulphate being even less soluble in water than barium sul n phate . It is therefore the least soluble sulphate know . Radium is distinguished from barium by th e lesser solubility of its chloride and bromide in water or in hydrochloric and th e t hydrobromic acid , and this is the principle of me hod an adopted in its separation from barium . The physical d

90 TH E CHEMI S TR Y OF TH E RADI O -ELEMENTS is often advantageous to precede this operation by a similar one , using a sodium hydrate solution containing a little carbonate which dissolves part of the lead and silica present . s The carbonate , washed free from sulphates, are treated with pure hydrochloric acid which dissolves the alkaline earths including radium . From the solution the latter may be precipitated as sulphates by sulphuric acid and reconverted b back into car onates as before , or sometimes more con v eniently they may be precipitated directly as chlorides by saturating the solution with hydrogen chloride . This is a very elegant method of great utility in the l aboratory , for the most probable impurities , chlorides of lead , iron ,

& c . calcium , , remain in solution and only the barium and radium chloride are precipitated , practically in the pure state , ready for fractionation . It has been applied successfully in 52 the treatment of the Olary ores . All these tedious and lengthy wet reactions could with advantage be replaced in many cases by simple reduction of r the insoluble sulphates to sulphides in a cu rent of coal gas , water gas , or other reducing atmosphere , followed by solution of the product in acids . The technical application of this method appears to pres ent difﬁ culties which have not yet been surmounted . Another suggested method is to mix the raw sulphates in the dry state with calcium hydride , both

finely divided , and to set fire to the mixture , whereby the ” sulphates are reduced to sulphides . ff fi The fractionation o ers no dif culty , at least in the initial stages when the quantities of materials are large . The chloride is dissolved in boiling water in one vessel , A , t to a j ust saturated solution and left to cool , the mo her o ff liquor is drained into another vessel , B, again concentrated to saturation , while the crystals are dissolved to saturation On B in fresh boiling water . cooling , the mother liquor of is drained into a new vessel , C , while that of A is drained

and . into B , used to dissolve the crystals The operation is n repeated till perhaps ﬁve vessels are obtai ed . After this the n number of fractions is kept constant while , alter ately , crystals are withdrawn from the process at the less soluble end and the mother liquor from the other end . The crystals contain practi cally all the radium , and the mother liquor only a small quantity . When the crystals become very small in RADI UM 9I

quantity , through repeated fractionation , gradually increasing quantities of hydrochloric acid are added to the water used to dissolve them , so increasing the volume of the solutions handled and enabling the crystallisation of only a few milli o i grams the salt to be conveniently carried out . The fractionation of the bromides is described as more rapid but . less regular . The chloride is a much more stable compound than the bromide , and parts with its halogen much less readily on keeping . ’ — S Mme mi W i . At c ts . o e gh ince Curie s determination , wonderfully accurate as it now proves to have been , a very careful revision of atomic weight has been undertaken by 69 H onigs ch mid . The method employed was the same as that Mm b e. RaCl A Cl R aCl A used y Curie, the ratios J g and z/ g f being found, the di ferences being mainly in minor points . n The mea value obtained , is so near the whole number , that for all practical purposes the atomic weight 2 26 may be taken as . The individual determinations did not differ from the mean by more than A similar set of determinations with the bromide give By spectroscopic comparisons it was shown that the ﬁnal bromide preparation employed must have contained less than of barium . About grams of the puriﬁed chloride was employed .

I n this connection , a revision of the atomic weight of uranium by the same investigator is of interest, in that it n sh o wed a ﬁgure , much nearer the whole umber than that previously accepted , It is, however, still far enough from it and from that calculated from the atomic weight of radium by the addition of three times the atomic weight of helium , to raise the question whether mass is accurately conserved in radioactive changes . There is, at least , reason to believe that in these changes small losses of mass may occur, and that, among the atomic weights generally, the small and irregular departures from whole members observed may be connected with differences of energy content . A heavy atom , for example, disintegrating n with evolution of e ergy into two lighter atoms , may suffer in the process an apparent loss of a small part of its mass . There have been a large number of new speculative attempts to ﬁnd regularities among the atomic weights on the idea 92 THE CH EMI S TR Y OF TH E RA DI O-ELEMENTS that the heavier elements are compounded out of the simpler ones, and the uncertainty whether mass is strictly conserved makes such efforts and the approximate accuracy attained more legitimate now than formerly .

Th In rnati onal Radi um Standard — The u se e te . of the 7 -ray method of comparing quantities of radium demands an absolute standard for comparison . A specially puriﬁed 2 2 specimen of radium chloride , containing nearly mg . of Mme a radium chloride , prepared by . Curie , was comp red by y -ray methods with several preparations used in H onig ' s hmid s c revision of the atomic weights, and , on practically n 2 2 perfect agreeme t being found , the mg . preparation was no w accepted as the I nternational standard , and is preserved et S e at the Bureau International des poids mesures , vres , near Paris . Copies of this standard have been supplied to the N ational Physical Laboratory , Teddington , and to the f o ﬁcial testing institutions of other countries , who undertake the determination of th e quantity of radium contained in preparations submitted to them . The former uncertainty as to the purity of radium compounds has in this way been entirely removed . Estimati on I ) Chemical Methods — The estimation of the radium in a preparation is accomplished by a variety of methods according to the circumstances . For mixtures of pure radium and barium salts , containing an important pro portion of radium in the absence of other elements, comple mentary to the spectroscopic test described above, is the Mme determination of the chemical equivalent . . Curie precipitated the solution of the chloride with silver nitrate , weighing the silver chloride and recovering the radium from the solution in the soluble form . The chloride of radium ° 1 0 h ro must be dried at 5 and weighed quickly , as it is yg S ir R n scopic . William amsay converts the chloride i to

bromide by ignition in bromine vapour . The only objection

to estimating the radium as sulphate , as in the case of

barium , is the labour of recovering the radium in soluble form . The solution of radium sulphate may conveniently be effected by repeated gentle ignition in a current of hydrogen - or coal gas , followed by solution of the sulphide produced in hydrochloric acid ; or by boiling with sodium carbonate

94 TH E CHEMI S TR Y OF TH E RADI O-ELEMENTS

R divided by the volume (V) of the flask , the radius being a R s given by Hence , of the above expres ion can be very easily found , and in most cases the first three s f In or four terms of the expres ion are su ficient . this way

- it is possible to compare accurately, by y ray methods, sub stances containing less than one -hundred -thousandth part by weight of radium , directly against a pure radium prepara

S . In tion , known in terms of the International tand ard a ' large number of tests in the author s laboratory , the results of which were checked by direct emanation tests, the method was found accurate and more reliable than the emanation method .

o - - As the y rays , and the same is equally true of the B rays , do not come directly from radium , but from its products , n escape of emanation must be avoided , and the preparatio must have been prepared at least a month . The only perfectly rigorous method is to seal up the preparations in s glas vessels a month prior to testing . But for the rapid approximate e s timation of the radium in mi nerals this pre caution can usually be omitted . This method lends itself especially to this kind of test , and is the one invariably ! employed by the author . A large lead electroscope , say - 20 20 . cm . high by cm . diameter, of wall thickness cm , the windows and exposed parts screened by lead to prevent the entrance of secondary radiation , is employed . The leaf system is the simple single leaf system insulated by a - sulphur bead . I n comparing pitch blendes and other minerals , a lump of 20 to 1 0 0 grams according to the richness is o n simply laid the top , and the increase of leak compared with that produced by a similar lump of a standard pitch blende , the uranium content of which is known . Thus , as an example

S tandard S pecimen tested

3 I ° ° = ! 4 x x x U , 4 0 I s

Old sh - b uildi n is bes t as the radi o -lead eet lead fro m the roo f o f an ancient g , , al a s resent i n reshl re ared lead wil l hav e deca ed and the natural lea o f w y p f y p p , y , k th e ins trument i ll th w erefo re be l ess. RADI U M 95 and the amount of radium is x gram per ton of mineral . For materials containing only of the order of one per cent . of uranium , larger quantities are necessary , and it is convenient to place them in curved shallow boxes of tinplate around the sides of the electroscope . For the standard of comparison the boxes may be reﬁlled with a mixture of th e material being tested with a known small proportion of - powdered standard pitch blende . This avoids all uncertainty due to the absorption in the material itself . The comparison of concentrated radium preparations in sealed tubes can be done readily by the same instruments, the distance of the preparation from the instrument being ‘ varied to suit the strength of the preparation . Provided a f proper lead electroscope is used , no di ﬁculty is to be anticipated from the strong secondary radiation generated - by the 7 rays in all surrounding objects , but otherwise it is important that nothing should be moved on the bench during the measurements except the active preparations n themselves , and that the observer should sit motio less in the same orientation throughout the actual observations .

I n the presence of thorium , the test gives still a reliable n measure of the tech ical value of the mineral . The separ able mesothorium contributes per unit about one -ﬁfth to one -sixth of the 7 -ray effect contributed per unit by the - uranium , whilst the non separable radiothorium probably s contributes a like amount . The radium eparated from thorium -containing uranium minerals contai ns also the n s whole of the mesothorium prese t . The possibility al o of the wilful adulteration of radium preparations by means of the less permanent mesothorium must not be forgotten . The fy -ray test gives really in such cases the measure of the penetrating radioactivity separable as radium rather than a n measure of the true radium . Thus if a mineral , contai ing n and equal percentages of ura ium thorium , appeared by this test to contain 30 milligrams of radium to the ton , the real radium content would be between 2 5 and 20 milligrams .

After the lapse of many years , when the decay of the meso n thorium has occurred , the activity would of course si k to In the true radium value . such cases one may take the 7 -activity three hours after the preparation has been dis 96 TH E CH E MI S TR Y OF THE RADI O-ELEMENTS

n n solved in water and evaporated to dry ess agai . This allows the radium emanation to escape , and after three - n hours the y activity is that due to the mesothorium alo e , but it is better to use the following method in all such

. S ee M esothori um cases ( also . ) ( 3) The E manatio n M ethod - A method applicable to solu tions containing from to gram of radium is to seal up the solution , which must be limpid , and acid , to pre o f vent the precipitation the radium , in a small distilling

ﬂask, and to allow the emanation to accumulate . After one month the equilibrium amount is present , and the quantity

‘ then does not alter however long the ﬂask is kept sealed . The emanation is boiled out of the solution into a pre v io u sl y exhausted gasholder , and the contents of the gas holder are admitted through a calcium chloride tube into

- an exhausted air tight electroscope . The leak after three hours from admission is taken , the gold leaf being positively charged . As standards may be employed similar ﬂasks containing small known weights of pitch - blende of known uranium content dissolved in nitric acid . If for any reason it is inconvenient to wait a month after sealing before test ing , the proportion of the equilibrium amount of emanation accumulati ng may be taken as 1 where 7\ has the value 1 71 0 0 0 t . . 75 (hour) , and is the time in hours (Tables of this Le R ad um 1 0 function are to be found in i , 9 9, The carrying out of the emanation method requires great care and circumspection , if reliable results are to be n obtained , owi g to the tendency of the radium , either in the standards or in the solutions being tested , to precipitate out in an insoluble form in which its emanation is retained . S b b ulphates, o viously, must rigorously e excluded from the n water and reagents employed . Old standards ca not be n relied upon for the same reaso , and it is necessary to calibrate the instrument used frequently with new uraninite In standards . addition there is still an uncertainty of some 3 Z in the exact ratio between the uranium and radium in - minerals . For accurate work, wherever possible , the y ray method is to be preferred , though in the case of materials suspected to contain mesothorium the emanation method furnishes a very valuable check .

— Radi oacti v i ty The radioactivity of radium itself, free

98 TH E CH EMI S TR Y OF TH E RA DI O-ELEMENTS

characteristic of the emanation itself . If the latter is at any time blown out and removed , there is again a rapid n decay of activity duri g the ﬁrst ten minutes , followed by a o ne more gradual almost to zero in about three hours . The existence of this most characteristic set of changes of radio activity affords an infallible qualitative test for th e presence of radium by means of its emanation in any quantity, however minute . The measurements of the a-rays afford the only way of following the progress of concentration during the frac tio natio n - of a radium bearing salt, for when freshly prepared from its solution the crystals possess about 1 8 of the

a - 8 — M equilibrium activity , but no , or 7 activity . esothorium a - does not give rays , and therefore its presence does not interfere in this case . The difﬁculties about this kind of measurement centre in the intense ionisation produced by a - the rays from even minute amounts of radium , and the impossibility of cutting it down satisfactorily by screens or by removing the preparation to a distance from the electro I an n scope . t c be overcome by increasi g the capacity of n the electroscope by means of a conde ser, but then the n danger is that the pote tial gradient , due to the charge

f . on the leaf , is insu ficient to saturate the gas With a fi n su f ciently high source of pote tial , a separate ionisation chamber , and a quadrant electrometer many accurate methods can be devised , but these requisites , especially the first , are not always to be found in a chemical n laboratory . The escape of emanation from the conce trated f neces radium salts is a very serious di ficulty , and it may be sary to convert the radium into sulphate to minimise this . One mode of procedure in this case would be to i ntroduce uniform films of the radium sulphate of definite area into an - air tight electroscope, and to exhaust the latter to a definite low pressure , measured by a barometer . At low pressures the ionisation is reduced proportionally to the pressure , and “ the saturation diffi culty is avoided . But it would pro bably be simpler to degrade the radium during conver sion into sulphate by addition of a known quantity of barium , so as to give a preparation of manageable activity . Naturally the greatest precautions have to be taken in working with bare radium preparations to avoid contamina RA DI UM EMANA TI ON 99

tion of the measuring instruments , Their introduction into the same room as the instruments is apt to spoil the latter permanently . Not only is there the ordinary contamination due to the escape of emanation and the coating of all the walls and obj ects of the room with ﬁlms of radio -lead and

- polonium , which is a well recognised danger, and which . must be g uarded against by carrying on chemical operations with radium as remote as possible from the instrument u n room , into which only sealed tubes of radi m may be take . But there is also an actual species of volatilisation from bare

ﬁlms of radium exposed in the open air, which possibly is a f kind of recoil e fect , and which results in the coating of all 72 the objects of the room with particles of radium itself . n O ce this has occurred , delicate instruments such as elec tr sco es n o p are useless , and must be replaced by less se sitive ones, which , as already explained , are less simple , as they require the use of high potentials , with separate ionisation chambers and measuring instruments . But if contamination is avoided , most if not all of the instruments required , except a the re ding microscope of the electroscope , can be and are ’ better made with one s own hands .

RADI U M E MANATION

t — 2 A tomi c weigh 2 2. D ensi ty — l I I (H : I

da s . P er i od of av erage l zfe 5. 55 y - P er i od of half change 5 days .

— 1 R adi oacti v e constant 5I (h o ur) .

— R adi ation a ways . f - of ai r. R angeo a ray s c m.

— Chemi cal analogue H o mo l ogue o f xeno n.

— P arent Radi um.

— D i si ntegrati onproduct Radium A .

o n The radium emanation , account of its comparatively long period and the ease with which it may be separated from inactive material and from its parent and products , and the large quantities of its parent that have been sepa rated i n the pure state , is by far the most completely investi

- - n gated o f the shorter lived radio eleme ts . Chemically , all three emanations are distinguished by that complete absence THE CHE MI S TR Y OF TH E RA DI O -ELEMENTS

of combining power characteristic of the gases helium , neon , n argon , krypto , xenon , so that they are not chemically n a . absorbed by y reagent On the other hand , they are all s - conden ed at liquid air temperature . I n the case of the radium emanation , the condensation and volatilisation occur with remarkable sharpness usually between the temperature ° ° 1 2 1 C . of 5 and 54 , but the phenomenon depends some o u what the conditions and the quantity of emanation . If a radium solution is kept for some time in a closed

ﬂask the emanation steadily accumulates with time , half of - the equilibrium amount forming in four days, three quarters and in eight days , so on , equilibrium being practically i n In reached three or four weeks . addition , hydrogen and oxygen are formed from the solution by the radioactive

. On decomposition of the water exploding the gases, a slight excess of hydrogen remains with the emanation . This may be admitted to a vacuous bulb immersed in liquid air . The emanation is condensed, and the hydrogen , together with any helium p roduced from the emanation , can be pumped away . As the emanation in presence of oxygen exerts a powerful oxidising action on organic compounds, for n n example , tap grease , the ema ation so obtai ed is usually contaminated with oxides of carbon , and may be puriﬁed n n by lo g co tact with freshly ignited lime or baryta . The volume of emanation , at in equilibrium with 1 gram 73 of radium , is almost exactly cub . mm . This latter value is the theoretical volume on the assumption that the emana tion molecule consists of single atoms, so that the agreement between the theory and experiment directly proves the monatomic character of the molecule . The spectrum con sists of a large number of bright lines , and is similar to that " n n . of xe o , its nearest analogue On account of its relation ship with the argon group the name “ niton ” has been suggested , but has not been generally adopted on account of the obvious advantages of its original name in expressing its radioactive relationships and of the disadvantage in pro posing a new name for one only of the three emanations k nown . Its atomic weight has been recently determined ' - n from its gas de sity , both by Bunsen s method , using its rate n 75 of effusio through a perforation in a plate , and directly by weighing on a micro-balance of q uartz sensitive to a

10 2 THE CH EMI S TRY OF TH E RAD I O-ELEMENTS

th e emanation to some extent . This is a possible source of

‘ a n th e su b error in cert in experime ts , for example , when sequent activity of metal plates which have been exposed to the emanation is being studied . n n The radium emanatio is disti ctly soluble in water, the coefﬁcient of solubility being about at ordinary tempera ture, and at It is m uch more soluble than this

. in all organic liquids , except glycerine . I n petroleum and f toluene , the solubility coe ﬁcients are and at the ordinary temperature . I t is less soluble in salt solutions " than in pure water . Its solubility in ﬂuids is governed by a precisely the same laws as thos e which apply to other g ses .

RAD I U M “ ACTIV E D E PO S IT

8 e s n A B t c c u f o s t u u i y mu mn mn a i i a u u u m i d i g n i m m 55 3 I s a d m d 5 d I ' ' g a m a 3 a 8 6 E R 4 R 3 R 2 3x 35 3?

E sti mated atomi c weight P eri o of av er ageIt!? " d 28 . 1 o rder o f 10

(mm. ) P eri od of half -change (mi n) w M t ode a. R adiati on a z ud Modeﬁ 8z y' R an e f a -ray s (cm g o 3. 9(calculated) ( ai r ) f Al = l = 75 nﬂA 13. 5 A1= 2 0 AI : Abso¢ti on $3 3 H 0 1 = d ents 9, 4 1b

Chemi cal analogue

m . Po l ni Th is uth o um. lli um. m. a I sotopi c wi th Po lo niu B “ RADI UM A CTI VE DEPOSI T ” 103

The radium active deposit of rapid change, so called to distinguish it from its products , radium D to radium F , which are more permanent , and are sometimes therefore “ " spoken of as the radium active deposit of slow change , comprises a group of three successive products , radium A , radium B, and radium C . The latter is believed to dis

in — integrate two ways . I n one a B ray is first expelled ’ a- producing radium C , which then expels an ray . I n the a - n C other an ray is first expelled , produci g radium , , which 3- then expels a , ray . The first member, radium A , is much

- shorter lived than the two succeeding members, and gives a- rays on disintegration . In consequence all experiments on d b d sh o rt time ra ium A must e carrie out within a very Q of its production from the emanation . If a negatively charged wire is exposed for a few seconds only in a relatively large t quantity of radium emanation , the activity of the wire at ﬁrs

a- is almost entirely due to the rays of radium A , and these decay to a very small value i n a few minutes after with drawal o f of the wire from the emanation , the activity the subsequent products formed being , for such short exposures , ’ u sually negligible . After twenty to thirty minutes exposure h e n to t emanation , the quantity of radium A attai s equi librium , and does not further increase . But the amounts of radium B and C on the wire go on increasing with the time of exposure up to three or four hours . Obviously the shorter the time of exposure the purer is the radium A deposited . Conversely with a wire which has been exposed any length of time to the emanation , no radium A remains w after twenty to thirty minutes from withdra al , the activity n at this stage being due to radium B and C . He ce , without chemical treatment, radium A alone or radium B and C together may be obtained at will by suitable choice of the time of exposure to the emanation and of the time elapsmg after withdrawal . The decay of the active deposit after withdrawal from the emanation naturally varies very much according to the time of exposure . There is always a rapid initial decay of

- th e a rays due to the change of radium A , which is the more marked the shorter the time of exposure to the emanation . This is followed by a period of very little change for about half an hour, as the decay of radium C is more or less 104 TH E CH EMI S TR Y OF TH E RADI O-ELEMENTS

balanced by its production from the radium B . Then follows a continuous decay to nearly zero i n about three or - n four hours , with a half period i termediate between that of and n n - radium B radium C . The most pe etrati g y rays are alo ne a measure of the quantity of radium C present , and an ini ti al therefore do not show y decay due to radium A , but rather an initial increase , or lag in the rate of decay , according to the time of exposure , as radium C is being produced by the changes of radium A and B . s in On heating the wire carrying the active depo it air, 8 0 is f R the volatility of the products very dif erent . adium B is strongly volatilised at about whils t radium C requires t a much higher emperature , and does not commence to volatilise till about As regards radium A , its true ° volatilisation temperature is probably between 8 0 0 and for if a wire , maintained at the latter temperature , is exposed n n to the ema atio it does not become active , however 81 s n s charged . The following volatili atio point are given radium A , radium B , radium C , As n o already explai ed (p . these v latilisation temperatures f n n30 are quite di fere t in hydroge . A practical method for obtaining radium C by its elf is to allow the radium A on n to s the wire to decay , and the expose the wire for ome minutes to a temperature somewhat above Radium B w o n to tal distils a ay , leaving radium C the wire , the activity of which the n decays exponentially with the half-period of

minutes . The other method mostly employed for this purpose is to dissolve the active deposit o ff the wire in boiling dilute hydrochloric acid , and to shake up the solution n n with ﬁ ely divided metallic ickel , or to plunge into it a R n o n copper or nickel plate . adium C alo e deposits the in n 8 2 d metal , leaving radium B solutio . The ra ium B, so left , of course immediately starts to produce a fresh amount of s radium C by its own change . Itself it produce only

- (B) and (7 ) radiation , but it is characterised by its power of generating with the lapse of time the powerful a B and - S s 7 radiation of radium C . imilarly , by electrolysi of the solution of the active deposit with a feeble current and large electrodes , radium C alone is deposited on the cathode . Radium B may readily be obtained from radium A by 8 ° was recoil . At one time it was thought that radium B

106 THE CH EMI S TR Y OF THE RADI O-ELEMENTS b completely , each type at the same rate, ut a more careful investigation with powerful preparations puts in evidence a residual activity , due to radium D and its products , which at ﬁrst steadily increases with lapse of time . By careful ‘ arrangement nearly the full theoretical 50 Z of this radium

D may be obtained from radium C by recoil , which is

a - proof that it is the product of the ray change . ' — - Radi um c . Application of the Geiger Nuttall relation shows that the a -rays must be derived from a change of very great rapidity , and this, the recoil evidence and the fact that the a- rays decay always at the same rate as the - B and 7 rays, leads to the scheme adopted , in which the “ ' " hypothe tical transitory a-ray giving member radium C n i tervenes in the direct line between radium C and D . Naturally its theoretical isotopism with polonium cannot be experimentally tested , owing to its ﬂeeting existence, but there is no reason to doubt this application of the rules . I t is a matter of considerable interest that the complete chemical character of an element , existing on the average probably not more than a millionth of a second, can be so completely predicted .

RADIO - LE AD OR RADI U M D

— E sti mated atomic weight 2 10 .

— P eri od of av erage l ife 24 years (P) .

— - R adi at io n (ﬂy and (y drays .

— A bsorpti on coeﬁ ci ents M AI 130 .

h m al n — I d C e ic a a gue so to pe o f lea . ' — P arent Radi um C .

D i si ntegrati onproduct Radi um E .

This element has not yet been separated or concentrated from lead, which , so far as is known , it resembles perfectly al l n in its chemical reactio s . As lead is an invariable con itu t st en of all uranium minerals , and in all except autunite is present in notable quantity , the lead separated from such mineral s always contains the radio-lead associated with the uranium . Conversely , common commercial lead contains traces of the element which causes it to be distinctly more RA DI O-LEAD OR RADI UM D 107

d a . F ra io ctive than most other metals or making instruments , a very old lead (such as from an old roof) should if possible be employed , as in this the radioactive constituent will largely

- have decayed . Although itself giving only a non penetrat

- ing (B) and (goradiation , its product , radium E , gives

- — B rays , and as the latter is of short period , the B radiation

M — is rapidly produced . ore slowly an a radiation makes its appearance , due to the production of radium F (polonium) . In consequence the lead separated from uranium minerals

— shows , a few days after separation , a marked B activity , and a- in the course of a few months an activity also . By purification the radio -lead is very easily obtained again inactive, but only to regenerate its activity in course of time . Owing to the large amount of lead in uranium minerals , radium D as yet is technically of little value , for no means of concentrating the activity exists . If it could be concentrated it would for many purposes be as valuable as radium itself . - r Another source of radio lead , however, is old adium . - The period of radio lead , although long, is still short enough to cause its production from radium to be easily detectable . of After the decay the active deposit of radium is complete , an extremely feeble activity is left, which then increases, the B-radiation for a few weeks and the a -radiation for a couple of years . This is due to radium D , which , after formation , E produces first radium and then radium F . In old radium preparations which have been kept in the solid state, or under conditions where the emanation is retained, the radio lead steadily accumulates , and may be separated with the E radium , from the polonium and radium , by precipitation with sulphuric acid . From the radium it may be separated by adding a little lead and then separating it by any con v enient method . - However, if a preparation of radio lead is required as concentrated and pure as possible , the simplest method of procedure is to remove the emanation periodically from a in - n radium solution kept an air tight vessel , and to i troduce it into a closed glass flask . As the emanation decays , radio lead is formed and accumulates as an invisible film o n the walls of the flask . Tubes that have contained extremely concentrated emanation from large quantities of radium 108 THE CH EMIS TR Y OF TH E RADI O-ELEMENTS

- show a sub metallic deposit on the glass , which perhaps may be due to visible amounts of pure radio -lead and its products . Recent researches have shown that it is radium D which is responsible for almost all of the soft (go-radiation which “ was at one time assigned to radium E .

RAD I U M E

ti mat at rme wei ht — 2 10 E s ed a g .

r li P er i od of av e age f e days . r h -cha n P e io d qf alf ge days .

i at i on— a Rad ﬂ ws.

— n c i nt . A bsor ti o oe e e Al . p ﬁ aﬁ 43 3 mi al nal o ue- o Che c a g Iso t pe o f b is muth .

— P arent Radi um D .

D isi nte r ati on er i od- Po l m i u m g p o niu ( Rad F ) .

- - and This short lived member gives out B rays , stands between a long-lived element producing only (B) -rays and a - a - n less long lived ray produci g element . It is therefore completely analogous to mesothorium 2 of the thorium series . At one time it was regarded as complex , consisting n E - of a rayless eleme t , radium 11 of half period days , - E which produced the B ray giving element , radium 2, with

- half period days, but later evidence has shown that it is single and in its chemical beh aviour is the isotope of n bismuth . On electrolysing a solutio of acetate of lead , n - in contai ing radio lead and its products , with gradually creasing current densities , the polonium is ﬁrst deposited - 2 on the cathode (at 4 micro amperes per cm . ) , then polonium E s 1 0 - and radium eparate (at micro amperes), while with d heavier currents all the products are deposite . This is an ’ example of von Lerch s partially true generalisation , that the successive products of a disintegration series become electro chemically more noble , and are deposited the more easily as we pass through the series . They also , as a general rule, become more volatile and more readily soluble in acids . The B— rays of radium E are absorbed exponentially by f aluminium , the coe ﬁcient of absorption being This is 8 - about three times greater than that of the , rays of uranium

1 10 TH E CHEMI S TR Y OF TH E R ADI O-ELEMENTS

tion for the ﬁrst ten years or more is small . S ince radio b - no lead is a y product of direct value , it should furnish the s n n On cheapest and mo t conve ient source of polo ium . precipitation with sulphuric acid the radium E and radium th e F remain in solution , and after a few weeks the former has completely disintegrated , leaving the latter by itself .

- On a large scale , the hot saturated solution of the radio lead nitrate may be crystallised and the polonium concentrated in the mother liquor . This, after the addition of a little E bismuth to hinder the deposition of radium , is electrolysed between platinum electrodes with a cathode potential not n n exceeding volt, or a curre t stre gth not exceeding n milliamperes per square ce timetre . The polonium ° may be removed from th e cathode by volatilisation at 1 0 0 0 and condensed on the surface desired , which is preferably 32 made of palladium or platinum . If the ordinary radium - containing residues of pitch blende are digested with hydrochloric acid , part of the n co tained polonium is dissolved , and may be precipitated o b with sulphuretted hydrogen . From the acid solutions tained i n n the worki g of the residues for radium , polonium may also be similarly obtained . About 3 kilograms of bismuth oxychloride are obtained from 1 ton of j oachimsthal - 1 pitch blende . The polonium may be separated ( ) by frac tio nal precipitation from solutions made very acid with hydrochloric acid , the polonium being enriched in the

2 i in v acuo precipitate ; ( ) by subl mation , the polonium being the more volatile ; ( 3) by fractional precipitation of the basic nitrate with water, the precipitate being enriched

Mme . ( . Curie) Very elegant methods have been devised by Mar kwald Radi o-Tell uri um c , who proposed the name for the substance before its identity with polonium was established by observations on its period . By immersing a plate of & c bismuth , silver , copper, . , in the hydrochloric acid solu tion , the polonium is practically completely precipitated . I n the same solution stannous chloride produces a small black precipitate consisting mainly of tellurium and con taining nearly all the polonium . If this is dissolved in not z too acid solution and hydra in hydrate added , the tellurium is precipitated and the polonium remains dissolved , and may be precipitated by stannous chloride . In this way 4 milligrams POLONI UM I I I

2 - were obtained from tons of pitch blende . Naturally it was intensely radioactive . A hundredth of a milligram sufﬁ ced to render a zinc sulphide screen plainly visible in the dark 8 6 to a large audience . The theoretical quantity of polonium in minerals is 1 milligram for every 1 4 tons of uranium

(element), assuming that no radium emanation escapes from Mme n the mineral . Latterly . Curie , separati g the polonium

- from several tons of pitch blende , obtained as ﬁnal product 2 milligrams , which were estimated to consist of polonium to the extent of 5 It showed several new lines in its spectrum , which are being periodically examined as the polonium disintegrates, to see if they disappear and give 8 7 f place to the lines of lead . But di ﬁculties were encountered with the lead derived from the paint o ff the walls of the reco m room , and the series of experiments has had to be men ed c . The exponential decay of the activity of polonium is

- and . perfectly regular complete , about one half per cent of

- the activity decaying daily , the half value being reached in

a - twenty weeks . Like ionium , it gives only rays of compara tiv el f y low range , no e fect being produced through a single n thin sheet of paper, even with the most inte se preparation .

« — n The ( wrays , rece tly put in evidence , are only just detect 88 able by delicate instruments from very active preparations . n It has proved an extremely useful radioactive substa ce, but for most purposes ionium would no w be preferred on account of its greater permanence . The product of polonium should 20 6 be isotopic with lead , and have an atomic weight of , C whilst the product of radium 2 should also be isotopic with

2 1 0 . lead , and have the atomic weight [ 12 TH E CH EMI S TRY OF TH E RADI O-ELEMENTS

THO RIU M (T H)

E E a e x “ . g E i n 5 8 8 k «1 3 Q 5 g g L 3 C é C. 0 ! x O 5 91 g 9 4: “ N N t— w

A to miewei ght— 2 ’ P eriod r av era e l e x ear P y g y y s ( ) .

— - R adi ation a rays . - n o a ra s P cm. o i Ra ge f y ( ) f a r. D i si ntegr ati onpr odu ct— M esothori u m 1

The chief sources of thorium at the present time are ( 1 ) S monazite sand from Brazil , N . and . Carolina, and the state of Travancore , India, from which practically all the thorium used tech nically is obtained it consists of 60 or 7 0 per cent . of monazite in ﬁne grains with other sands , com o f posed granite , rutile , zircon , magnetic and titaniferous iron b ; 2 stone , not attacked y acids ( ) thorianite, a rare mineral n 60 0 . from Ceylon , which co tains as much as to 7 per cent Th O b of 2 in a form solu le in moderately strong nitric acid U 1 0 20 . 0 . and from to per cent of 8 8 Formerly thorium was o r aneite obtained principally from the silicates , thorite and g , found in Norway , but these sources are exhausted for M technical purposes . onazite is mainly a phosphate of the n raeso d mi u m neo d rare earths , cerium , la thanum , p y , and y

. U mium , with variable amounts of thorium sually the com mercial to . Tl10 sand contains 4 5 per cent of 2, having been t o concentrated by washing this percentage . I n addition ,

” r small quantities of i on oxide, lime , alumina , and silica are present . th e In the ﬁrst stage of technical treatment of this sand, it is heated with twice its weight of sulphuric acid . The

i . cold mass s dissolved in water , and left to settle The solution is then fractionally precipitated with magnesia, the

1 14 THE CH EMI S TRY OF TH E RA DI O-ELEMENTS

to separate thorium from actinium and its isotope , meso thorium 2 . i n i n Alike the old , now obsolete , as the present tech nical s n method of purifyi g thorium , the peculiar solubility relations of thorium sulphate in water have been largely applied . The older method consisted in volatilising the excess of sulphuric t acid from the material being reated , and in dissolving the anhydrous sulphates in ice -cold water — a tedious operation — and in heating the solution till the hydrated thorium sul t t phate was precipi ated . The latter was hen dehydrated at ° n 30 0 to a d the process repeated . In present p rac tice the sulphuric acid is always kept in great excess in n n mineral the i itial treatme t of the , but the sulphate method n may be employed at the ﬁ al stage of manufacture as follows .

The thorium hydroxide is dissolved in hydrochloric acid , so n n 0 Th0 that the solution co tains not more tha 3 Z 2, and sulphuric acid is added to the extent of a half per cent . more n , b than the equivalent quantity the temperature ei g kept low , ° i s o n and n any case below 40 as a maximum . Under the e c diti o ns Th S O 8 H 0 the hydrate ( 4)2 2 is precipitated , departure from the conditions causing the separation of the tetrahydrate , in reci i which is every way less easily manipulated . The p p t ted n a sulphate is reco verted into hydroxide, and the process ” repeated as often as necessary to remove all impurities .

Thorium forms a curious compound with acetyl acetone , s and which is oluble in chloroform and alcohol , i n can be distilled a vacuum , and so can advantageously be employed for the puriﬁcation and separation of the element . It may be mentioned that in the fusion of refractory n minerals , for example , with sodium carbo ate, the thorium , n ThO if present , is converted i to the highly insoluble oxide, and its presence is apt to be overlooked . in Thanks largely to the thorium industry , which a pro n s duct u usually pure is essential , there exi ts , therefore , a great variety of exceedingly good and sharp methods for n the separation and puriﬁcatio of thorium , and it must be n u derstood that ionium , if present, and radiothorium always remai n uns eparated from thorium in these processes as far n as they have been exami ed . A glance at the disi ntegration series above shows that radiothorium and its products are responsible for by far the TH ORI UM 1 1 5

larger part of the radioactivity of the whole series, just as radium is for that of the uranium series . The three bodies preceding radiothorium in the series yield only a very a- and 8 small proportion of the rays, about half of the , and - y rays . Thorium , free from radiothorium , has not yet been prepared , and although this is now theoretically possible , by the repeated removal of the mesothorium as fast as it forms over a period long enough to allow the radiothorium present to disintegrate completely , the operation would require many years . For this reason , and also on account of the ionium always present from the uranium of the original mineral , it f - is di ﬁcult to examine the a rays of thorium itself . The

a- range of the rays , from a thorium preparation in which the radiothorium has largely decayed , is , however , distinctly 14 a - less than that of the rays of ionium . The non -separability of radiothorium and thorium results in all thorium preparations in the ordinary state yielding a-rays of the same order of intensity as that of n - uranium preparations , but more penetrati g , and B rays - about one tenth as intense, and l ess penetrating than those

f - - of uranium . The y rays , like the ﬁ rays , are relatively feeble , but those of thorium C are the most penetrating 2 known , while those of mesothorium are only slightly less

v - penetrating than the radium C y rays . Freshly prepared n from minerals , and retai ing therefore the full equilibrium amount of radiothorium , the activity of thorium salts attains a maximum after one month (due to the accumulation of

d . X thorium X), and then stea ily decays Thorium and mesothorium 1 resemble radium chemically , and are always separated completely in the processes of manufacture . The absence of the mesothorium initially causes the a-rays due to radiothorium and its products to decay to a minimum , which

' - max imu m v alu e is about one half of the , in about four and a half years , when further decay is j ust balanced by fresh radiothorium resulting from the now partially regenerated n mesothorium . The activity must then i crease again for several decades until the maximum value is regained . Thus the radioactivity is a complex function of the age of the thorium preparation , and for this reason alone thorium is entirely unsuited for use in the preparation of radioactive standards . 1 16 THE CHEMI S TR Y OF TH E RADI O -ELEMENTS But in another respect the radioactivity of this element n is quite differe t from that of uranium . All thorium prepara tions give out , according to their nature , more or less of the thorium emanation , the volatile product of thorium X with - period of half change of about one minute . If the electro

- scope is air tight, this emanation goes on accumulating for some ten minutes after a thorium preparation is placed inside , so that the activity on this account rises over this period, but may be reduced again to its ﬁrst value by blowing out the air of the instrument . The proportion due to the emanation is the greater the greater the mass n a- of the preparatio , whereas the proportion due to the rays of the solid substance is ﬁxed only by its surface . I n con sequence, the effect of the emanation is more marked with thick than with thin layers . After being exposed to the n emanation for some time , the i side walls of the electro “ ” scope become radioactive through the active deposit, which , once formed , takes some days completely to decay . Commercial thorium oxide has an emanating power

- about one third that of the hydroxide , but by ignition the n ema ating power is permanently reduced , the more the higher the temperature employed , until it reaches a limit at - S about one tenth of that of unignited oxide . olid thorium nitrate has even less emanating power than this . I n these cases the molecules of emanation diffuse out of the solid so slowly that the vast majority disintegrate withi n the solid itself and never escape . Naturally the radioactivity of the solid non -emanating compounds is higher on this account than that of those from which the emanation escapes easily . On dissolving these preparations the differences in their emanat ing power disappear .

M E S OTHO RI UM 1

E sti mated atomic w i h — eg t 228 . P er i od ( av er a e l e r ff g if yea s .

- — P eri od o ha chan e e rs . f lf g s. 5 y a

Radi ation— Ra les y s.

Chemical analo ue— f i u g Iso to pe o rad m.

a t— m P ren Tho ri u .

D i s i nte r ati on roduct — M th o ri um 2 g p eso . This important substance was discovered some years

1 18 TH E CH EMI S TR Y OF TH E RA DI O-ELEMENTS the insoluble material left after treatment of the product with water . The period of mesothorium is several hundred times shorter than that of radium , so that in a preparation having n its activity equally distributed between the two constitue ts , the mass of the chemically identical radium must be many hundred times that of the mesothorium . This fact seems to interpose an almost impenetrable barrier to the further study n E & c . . of the spectrum , atomic weight, , of the eleme t ven when prepared from a mineral like Ceylon thorite , which has only a very small proportion of uranium , the radium must exceed the mesothorium in absolute quantity , though its radioactivity might be quite negligible by comparison . The preparations prepared technically from monazite may be and fractionated till free from inactive matter, then are stated to be four times as active as pure radium compounds, the activity due to the mesothorium being three times as n great as that due to the radium . The compositio of such ‘ preparations i s estimated as 1 Z mesothorium to 99 Z ” radium , by weight . Mesothorium 1 shares with actinium the distinction of being the only radio -element known in the change of which n no radiatio whatever has yet been detected . Itself in

Group I I and its product in Group I I I , it obeys the rule for - a B ray change , and it is most probable that in its change a

- 8 . , ray is expelled , too feeble yet to have been detected

M E S OTHORI U M 2

m i h — 2 E sti mated ato ic weg t 28 .

P eri od of av erage lif e h o urs .

P eriod of halft hange ho urs .

— - R adi at ion and ra s . S , (7 ) 7 y

bsor ti on co cients— Al ro m to A p eﬁ aﬁ f

— o ni u Chemical ana l ogu e Iso to pe f ac ti m.

— P ar ent Meso tho ri um 1 .

- D i si nteg rati onproduct Radi o tho ri um.

i n Mesothorium 1 behaves like a alkaline earth , whilst mesothorium 2 behaves like a rare earth . The latter is in ME S OTH ORI UM 2 1 19 consequence readily separated from the former by adding ammonia to the solution . A trace of some other inactive material precipitable by ammonia must be present , naturally , to serve as a nucleus in the filtration of the precipitate , and a small trace of zirconium is usually employed . The 93 zirconium brings down also any radiothorium present . ne H owever , by repeating the process at an interval , say , of o i n or two days , which the mesothorium 2 is to a great n n extent regenerated , the first precipitate alo e contai s much radiothorium , the mesothorium 2 regenerated in the interval being under these circumstances the more free from radio thorium the shorter the interval . But since radiothorium a- i 2 - gives rays, whereas the mesothor um gives only B rays , a- n some radiatio is always initially shown by the preparations, and for this reason the growth of radiothorium from f mesothorium 2 is di ficult directly to prove . The period of the former being many thousand times greater than that of

rowth a- the latter, the g of rays due to radiothorium is very small . A fuller examination of its chemical character revealed the fact that it is chemically non -separable from or isotopic with actinium , and may be separated, therefore , from thorium by precipitating the latter, in neutral solution , either

- - with meta nitro benzoic acid , potassium hydrazoate, or hydro 2 gen peroxide . In this way mesothorium has now been . n obtained in a pure condition , i itially almost free from a - activity . I t has been established that the growth of a- 8 « - activity from it, as its , and y activity decays , proceeds according to the theoretical assumption that radiothorium ' is 94 the direct produ ct of the change . Although not yet pre

a - pared absolutely free from rays initially , these appear to come from the last unremovable traces of thorium C and no t from the mesothorium 2 itself . The B— rays of mesothorium 2 decay regularly with the d b period given , the ecay eing practically complete in two or

— three days . The B rays are of moderate penetrating power, n co but are not absorbed exponentially , the absorptio efﬁcient for aluminium increasing from to - as absorption proceeds . The 7 rays are , relatively to the - - d , d B rays , about one half as powerful as those of ra ium an

. are rather less penetrating , especially in the case of lead

For other metals the difference is not so marked . I n this [ 20 TH E CH EMI S TR Y OF TH E RA DI O-ELEMENTS

- n the 7 rays resemble rather those of ura ium X . A care ful comparison of the absorption of the 7 -rays through lead with those from pure radium will reveal the contamination of radium by mesothorium wi thout the necessity of opening n 95 the tube containi g the preparation .

RAD IOTHOR I U M

a — E sti mated tomi c weight 228 .

r — P eri od of av e age lye 1063 days years ) .

- — P eri od of ha lf ch ange 737 days years ) .

— R adi at io n aways . - an e a ra s c m. o f a r R g qf y i .

— Chemi cal ana logue Iso to pe o f tho ri um.

— Meso tho rium 2 P arent .

i s i nt r atio n oduct— Tho ri um D eg pr X .

- This radio element , which resembles perfectly its original in parent chemical nature , would , like the isotope of uranium , have remained experimentally unknowable but for the relatively short life of the intermediate substance meso o thorium . The latter can easily be separated b th from thorium and from radiothorium . Hence , after separating it from a thorium mineral , mesothorium is left to itself to produce radiothorium , which is then separated by adding to the solution a trace of thorium , or , if that is objectionable ,

n . of zirco ium , and precipitating this out with ammonia The precipitate retains the radiothorium and the solution the mesothorium . This is the only known way in which radio u n thorium can be prepared . The earliest preparations n doubtedly were prepared in this way unwitti gly, m esothorium n n “ not the havi g been discovered . The period has been determined by direct measurements of the rate of decay .

When prepared in the manner described , the radiothorium 2 at ﬁrst retains the mesothorium , which decays completely in the course of two or three d ays . I n the meantime thorium X is being produced , which causes an increase of the activity over a period of three or four weeks . The emanating power of the preparation grows with the growth nth a and of thorium X . After a mo the tot l activity ema to nating power reach a m aximum , and then slowly decay

THE CH EMI S TR Y OF TH E RADI O-ELEMENTS the whole of the emanating power and the greater part of l the radioactivity , is left in so ution . Whereas if the pre

c i itant &c . p is a carbonate , phosphate , , no separation is f e fected, and the precipitated thorium possesses the same radioactivity and , in solution , the same emanating power as a initially . If after precipitation of the thorium with m monia the ﬁltrate is evaporated to dryness and the ammonium

- salts volatilised by gentle ignition , a non volatile residue is n n obtai ed , very small in quantity but i tensely radioactive , the activity being that of thorium X . I f the thorium com pound used is , for example , one like thorium nitrate , and has been preserved in the solid state , the emanation is n th retained , and nearly the full amou t of e later products of “ the active deposit thorium B to D will be present . Whereas if a solution of thorium which has been kept for v the last two days in an open essel is used , these products at will not be present , least in their full equilibrium quan tity . On precipitation with ammonia the thorium X alone is left in solution , the other products , if present , being pre i itate c p d with the thorium hydroxide . The activity of the precipitate thus decays for the ﬁrst twenty -four hours until the regeneration of thorium X overpowers the decay due to the disappearance of the active deposit . After that time the activity steadily rises and attains equilibrium in three or four a- 2 0 weeks . At the minimum the rays have about 5 to 3 per cent . of their equilibrium value , whilst the penetrating rays are practically absent , as these come entirely from the “ " active deposit . The emanating power of a solution is a s direct mea ure of the amount of thorium X , so that if the freshly precipitated thorium hydroxide is free from thorium

X , when dissolved and tested for emanating power it will be i found to have practically none . Conversely the activ ty of thorium X , especially after ignition , when the emanation is n - retai ed , increases for twenty four hours and then decays regularly to zero in three or four weeks , according to the exponential law with the period of thorium X . The emanating power of the solution from which a thorium salt has been precipitated with ammonia is at

ﬁrst the same as that of the original thorium in solution , but this decays exponentially with time to zero , according to the period of thorium X . The emanating power of the TH O RI UM x 1 23 thorium hydroxide meanwhile steadily rises from zero to

b . the equili rium value If, instead of ammonia , pyridine , - - fumaric acid , or meta nitro benzoic acid is used as the

precipitant , thorium B , as well as thorium X, is left in the

solution . As thorium B is by far the longest -lived member

of the active deposit group , these reagents are to be reco m mended when it is desired to prepare quickly thorium

hydroxide free from both thorium X and its products .

- Four precipitations with the last named reagent, at intervals f of two hours , are stated to e fect this , and to give a thorium

a - - hydroxide of minimum activity , no B activity and no

emanating power, the activity of which then increases regularly without the initial decay characteristic of the n 97 ordi ary precipitate obtained by ammonia . S ince thorium B and thorium C are the isotopes of lead

- - and bismuth , respectively , and meta nitro benzoic acid pre c i itates p thorium C , leaving thorium B in solution , the same ff o f lead d b reagent should e ect a similar separation , an is

muth . On testing this deduction it was found to be correct, and a separation of these metals may be effected in this

way . This is an interesting example of the opposite to the

usual case , the determination of new reactions of old

elements by proxy , from the reactions of their radioactive “ isotopes . The isotopism of thorium X and radium follows from a number of indirect observations rather than from any

systematic investigation . The close resemblance was shown ﬁrst by some experiments on the crystallisation of various M salts from saturated solutions containing thorium X . ost salts crystallise leaving the active material in the mother liquor , but the barium salts on crystallisation preserve much 98 the same proportion of the thorium X as the mother liquor . Of course it should be actually concentrated in the mother liquor, just as radium is, but the point has not yet been speciﬁcally investigated . Out of acid solutions thorium X cannot be separated by electrolysis or by the action of metals , such as copper, zinc , “ ” nickel . Only the active deposit is separated . But out of alkaline solutions all the active substances may be deposited by these means . Thorium X follows von ’ Lerch s rule , and is less easily deposited, or is electro 124 THE CHEMI S TR Y OF TH E RADI O -ELEMENTS

“ chemically less noble than its successive products . It h as not been volatilised . The period of thorium X is almost identical with that of the radium emanation , and this is the only case of the kind among the known radio elements . This , together with its isotopism with radium , leads to some curious results in the separation of thorianite ,

— when the radioactivity is studied by means of the 7 rays .

Preparations obtained , for example , by precipitating barium sulphate in the solution may appear to be of constant activity , when what is really taking place is the s imultaneous decay n of thorium X and the regeneration of radium ema ation , leading to a decay of the T rays due to thorium C and a w gro th due to those of radium C .

THO R IU M E MANATION

i m h — E st ated atomic weig t 220 .

r i — e ds P er i od of av e age l fe 78 s co n . - P eri od of half change 54 seco nds . 1 Radi oacti v econstant 128 (sec. .

' — - R adi at i on a rays. n - o f ai r Ra ge of a ray s cm.

— a a i Chemical analogu e Iso to pe o f radium em n t o n.

— Parent Tho ri um X .

— D i si ntegrati onproduct Tho ri um A .

The short period of the thorium emanation and , in con sequence , its rapid reproduction after its removal from thorium or thorium X preparations by a current of air , are the features that distinguish it chieﬂy from the radium emanation . Apart from this the two emanations are very

a - n similar , both giving rays on disintegratio , producing active n deposits , being conde sed at liquid air temperature and absorbed by charcoal at ordinary temperature b ut not by chemical reagents . The condensation of the thorium emana tion under ordinary circumstances is not so sharp as that of s the radium emanation , conden ation commencing as high as and being complete at Its coefﬁcient of f dif usion is similar to that of the radium emanation , being about indicating that the molecular weight is high . Direct measurements by the effusion method have given the

1 26 TH E CH EMI S TR Y OF TH E RAD I O-ELEMENTS

THOR I U M “ ACT IV E D E POS IT

n g E 0 0 a gf ‘ a g s s a 5 E g if s uJ R is e 15 Q 1 S 2

Th e

E sti mated atomic wei gh t P m od of “mm“ h rs . 8 min sec . . 7 min.

sec . h rs P ( ) min.

Range of a -ray s m (c . o f ai r)

A dsorpti on coef cients

Chemi cal analo ue g .

m. d . is mut I ni u h P lo i um. soto i i t . p c w h Po lo Lea B o n Thal li um.

Th ori um L — The ﬁrst product of the thorium emana 100 tion , thorium A, was the last one to be discovered . It had previously been known that the actinium and thorium

a - emanations emitted their particles in pairs , as shown by the appearance of double scintillatio ns on the zinc sulphide screen . In the cas e of actinium the emission appears simultaneous, whereas in the case of thorium a distinct but very short time interval separates the two members of 101 th e , the pair . If, in a dark room , thorium emanation is allowed to diffuse from a radiothorium preparation into a small cylindrical vess el carrying an axial electrode coated “ THORI UM A CTI VE DEPOSI T l 27 with zinc sulphide and connected to the negative pole of 1 0 0 0 a battery of volts , the application of the ﬁeld is aecom anied n end p by the i stant brightening of the of the electrode .

On disconnecting the battery the luminosity instantly ceases . ld The ﬁe attracts the negatively charged A atoms , which cause the screen to glow . On breaking the electrical con necti o n n , that attracted decays almost insta tly . Another way of demonstrating the existence of this excessively short lived ﬁrst member of the thorium active deposit is to carry n i n an e dless wire , negatively charged , through small holes ebonite stoppers closing a tube containing a source of thorium emanation , and to drive the wire round by a motor .

On applying a zinc sulphide screen , the wire is found to be active while the motor is driving it , and the activity along the moving wire is found to fall o ff the more quickly the slower the wire is driven . By this or similar devices , the - period can be accurately determined . The half period, 1 /7th of a second , is the shortest known , except for an analogous much more rapidly decaying product of the 102 actinium emanation . Both are entirely analogous to radium A of the radium active deposit . In all ordinary experiments the effects of thorium A add themselves to those of the emanation , and are indistinguishable from them .

ri m B — Th o u . The second product of the thorium emana tion , thorium B , may be considered almost rayless , the very

- soft (3) rays it produces not being at all powerful . In consequence , a negatively charged wire exposed for a short n time to a very powerful source of thorium ema ation , such as can be got by using radiothorium , and then withdrawn , possesses practically no activity a second after withdrawal , but in the course of a few hours a powerful activity due to thorium C and D develops . This activity attains a maxi ln 2 2 mum 0 minutes, and then decays slowly at ﬁrst, but,

- after 5 hours , exponentially to zero with the half period of the n in hours , decay bei g practically complete two or

a and a three days . I n these changes the B r rays vary very nearly uniformly , the period of thorium D , the B and - n 7 ray producing body , being too short to enter i to con sideration . According as the time of exposure to the emanation is increased, the initial rise of the activity becomes 128 TH E CH EMIS TRY OF TH E RADI O -ELEMENTS

’ less and less marked , until after several days exposure , when i n the whole of the products are initially equilibrium , the existence of thorium B makes itself felt merely by a tem o rar p y lag , over the first five hours after withdrawal , of the decay of the activity . This example illustrates well one i n po nt in the theory of successive cha ges . The course of the curves of activity of the thorium active deposit would be identical in every respect if the periods of thorium B and thorium C were exchanged in the series , that is to say , if the first rayless body had the shorter instead of the longer of i n n the two periods . H s o ly by separating o e of the pro i n n ducts the pure state , and measuri g its period of decay , that the choice can be made as to which product has the 103 longer period . The separation of thorium B from thoriumC can be ff e ected in various ways . By heating the active wire for - a short time to a red heat , the thorium B alone distils away f without any immediate ef ect on the activity of the wire , 104 which , however, now decays much more rapidly . The sublimate is at first inactive , but acquires an activity reaching a maximum in four hours , and then decaying - s with the hour half period . The ublimate, therefore , f is thorium B . I the wire is heated above thorium C ° 1 0 0 0 also volatilises, but heating for a few minutes to removes all the thorium B and leaves some of the thorium - C , which now decays exponentially with the half period of

55 minutes .

Th ri um — o 0, Thorium C may be removed from a solu tion of the thorium active deposits in acids by adsorption with animal charcoal , the thorium B remaining in solution . The same is the case whe n the solution is shaken up with

ﬁnely divided nickel or exposed to a nickel plate, or when it n is electrolysed , thorium C bei g always the most readily Z deposited . inc , on the other hand , deposits both thorium B and C from a solution , but not thorium X . s The isotopi m of thorium B , C , and D with lead , bis e muth , and thallium , r spectively , has been established by “ direct experiment . If we stop our analysis of the active deposit at thorium o C , and regard it for convenience as a single pr duct like

a — all radium C , it appears to give B and 7 rays, three

1 30 TH E CHEMIS TR Y OF TH E RA DI O -ELEMENTS

far as appears , is the stable or ultimate member of this ” branch . The whole scheme depends on the homogeneity of thorium C and the impossibility of separating it into two th e products , one giving long and the other the short range

- a rays . There is some evidence that such a separation may b f b yet e e fected , ut , for the present, the scheme may be regarded as pretty ﬁrmly established .

Th ori um D — . The isotopism of thorium D with thallium was ed b Law n im pr icted y the Periodic generalisatio , and mediately veriﬁed in a number of characteristic tests . It - i n n may be precipitated as platino cyanide , prese ce of potassium or of thallium , and it is precipitated by hydrogen in sulphide , neutral solution , in presence of a member of the “ second group to act as a nucleus .

If in the separation of thorium C by electrolysis , or n O shaki g with nickel , the peration is accomplished in a few seconds , the thorium C is at ﬁrst free from thorium D . a- Its rays decay exponentially with the 55 minute interval ,

— whilst its 7 rays are at ﬁrst zero , but increase to a maximum - with the minute half period of thorium D . Thorium D is more easily soluble in acids and is more volatile (being completely volatilised by heating the active wire for 30 seconds in a Bunsen ﬂame) than any of the other pro 108 - ducts . Thorium D is the last known member of the t i' series . The decay of the horium active deposit is regula and complete , no feeble residual activity remaining as in the 109 case of radium . I n both branches the ultimate products s hould be isotopic with lead and have the same atomic weight , A CTI NI U M 1 31

ACTI N I U M

E x C s . S v) . E , 9 ? g 0 ° - 2 ? 6 8 “ s 25 0 s = “ 3 . é s 3 E R 39 El?

— n Atomi c wei ght U nkno w .

— o w P eri od of av erage l if e U nkn n.

— P arent U n no wn. Pro b abl urani um i s the ultimat r k y e pa ent.

uc — R adio -actini um D i si ntegrati onp rod t .

Chemical anal o ue— H o mol o ue o f lanthan g g um. Inb asici ty b etween m lanthanum and cal ciu .

' As the above headings indicate, actinium is still very l l . Y d D ebi erne itt e known et its iscovery was made by , in - the iron group separated from pitch blende , very shortly after the discovery of polonium and radium . The total absence of knowledge of its most important radioactive char acteri stic , its period , which at present is wholly incalculable, alone makes it unique . All that can be said is th at the activity of the series appears to be permanent within the few years it has been known , and therefore its period pre s umab ly must be at least several times as long as that of polonium . This is perhaps the most important datum in radioactivity still lacking . Next perhaps in importance is the atomic weight, the absence of knowledge of which , of course, extends to the whole series , comprising eight a- members, of which ﬁve emit particles . According as the branching of the uranium series occurs at uranium 1 or 2 uranium , the calculated atomic weight of actinium itself 2 26 fﬁ is either 2 30 or , and there is not yet su cient evidence to d ecide between these alternatives Actin i um i s a constant constituent of uranium minerals , so far as these have been examined , and this points to uranium being the ultimate parent , but no production of actinium from uranium or 132 TH E CH EMI S TR Y OF TH E RA DI O-ELEMENTS

any of its products has as yet been observed . Its peculiarity a - is its great rarity . The activity of uranium minerals ne is times that of the contai d uranium . Of this all but is contributed by the uranium -radium -polonium series a- proper, the several activities of the various ray products being in agreement with the view that each of the six a - t n products expel one par icle each , per atom disi tegrating . Now four or ﬁve members of the actinium series expel a - a- particles , and yet the total proportion of the radiation of uranium minerals contributed by the actinium series is estimated as only out of this Were the

- - actinium series in the main uranium radium polonium series,

a - its activity should be similar to that of the radium series . It seems most probable that one of the members of the uranium series has two ways of disintegrating , the one mode , in which by far the greater proportion of the atoms disin te rate i g , giv ng ultimately polonium and the other giving actinium . I n consequence of what has been said , it will be clear that actinium must necessarily be excessively scarce even compared with radium ; To produce an actinium preparation , which , when in equilibrium with its products , shall have the activity of a given mass of radium in equilibrium , some ten or twelve times as much mineral must M be worked up . oreover, the ﬁnal complete concentration

& c . s of actinium from lanthanum , , h as not yet been succes fully accomplished , so that actinium preparations are at once excessively costly , and at the same time disappointing in the intensity of their activity , as compared with radium . a Actinium itself is rayless, and its ﬁrst product, r dio

- actinium , has a period of half change of days . As - often separated from minerals, free from radio actinium , n acti ium preparations are hardly at all active , but the activity and emanating power increase enormously over a s n period of everal mo ths, a behaviour which is characteristic a of the substance . I t is thus very e sy to overlook its presence , and this , coupled with the fact that, as ammonium salts accumul ate in an actinium solution , it becomes less and less completely precipitated with ammonia , very special precautions have to be taken in working wi th actinium not to lose this rare and diﬂi cul tly won substance altogether .

I n no case is the maxim , never to be in a hurry to throw

134 TH E CH EMI S TR Y OF TH E RA DI O -ELEMENTS

- mother liquor . The rare earth element most closely asso c i a d te to actinium is lanthanum , and by this latter frac tio natio n , with the double magnesium nitrate , part of the lanthanum may be removed in the crystals free from actinium . Ionium may be separated by precipitation wi th sodium thiosulphate, in presence of thorium , the actinium remaining in solution . Auer von Welsbach places actinium between lanthanum and calcium in chemical properties . H e observed that in the presence of ammonium salts the precipitation of actinium is far from complete, but it is completely precipitated in presence of manganese from basic solutions as a manganate . This reaction he found of great service in the separation of actinium from pitch 87 blende residues on a large scale .

Observations on the penetrating rays of actinium , pre pared many years previously , have shown a marked decay of the activity during the last three years , amounting to m about 1 0 % . This indicates either that the period of n average life of the element is o ly about thirty years , or that , as in the case of thorium , an intermediate product n - of long life exists between acti ium and radio actinium . The ﬁrst explanation makes the continued failure to detect ﬁ s a production of actinium dif cult to under tand , for this should be easy to do if the period is only thirty years . The h second explanation as only analogy to recommend it . On l genera grounds, since preparations times as active as uranium have been prepared , the period cannot be greater b 1 o o o o th i s b than a out /3 , of that of uranium , that a out

years .

RAD I O -ACTI N I U M

P eri o d of av erage life days . - P eriod of half change days .

R ad — a and -ra iation (ﬁ) ys . - r. Rangeof a ray s cm. o f ai

Chemical ana lo ue— Iso o o f tho ri g t pe um.

— P arent Act ini um.

— D i si ntegrationproduct Actinium X .

Radio -actinium may be separated from its parent and its A CTI NI UM x 135 products by forming in its solution a ﬁne amorphous pre ci itate p , such as sulphur , by adding a little sodium thio 112 n sulphate to the strongly acid solution . The actinium a d

- actinium X remain in solution , whilst the radio actinium is precipitated . The separation , however , was originally de scribed as rather uncertain . The subsequently discovered isotopism of radio -actinium and thorium shows that the uncertainty was probably due to the presence or absence of 34 thorium in minute traces . I n presence of thorium the separation works perfectly . When an actinium preparation is treated with dilute

d , hy rochloric acid a small part usually is undissolved , and this contains a larger proportion of the radio -actinium than

- the solution . Freshly prepared , the activity of radio actinium

- a - consists entirely of non penetrating and (3) rays , which increase , in about three weeks , to a maximum of two or ao three times the initial value , due to the production of tini u m X and its products . The penetrating rays and the emanating power of the preparation increas e from zero to a x c ma imum in the same time , and then the total activity de ays - exponentially with the half period of about twenty days , the decay being practically complete after four months . Acti nium itself is often not completely precipitated by ammonia , - whereas radio actinium appears to be more easily precipitated . By fractional precipitation of an actinium solution with n - i n ammo ia, the radio actinium is concentrated the ﬁrst fractions . The behaviour of the substance is not very deﬁnite , R - ex ce except in the presence of thorium . adio actinium is p tional a - , with radium , in giving both and (B) rays, though in each case the product obeys the rule as though only an - a particle were expelled .

ACTIN I U M X

o av ra e l e a s P er i d of e g if d y .

- — P eri od of half change I days .

— - R adi ation a rays .

- — an e o a ra s J c m. o f air R g f y 4 7 .

— Chemi cal analogu e Iso to pe o f radium.

— - P arent Radi o actini um.

— manati o D i s i ntegr ati onproduct Actini um e n. Actinium X resembles thorium X completely in chemical 1 36 TH E CH EMIS TR Y OF TH E RADI O-ELEMENTS

98 e nature , and may be separat d from actinium or radio 113 n f actinium by precipitation with ammonia . The o ly di fer ence between this separation and that of thorium X is that the “ active deposit of actinium is separated with the actinium X, but in the ignition of the ammonium salts it is volatilised . The result is therefore similar, so far as the ignited preparation is concerned . I nitially it is free from ” a active deposit , and immediately st rts to produce it . The a- - - rays increase in intensity about one third , and the B rays grow from zero to a maximum , in three or four hours, and then the total activity and emanating power decay ex iall - po nent y with the half period of ten or eleven days . The h actinium hydroxide so obtained is , like the thorium y d ro x ide - - n precipitated with fumaric or meta nitro be zoic acid , " n i itially free from the active deposit , and in consequence the activity rises from preparation without any initial decay .

a - 28 The activity is initially about z of the equilibrium value , - n which is due entirely to radio acti ium , but owing to the i ndeﬁniteness already referred to in the behaviour of the latter substance , it is apt to be separated to some extent from the actinium in the chemical Operations for removing actinium X . As in the case of thorium X , the emanating power of a solution is the direct measure of the amount of actinium X present therein at the time of the measurement .

ACTI N I U M E MANATION

' P eri od e av er a e l i e seco nds y g f . ri od h al -chan e d P e of f g seco n s .

n ta t— R adioacti v eco s n O . 18

— - Radmt ion a rays .

-ra m ai R an e a s c . o f r g of y . Chemical ana lo u e— I t f radium emana i o g so o pe o t n.

— P arent Acti nium X .

at r — i D is i ntegr i onp oduct Acti n um A .

The emanation of actinium , except that it has a still shorter n period than that of thorium , resembles the other two ema a n tions closely . Acti ium preparations generally consist for t the most part of hydroxides of the rare ear hs, which evolve and t their emanation with great facility , and this the shor period account for the beautiful effects due to the emanation

138 TH E CH EM I S TR Y OF TH E RA DI O-ELEMENTS The main differences between the active deposit of t actinium and hat of the other series are , ﬁrstly , the short n ness of the period of acti ium C , which is shorter than that of actinium D , and secondly the fact that what corresponds with the minor branches in the other two series is here , if not the only branch , the only branch the existence of which

. S ee . . has been put beyond all doubt ( Part I I , p The actinium active deposit bears the closest analogy to the thorium active deposit, the periods of the B and C members being , however, much shorter, and that of the D member somewhat longer th an in the latter case . The period of the active deposit as a whole is very similar to , b n eing rather longer tha , that of the radium active deposit after the rapid changes due to the A products are at an end . There is a slight initial lag in the decay of the actinium active deposit as a whole , similar to , but much less pronounced than , that in the case of the thorium active deposit . After this initial lag the decay continues exponentially to zero with - the half period of minutes due to actinium B , the - longest lived member of the group .

Acti ni um A — . The recently discovered ﬁrst member ,

a- actinium A , gives rays , and has the shortest period so far - h as actually measured . The period of half change been measured accurately by a rotating disc method , and found h 102 to be exactly 1/500t of a second . Actinium A may be put into evidence as a distinct product of the emanation by methods similar to those described under Thori um A . I t is 11° n 1 0 of interest that Giesel , so l o g ago as 9 3, working with a powerful emanium preparation , observed that the emana tion could be influenced and directed by an electric field . If the preparation was contained in an earthed vessel with an opening of any shape, on bringing a zinc sulphide screen , O negatively charged , opposite the pening, a bright image of n the latter appeared on the scree as a phosphorescent patch , and disappeared instantly the field was interrupted or ” - E R . screened . He termed the phenomenon the ays This experiment is very analogous to that already described

Thori um A under . I n all ordinary work actinium A and the d f emanation act together as a single pro uct, and their ef ects are superimposed .

Acti ni um B — n . Actinium B is more volatile tha actinium ” A CTI NI UM “ A CTI VE DEPOS I T 139

C , and on heating a wire or plate coated with the active deposit strongly for half a minute the a -activity is not itself volatilised but decays with great rapidity after the heating, with the period of actinium C , the actinium B being almost - completely volatilised . The B activity of the wire or plate after heating , on the contrary , rapidly increases to a maxi mum and then decays with the somewhat longer period o f actinium D . The latter substance is thus volatilised with the actinium B .

ni um - Acti 0. At one time it was thought that actinium C , like th o ri u m a- d f g C , gave out two sets of rays of i ferent , but, in this case , very similar ranges . This , however, has been a- shown to be unfounded . Among the rays of range n cm . , a very small proportio , has been found with ’ u Acc range cms . , which may be due to a prod ct , , 117 B u analogous to that in the other series . t this still awaits conﬁrmation .

A tini um D — c . Actinium D can also be prepared pure by recoil from the active deposit , the active plate being positively O charged and pposed by a similar negatively charged plate, at a short distance, on which the recoiled product is received . The yield may be as much as 50 z of the theoretically « - possible . I ts activity is solely conﬁned to B and y rays, and - decays exponentially with the half period of minutes . Actinium D is alone withdrawn from an acid solution of the active deposit by boiling with animal charcoal or platinum 118 black . I t has been experimentally shown to possess the reactions of thallium in the same way as thorium D . co m lete The decay of the active deposit is p , and is end regular right up to the , and there is no direct evidence whatever as to the identity or atomic weight of the ultimate

t . product , which , theoretically, should be the iso ope of lead 140 TH E CH E MI S TR Y OF TH E RA DI O -ELEMENTS

T H E ULTI MATE PRODUCTS— ATOM IC WE IGHT OF LEAD

The remarkable fact emerging from the generalisation , shown in Fig . 3, that all the series end in the same place in the Periodic Table , namely, that occupied by lead , and there fore should b e chemic ally identical with and non -separable from lead , has stimulated investigation of the atomic weight of lead derived from radioactive minerals . The results , so far as they have been obtained , appear favourable to this hypothesis . As the result of a preliminary examination of the atomic weight of lead derived from Ceylon thorite, a mineral to the unique character of which , as an almost pure thorium mineral , frequent reference has already been made , an atomic weight of relative to that of ordinary lead as 119 was obtained . Assuming that the ultimate products of both the uranium and thorium series are completely stable , and accumulate in geological time , which is the point most in b ad dou t , the 70 of le present in Ceylon thorite, if entirely i n of radioact ve origi , should consist of about ten parts of “ “ ” thorium lead per one part of uranium lead , and its atomic weight should therefore be about The question is being re-examined with a considerably larger quantity of the mineral . a Almost immediately fterwards , careful estimations of the atomic weight of lead from a variety of uranium and thorium n minerals by several indepe dent workers were published . 12° I n one set the following results were Obtained

Lead from ranini aro na U te (N . C li ) Pitch-b lende (J oach imsthal) Camo tite (Co lo rado ) T h o rianite (Ceyl o n) Pitch -b lende (Co rnwall) Ordinary lead Another worker 121 found that the atomic weight from uranium minerals varied from ' to and is

14 2 TH E CH EMI S TR Y OF TH E RA DI O-ELEMENTS

POTAS S I U M AN D RU BI D I U M

A few words in conclusion may be said regarding the a s radio ctivity of the e two elements, though it is doubtful at the present time whether we have to do here with cases of atomic disintegration of the type established for the

- other radio elements . All potassium salts show a feeble - h - 1 1 0 0 0t . B activity , about / p art of the B activity of uranium The fact that the s o -called “ potassium cyanide of com merce is often exceptional is due to these preparations being largely sodium cyanide . The activity appears to be a speciﬁc atomic property of potassium , and has not so far been concentrated from it . Of the other alkali metals r a and thei salts , lithium , sodium , and c esium possess no R detectable activity . ubidium , however, possesses a B activity similar to that of potassium , but distinct from it . The speciﬁc activity of the two elements for thick layers - of their salts is very similar, but the potassium B ray is about nine times more penetrating than that of rubidium . Hence the rays come from greater depths in the case of potassium salts, and the activity of rubidium , allowing for the absorption of the radiations of the salt itself, must really be much greater than that of potassium . The potassium B-rays are described as heterogeneous and of varying degrees of penetrating power . The most penetra ting are nearly as penetrating as those of uranium X .

They are easily deviated by an electric field . On account of their feebleness the se supposed cases of radioactivity are difli cu lt to investigate . The alkali metals under the influence

- of light emit cathode rays, that is to say , they possess photo electric sensitiveness . No special examination seems to have been m ade of the inﬂuence of light on the radioactivity of fi potassium . It is dif cult to believe that the atoms of potassium and rubidium are really disintegrating like those of uranium and thorium until products of the disintegration have been isolated or similar further e v idence of the pro cess is obtained . Naturally the facts may be explained by POTAS S I UM A ND R UB I DI UM 143

supposing the existence of minute quantities of new elements , not yet separable chemically from potassium and rubidium respectively, as the cause of the phenomena . But of the existence of two new speciﬁc types of B-radiation it is not 123 possible to doubt .

146 THE CHEMI S TR Y OF TH E RA DI O -ELEMENTS

i . M . 1 1 etc er P a 26 6 . Fl h , h l g , 9 3 , 74 2" Rut er o r and S od i bi d. 1 0 6 f y , , 5 1. h d d 9 3 , 5 ‘ 9 R ut er o r ature 1 06. 74 6 C em. e s 1 0 f , , 9 , 4 ; w , 99 6. h d N 3 h N 9 9, , 7 3’ P H . . S c m t h s ik l . Z i a e t h r. 1 8 1 sc 0 9 8 . W h id , y , 9 , , 4 3° M a . 1 12 S c ra er i . 24 12 Russe i bi , P , 9 , ; , d 1 . h d h l g 5 ll , 34 3 ‘ w a ic and Russ i bi d. 1 1 1 12 C k e , 27 . h d ll , 9 4 , P anet nd v n H s n a o e e M o h . 1 1 160 y , ats , 9 34, . h v 3, 5 ”3 H a n Ph s ikal . Zei tsch r. 1 0 10 8 1 Ha n and M ei n r B h , y , 9 9, , h t e , er. d .

Ph sikal . G es . 1 0 Ph sikal Zeit r 11 . sch . 1 0 y , 9 9, , 55 ; y , 9 9, 10, 697 ; R nd M k w r P us s a a o e roc . Ro . S o c. 1 0 0 P A 2 M a . 82 i . , y , 9 9, , 5 h l g , 1 10 1 100 8 Mako wer and E 9 20 ans i bi d. 88 9 , , 75 v , , 2 Wer

m t. tonstein Co ren . 1 10 150 86 151 6 Le Ra ium 1 12 , p d , 9 , , 9; , 4 9; d , 9 , n 6 A n. P i 1 9 s ue 1 . , ; hy q , 9 4 1, 347 3‘ ec T r n . m. S o c . 1 1 8 1 n a s e 103 a d 10 2. Fl k, Ch , 9 3, , 3 5 "1 Russe em. e s 1 1 F a ns Ph l a i . , C w , 9 , 107, 4 ; s ka Zeitsch r. 1 1 ll h N 3 9 j , y , 9 3,

14 1 1 1 6 S o em. e s 1 1 107 a r. Ra io , 3 , 3 ; ddy, Ch N w , 9 3, , 97 ; J h d

ak iv itat 1 1 188 . t , 9 3, 10, B o ltwo od Amer . ci . 1 6 . S 0 22 1 0 24 0 . , , 9 , , 537 9 7, , 37 3" J A er v n S u u o e sbac itz n s . ber . A a . i s n W , W s . Wie 1 10 119 l h K k d , 9 ,

1 M o natsh . 1 10 51 1 1 . , 9 , , 59 3 ° i . 6 2. o rin P h sikal . Ze tsch r 1 1 G h g , y , 9 4, 15, 4

E b ler and e ner Ber. 1 1 1 44 2 2. F ll , , 9 , , 33

Ritze Zei tsch r h s ikal. em 1 0 2 . l , . p y Ch , 9 9, 67, 7 5 F reun a d r P ic eum nn an aem e h s ikal . Zeits 1 1 , f , ch r. 4 15 dl h N K p y , 9 , ,

537~ ‘ 2 A . n . Co eh B r. 1 8 1 1 , e , 904, 37, ‘ 3 M 1 1 h r. 1 1 . ei n r Ph i l . Ze t 0 t e , y s ka i sc , 9 , 12, 94

. 26 0 1. nOﬂ r . D . R . P . K e Co O , , 95 ‘ 5 anet n tsh . 1 1 P and v o n H e es y Mo a , 9 3, 34 1 . h v , , 593

H v . 12 628 . v o n e es i M . 1 y , P a 23, h l g , 9 ‘ 7 anet and v o n H e es Mo natsh . 1 1 160 . P h v y , , 9 3, 34, 5

P ne o o i Z it h . 1 1 2 a t , e sc r 9 3, 13, 1, 97 ; G od ewsk i bi d 1 1 h K ll d l i, , 9 4, w 1 22 A i . raco 1 A 6 u . ca . Sc L u 14, ; B , C , 3, , 33 e Ra i m 1 1 9 ll d 9 d , 9 3,

10 2 0 P . M a . 1 1 27 618 . , 5 ; hil g , 9 4 ,

So and F ec a r no t et ub is e . ddy l k , p y p l h d 5 ° o . s ica cm. 1 10 14 476. Ll yd , J Phy l C , 9 , , ‘ 1 F a ans r 1 1 86. j and Beer, Be . , 9 3, 46, 34 '

l . . s 1 . Rad iﬂ . R . N . S a e 1 1 c . and roc S , , 9 3, 47, 4 , J P W l 5

n r. . A is . n 1 H a tin er and U ri c S itzu sbe a . s e 08 l h , g K k d W Wi , 9 , 117, i a 61 i i . 5‘ McCo P i M a 1 06 11 1 6. y , h l . g . , 9 , 7

8 8 1 10 . i . 1 2 od . Ma 0 1 S dy , Ph l g , 9 9 8, 5 ; 9 20, 34

. 00 0 S o nd R us e i ro o es r . Ro . S o c 1 a s . C k , P oc y , 9 , 66, 4 9; ddy ll , Ph l

M a . 1 0 620 . g , 9 9 18, “7 . 1 10 0 . o . sica em 9 , 14, 9 Ll yd , J Phy l Ch , 5 man R i aktiv itat 1 0 6 26 . K eet a r. ad o , J h , 9 9, , 9

e in Ph i l Zei s ch r. 1 0 8 . s ka . t 8 L v , y , 9 7, , 5 5

Ma 1 0 620 . S o and Russe . 18 ddy ll , Phil 9 9 ,

ar o n i hi 1 1 2 2. Rich ds , d. , 9 4 5 '

Anto no ﬂ P . M a . 1 1 1 22 1 1 1 26 10 8 S o , hil g , 9 , 43 9 3 , 5 ; ddy , ‘ ’ d M ner Ph s ikal i bi d 1 1 21 H a n an e t . Zeitsch r. , 9 4 27, 5 ; h i , y ,

M . 1 0 . Rut er o r . a 14 h f d , Phil g , 9 7 , 733 S o Le Ra um 1 10 2 Ro a Inst tution ecture M a 1 ddy, di , 9 , 7, 95 y l i L , y 5,

1912.

Rut er o r . Ma . 1 1 28 20 . h f d , Phil g , 9 4 , 3 m 1 1 1 2 6 H eimann and Marckwald M e. e tsc Le Ra u , 8 ll Gl di h , di , 9 , 5 ; ,

h i sch . 1 1 0 P y s kal . Zeit r , 9 3, 14, 3 } REFERENCE S 147

“7 D e F o rcr n a d o m . t ren . 1 1 1 , C , 152 66. p d 9 , , “8 E bler and en er Z h n eitsc r. a o r B , . C em. 1 1 83 1 . d , g h , 9 3, , 49 ‘ 9 M me. ie o ur m t. ren . 1 12 C , C p , 135 161 1 0 145 22 H o nig . d 9 , , ; 9 7, , 4 ; s c mi Mo n ats h . 1 12 33 2 2 1 1 28 1 h d , , 9 , , 5 ; 9 3, 34, 3, 35 . 7 ° Com a re al s o v o n S ch weidler Ph k l . Z h S I a eitsc r. 1 12 . p , y , 9 , 13, 453 7 1 Ho t o o P M i . a . 1 0 S l w d , h l g , 9 9 o i bzd. 1 0 18 8 6. 5 , 599; y , , 4 dd 9 9 , 7 2 Rut er o r R a io a i i ct t z u d e i i 8 . h f d , d v y , d t o n, 46 7 3 R ut er o r P i M 0 . a . 1 8 f , g , 9 16 00 ; 1 1 28 20 ; G ray h d h l , 3 9 4 , 3 and Ramsa T r ns a . em. S o c . 10 b y , C D e ierne C o m t . h , 73 ; , p

ren . 1 0 d , 9 9, 148 , 1264. 7 “ Ro gs i M 1 . a . 0 1 20 2 a s n , P g , 9 9 7, W t o P ro c . R o . S o c . , 1 0 h l , y 9 9,

g 0 . A , 5 7 5 D eb i ern m n e o t. re 1 10 , C . 150 1 0 . p d , 9 , , 74 7 ° ra nd R ms a a a ro . R . c o S o . 1 1 1 6 G y y , P c 84A . y , 9 , , 53 7 7 ra and R amsa T rans . em. S 1 0 G y y , C o c . 0 95 1 Rut erfo r h , 9 9, , 73 ; h d ,

M . i . a 1 0 2 Ph l g , 9 9 17, 7 3. 7 ° R m a sa nd S P . a o ro c . Ro . o 1 0 y y , S c , , 72A 20 73A 46. dd y 9 3 , 4 , 3 7 ° M le. R ams te t Le R a ium 1 1 1 1 2 B P i M . 1 1 , , , 8, ; o y e . a l d d 9 53 l , h l g , 9

2 . 21, 7 2 9° P . C u rie and D ann C m t r n 8 e o . e . 1 0 . , p d , 9 4, 138 , 74 91 M ak o wer Le R a i um 1 , d , 909, 6, 50 . “2 v n er o c A P i 0 . , nn. y s . 1 6 20 L h h k , 9 , 345 “3 H a n and M i n r B D h . 1 0 e t e er. . sik al . G es h , p y , 9 9, 11, 55 H a n an M i n d e t er Ph s ikal . Zeits ch r. 1 0 6 F a ans i bi d h , y , 9 9, 10, 97 ; j , ,

“ 5 R ut er o r nd Ri n P 2 . a a . 1 1 f c ar so i . M 26 4 h d h d , h l g , 9 3 , 3 9 ° M ar kw l c a d B . m. G 1 2 228 and 2 1 0 er . c e es . 3 , d h , 90 , 35, 5 4 39; 9 3, 6, 6 2 2 6 . “7 M me. urie an D 1 10 86 d b i rne m t r n . . C e e C o . e 150, , p d , 9 , 3 “3 i w c and R 1 1 1 12. C a us se P i . M a . 4 27 h d k ll , h l g , 9 , 9° n H 10 66. o a d m its ano r em. 1 2 e o t a Ze ch r. . 67 K pp l l k p , g Ch , 9 , , 9° H a n Ber . c em. G es . 1 0 1 62. h , . d h , 9 7, 40, 4 “1 S r n m. M ar kw ld B r 10 20 o T a s . e . m. G es . 1 c a , e . c e 43 4 ; y , C d h , 9 , , 3 dd h S o 1 1 1 c . , 9 , 99, 72. “2 H a n h m. Z s h r. 1 1 8 . C e ei t c 1 , 34, 4 h , , 9 5 “3 n h H a P s ikal Zeitsch r. 1 0 8 2 6. h , y . , 9 , 9, 4 9‘ C r ns n P i M a . 1 1 12. a to , h l . g , 9 3 25, 7 95 0 H ah n Le Ra i u m h M a . 1 1 1 1 Rus s e and S o y P i . 21, 3 ; , , ll dd , l g , 9 d

9“ i 61 H ah n B er 1 0 1 R m . s . 1 0 ii . 3 a sa Ch im. hy , 7 ; , , 9 , 8, 337 y , J p , 9 5 5 n 1 0 40 1 62 0 a r . R adio akti v itat 1 0 2 2 E s ter a d 9 7, , 4 , 33 4 ; J h , 9 5, , 33 l

l Ph ik 1 anc i bi d. 620 Atti . R . G ei te s al . Zei tsch r. 06 7 B , ; , y , 9 , , 445; l ,

Acca . incei 1 0 16 i . 2 1. d L , 9 7 , 9 97 82 h i em. 1 0 6 . P s kal . S chlundt and M o o re . Ch , 9 5, 9, , J y “3 no r em. 1 0 61 8 63 S trOmh o lm and S e b erg Zeitsch r. a . C , 9 9, , 33 ; , v d , g h

197 . 9’ M t r n . 1 1 1 28 . i o m . e 5 e. es e 1 3 ll L l , C p d , 9 , , 3 1 0° 621 . P i . M . 1 1 1 22 R utherfo rd and Geiger h l ag , 9 , , 0 ” 1 10 ei er P h 1l. ei er and M ars en Ph s ikal . Zeitsch r. 1 11 7 ; G g , G g d , y , 9 , ,

M a . 1 1 1 20 1 . g , 9 22, 10° 6 . 1 1 1 22 2 . M o se ey and Fa ans P i . M a , 9 , 9 l j , h l g 103 i i o n 1 . R ut erfo r Radl o activ it z u d e t , 35 h d , y , d

628 . t r P i . M a . 1 0 9 M i s s S la e , h l g , 9 5 ,

M a 1 06 11 . n P il . . H ah , h g , 9 , 793

s ikal . G es . 1 0 11 . er. . h H ah n and M eitner, B d p y , 9 9, , 55 0 107 M ars n and D arwm S o c . 1 12 Barratt P ro c . P y sica , , , h l , 9 de

1 12 A 1 H a n and M ei tner Ph 51kal . P ro c. Ro . S o c . 87 7 , y y , 9 , , ; h

h r. 1 12 0 . Zeitsc , 9 , 13, 39 148 TH E CH EMI S TR Y OF TH E RA DI O-ELE MENTS

v o n er nd v o n ar ber z un . A c a t S it sber . i i n a . ss . e L h W g, g K k d W W , 1 575. 1“ v o n erc i bi d 1 0 116 [ ° L h , , 9 7, 443 "0 B , o mer . 1 1 ltwo od A . 08 25 26 , J 9 ) , 9 111 M uri L Ra 1um 1 1 1 . me. e e 5 C , ’ d 9 353 112 h n . 1 0 - m G es . 160 Ph Ik l Z I er. c e h 06 H a , B d h . , 9 6, 39, 5 YS a C tS C r i 19 , 8 7, 55 1 13 s i i M a 1 0 V 1 G o dlew k Ph l g 9 5 ( ), 10 44~ 111 6 Ru s d r . 2 R um 1 s B ruh at C m t. en 148 6 Le a 0 6 , p d , 9 di , 9 9, , 7 ; ,

i . M . 1 0 vi . Ph l ag 9 9 ( 17, "5 ino sh i ta i . M a . 1 08 16 121. K , Ph l g , 9 , m . m G es . 1 0 2. ese Ber . c e . 36 Gi l , d h , 9 3, , 34 "7 M ars en and i so n ature 1 1 92 2 . d W l , N , 93, , 9 “ 9 and M eitner Ph s ikal . Zei tsch r. 1 08 6 H ahn , y , 9 , 9, 49 1 " d H m T rans . h em. c . 1 1 1 0 2. S o ddy an y an, C S o , 9 4, 105, 4 120 2 m . m. S . 1 1 1 o t R i ch ar s and emb ert . Amer. e oc 4, 36, 3 9; C d L , J Ch , 9 p 8 . 1 1 2 rend , 9 4, 159, 4 121 n 1 Cu rie C o m . re . 1 M aurice , p d , 9 4, 1“ i mid and M e. H o ro witz i bi d 1 6 H o ngsch t ll , , 79 28 m m be and oo P ro c . amb . P i 8 00 1 0 14 1 a b e C a p ll W d , C h l , 9 7 . 5 ; C p ll

2 1 1 1 0 1 1. ; 9 9, 15

1 5c> TH E CH EMIS TR Y OF TH E RADI O-ELEMENTS

f in ra io ‘ H EL U M roductio n o , Pi tCh b lende 2 71 8 I I , p d r 31 5 1 541 1 51 941 332 act e c an e 1 10 1 iv h g , 7, 9 , r ra io acti e de H y ro gen e o xi e, at noc ani es 1 6 d p d d v Pl i y d , , co m o siti o n o i 1 eoc ro c a o s p , 5 Pl h i h l , 9 o o ni um — 11 1 1 2 2 P l , 109 1 , 3, 4 , 9, 4, 3 , 6 10 4 5» 491 59) 41 751 7 - ements us e — IN D I CATOR , rad o el d Po tas sium 14 1 2 S i , 2 43,

as 62 r b m s m scure — , P o le s , o e o b , 65 69 Internat o na ra i um stan ar th e t ﬁ i io n f i l d d d , , P uri y, de nt o , 44 2 P r m r ite 88 9 y o o ph , Inte enetrat o n o f ato ms rp i , 9 Io i es ra oact e ec o m o si tio n d d , di iv d p - o f 16 U ANTITIE o f radio elements de , Q S tectab le 16 20 Io n s in actio n o f ra s 1 6 8 1 , , i g y , , , , 5 — 28 1 2 Io nium, 84 87, , 5 , 5 , 75, 1 1 5 R D O ana si s o f 6 A IATI NS, ly ,

I so to es 1 6. p , 5 5 Ra io -actini um — 2 d , 134 135, 7

Ra i o acti e c an e — 6 d v h g , 4 1 co nstant, 8 L D ato mic e t o f 140- 141 EA , w gh , — i e ui b rium 23 26 q li , ea use o f, fo r ns truments 107 L d , i , R a i oact t is co er o f 1 d ivi y , d v y , i s nte ratio nt eo r o f 128 d i g h y , 5, ” - i r ﬁn i Radio chem cally pu e, de it o n M O H O U M — 118 1 ES T RI , 3, 3 , 49, Of, 44 - m nt nd ri i l w Radio ele e s a pe o d c a , 2, 2 - 12 1 60 , 118 120, , 3 , — 10, 34, 47 56 Min r s s ar i n o f ra io -ele e al , ep at o d w — ne , 3, 45 52 nt ro m — me s f , 53 56, 75 - s R Radio l ead , ee adium D test n o f ra io acti e - 6 i g d v , 94 9 Ra io t o ri um - 1 2 0 d h , 120 21, 3 , 33, 4 , M o naz ite 86 1 1 1 1 , , 112, 7, 4

i n l adhu . 87

N W io - e e 1 — E rad el m nts , , 1, 45 52 3 ’ i e o it R - act e s ( a A, B , C , C , Nl to n 100 v d p , C 102" 106 1 2 2 2 l o menc ature o f ra o-e ements 1 2)1 1 , 51 9) 3 i N l di l , 3 - 6 No n-se arab e e ements 1 1 — 56 5 p l l , , 46 uc eus ato mic 10 1 N l , . , 5

— 2 E , 108 109, 1

L Y o res 2 6 0 emanation 99— 102 6 O AR , 7 , 7 , 9 , , 3 , 37, 53 Oran eite 1 12 F see P o o ni um g , , l

ni o n r ia i ns R n f a-ra s 8 — Oxidatio n of tr ge b y ad t o , a ge o y , , 27 29 r i 2 s reco il part c les , 4 Ratio o f radium and uranium in m nera s 2 8 i l , 4, 7 P G o er o f ra s 1 ENETRATIN p w y , 7, 3 R ec i met o s o l h d , 41, 44 Peri o o f a era e e 26— 2 g f , 18, 9, d v li 33 - R u i i um ° 2 b d , 142 143, - n 1 half c ha ge, 8 eri o ic l aw and ra o-e ements 2 P d di l , , - i s 10 S C ER NG o f a art c e , 9 , 34a ATT I p l oto ra ic actio no f ra s 6 S i ca acti o n of ra s o n 16 Ph g ph y , il , y , 1 51

S o ub i t o f ra ium emanatio n 102 U ranium X and X 9- 82 12 2 l li y d , 1 2, 7 , , 5,

S er ca ab s o r tio no f — ra s 26 2 0 1 6 8 ph i l p y y , 93 a 3 1 4 1 471 5 1 57: 51 771 7 S tan ar in i ium 2 Y - d d , ternat o nal rad , 9 , 82 84 S tan ar s - 2 2 0 8 d d , a ray , 9 9 5 1 3

S tru — 1 cture o f th e ato m, 9 11, 5

V LOC Y o f a - E IT rays , 8 T H O 2 6 112 1 0 -ray s 1 1— 12 RIANITE, 7 , 7 , , 4 ﬂ , T o V o ati i satio nmet o s 0 h rite, 1 12 l l h d , 4 e o n 86 1 1 1 0 V o ati i t effect o f atmo s er n 1 C yl , 54, , 7, 4 l l y , ph eo , 4

T o ri um — 2 1 28 6 V o ume o f ra ium emanatio n 2 8 h , 112 116, , 9, , 3 , 37, l d , 4, 3 , 2 100 SI , 5 ’ acti v e e sit Th -A o ( , B , C C , d p ,

and D 12 — 0 26 2 0 ) , 6 13 , , 9, 4 , 59,

- 61 6 R ra i oac tiv e eco m o sitio n 5 WATE , d d p emanati on 124- 125 6 o f 1 8 , , 4, 3 , 37 , 4, 3 X 122— 124 2 “ i emi , , 4, 3 , S W ll te, 6

U L M r ucts — - E o , 140 141 1 1 1 1 X RAY s ectra 10 1 TI AT p d , 3 , p , , 3 U rani um - 1 1 28 , 70 79, , 9, and t o rium b e a io ur f h , h v o mix tures o f 8 1 126 1 , Z NC su i e 6 , 33 I lph d , ,