LXXXI.-Connectio~~ Between the C~~Ystalloyr~Z~Hicul Clzarcicters of Isomolpjhous Salts Ccnd the Atomic Weight of the Metals Contuined - DocsLib

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846 TUTTON : CONPARhTIVE CItYSThLLOGRhP€I1CAL STUDY Oh'

LXXXI.-Connectio~~ between the C~~ystalloyr~z~hicul Clzarcicters of IsomolpJhous Salts ccnd the Atomic Weight of the Metals contuined. A Comjmratitv C~~ystallogrc~~hicclStudy of the Nomzal Selenutes of Potassium, Rubidium, and Casium. By ALFREDEDWIN TUTTON, Assoc. R.C.S. INthis communication are presented the results of a detailed investigation of the morphological and physical characters of the normal selenates of potassium, rubidium, and czsium, of an identical, and therefore strictly comparable, nature to that of the analogous sulphates of the same three alkali metals, the results of which were laid before the Society in the year 1894 (Trans., 1894,65, 628). Only the more easily procurable of these selenates, the potassium salt, appears to have been hitherto subjected to a crystallographical study, beyond the cursory inspection which revealed the fact that they were isomorphous with the corresponding sulphates. Potassium selenate was included by Topsoe and Christiansen in their well-known investigation published twenty-three years ago (Ann. Chim.phyls., 1874, [ v], 1, 1). The densities of all three salts were determined by Pettersson in 1872-1876 (A'ova Actcc B, Xoc. Upsaka, [iii], 1873 and 1876), with small quantities of material. The investigation of the selenates has been attended by far greater difficulties than beset that of the sulphates, on account of the fact that they are so much more soluble in water than the latter salts as Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. to be in no small degree hygroscopic. At the very outset of the investigation, the progression from potassium to cmium was unmistakably manifested with regard to this property ; for while the deliquescence of the potassium salt is only suficiently rapid to prove inconvenient, that of the rubidium salt is so much more marked, that it was rarely possible to complete the goniometrical measurement under ordinary conditions, exposed to the atmosphere, with one and the same crystal, the rare occasions being on remarkably dry days; and the rapidity of deliquescence of the c&um salt is 80 extreme that a crystal usually becomes a dropof solution in a few minutes after its exposure to the air on ordinary moist days. The manner in which this difficulty has been overcome mill be described at a later stage. The potassium selenate employed in this investigation was supplied specially for the purpose in a high state of purity by Merck. It was subsequently repeatedly fractionally crystnllised, and the crystals found to bo free from any appreciable traces of impurity. The View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CACSIUM. 847

rubidium and cssium salts were prepared by the author from the pure carbonates of the metals and pure selenic acid, all of which were also specially supplied to the author by Jlerck. The carbonates were dissolved in water and the solutions decomposed by the correct quantity of selenic acid. The solutions of the selenates were then allowed to crystallise, and the crystals obtained several times fractionally re- crystallised. The crystals thus obtained proved, as will be subsequently shown, to be pure. Crystallisation of solutions of the selenates is almost impossible in the open air in ordinary moist conditions of the atmosphere ;the potassium salt, being least soluble, of course shows the greatest tendency to crystallise, but concentrated solutions of the ccesium salt may remain exposed for weeks and months without depositing crystals. Indeed, saturated solutions of both rubidium and cmium selenates appear to attract moisture from the air. Hence, the crystallisation of the salts was effected over oil of vitriol under reduced pressure. Half a dozen large receivers were independently connected with the same double- barrelled air-pump, so that several clearly labelled crystallising dishes containing solution could be placed over vitriol in each, only crystals of any one salt being grown in the same receiver, to avoid all chance of admixture. The receivers stood on ground glass plates, and connection with the air pump and manometer was made by suitable glass connections provided with stopcocks. Thus all use of metal mas avoided, and as the whole arrangement stood on a separate table covered with thick non-conducting baize, all chance of sudden thermal disturbance of the crystallising solutions was avoided. The pressure was only reduced sufficiently to permit of slow crystallisation, in order to Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. obtain the most satisfactory crystals. The author desires to express his thanks to the Research Fund Committee of the Society for their grant to defray the cost ol the large quantities of the expensive chemicals employed in the investi- ga tion, Gyavimeti.ic uncl Xpectyoscopic Ancclyses. Each of the three selenates was gravimetrically and spectroscopically analysed, as has been the author’s custom throughout the whole of these investigations, in order to afford the highest guarantee of their purity. The gravimetric analysis consisted in estimations of the content of selenium. The method found most convenient in the cases of potassium and rubidium selenates was the reduction of tbe selenates to selenites by prolonged boiling with hydrochloric acid, and subsequent precipitation of the selenium from the selenites by means of sulphur dioxide gas. The clear characteristic crystals selected from one of the crops 3L2 View Article Online

848 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

used in the goniometrical and optical work were in each case finely powdered in an agate mortar, and the powder rendered perfectly anhydrous by heating in a current of dried air to 150" for some hours, employing the apparatus, and taking the extreme precautions to prevent contact with the moist air, which will subsequently be more fully described in connection with the density determinations. The weighed quantity of salt was dissolved in distilled water, and boiled with about an equal quantity of pure hydrochloric acid, in a flask fitted with an upright condensing tube, to prevent loss by spirting and retard loss by evaporation, for a couple of hours, when all trace of liberated chlorine had disappeared. The contents of the flask were then transferred with washings to a large three-bulbed U absorption tube, which was connected on one side with a large flask containing a concentrated solution of sodium sulphite into which sulphuric acid could be allowed to fall from a dropping funnel, and on the other with an empty flask fitted with doubly perforated stopper, through whose second hole was inserted a long tube to lead excess of the sulphur dioxide into the flue of the large fume cupboard in which the operation was performed, Sulphur dioxide was then liberated from the generating flask by running in the vitriol and applying heat, and a stream of such rapidity was maintained that a few bubbles escaped absorption every minute. Towards the end of the precipitation, when the red selenium had coagulated to a mass in the lowest bulb on the bend of the U-tube and the liquid began to clear, the absorption apparatus was gradually warmed to about 60", the flame was then altogether removed from the generator, and the reaction was left to complete itself for another couple of hours, the contents of the bulbs being surrounded on each side by a flask filled with the reducing gas. The precipitate Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. was subsequently collected on a weighed filter, thoroughly washed with hot distilled water, dried at SO", and weighed. This method, which proved eminently satisfactory in the cases o€ the potassium and rubidium salts, was found to be inapplicable to cmium selenate. The much greater stability of the caesium salt is strikingly indicated by the fact that, in the first place, boiling with hydrochloric acid appears incapable of reducing it to selenite, there being little or no indication of liberated chlorine, and, further, barely a trace of sele. nium is precipitated by sulphur dioxide. Under these circumstances, a method was tried, and found to answer admirably, which has recently been described by Pierce (Zed. ccnorg. Chem., 1896, 12, 409). It consists in adding to the highly diluted hydrochloric acid solution a considerable excess above the calculated quantity of potassium iodide solution, and boiling for upwards of half an hour until all free iodine is expelled. The precipitated selenium, in the state of black powder, is then transferred to a weighed filter, dried at loo", and weighed. View Article Online

NORMAL SELENATES OF POTASSITJM, RUBIDIUM, Ah’D CfiSTUM. 849

The analytical results obtained mere as follows : Potccssium SeZenccte.-l*O453 gram K,SeO, gave 0.3732 gram Se, corresponding to 35.61 per cent. The calculated percentage of Se in K,SeO, is 35.69. Rubidium 8eZencite.-l.4494 gram Rb2St.0, gave 0.3686 gram Se, corresponding to 25.43 per cent. The calculated percentage of Se in Rb,SeO, is 25.16. Cmsiuna SeZenccte.-l-6667 gram Cr;.,SeO, gave 0,3235 gram Se, corresponding to 19.41 per cent. The calculated percentage of Se in Cls2Se0, is 19.30. The above numbers thus indicate that the material of the three selenates which was employed in this investigation mas analytically pure. The rubidium and casium salts were each further spectroscopically tested for traces of the other two salts, in the same manner as in the case of the sulphates (vide that memoir, p. 631). No traces of the ciesium lines mere detected in the spectrum of the rubidium salt, and, likewise, no evidence of rubidium in the casium salt was afforded, although very brilliant spectra were obtained. Moreover, neither salt exhibited potassium lines, while, in the case of the rubidium salt, the two red rubidium lines mere most brilliant; the position which the red potassium line should occupy had previously been well located by the same excellent method as is described in the previous memoir concerning the sulpbates on the page just quoted. If potassium had been present in traces, its red line mould have been at least equal in intensity to the red rubidium line, as has been shown by Erdmann (ref. given, Zoc. cit.). Hence there can be no question as to the purity of the material of Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. the salts employed in this investigation.

Beteyminations of Solu bizit?. The solubility of the three selenates in water mas determined with great care with relatively large quantities of the salts. Strong solutions were first concentrated over oil of vitriol under reduced pressure until crystals commenced to be deposited. The vessels containing the solutions were then transferred to the interior of an empty bell-jar standing simply on a glass plate, and allowed to stand for several days at the ordinary atmospheric pressure, in order that, under these normal conditions, equilibrium between solution and deposited crystal might occur, any excess of the latter being re-dissolved or supersaturation of the former discharged by further deposition. Suitable quantities mere then transferred to weighed vessels, the temperntiire being noted weighing bottles of considerable size were used in the cases of the View Article Online

850 TUTTON : COMPARATIVE CRYSTALLOGRAPHTCAL STUDY OF

potassium and rubidium salts and a large platinum crucible in the case of czesium selennte. The three vessels and their contents were then closed and weighed to ascertain the amount of solution taken in each case, and subsequently opened and placed under separate vacuum receivers containing dishes of vitriol, and, after exhaustion of the receivers, allowed to evaporate in V(ICUOas far as such a process could be carried. It was found possible to evaporate thus to complete dryness in a few days in the cases of the potassium and rubidium salts, but after a month a notable quantity of the solution of the much more soluble cmiurn selenate still remained about the large crop of crystals of that salt. The last traces of water were removed from the deposited potassium and rubidium selenates by heating the weighing bottles to 150° for a whole day in an air bath, a slow current of well-dried air being aspirated through the bottles the whole time by means of a water pump under such conditions that the pressure within the bottles was considerably less than atmospheric. Towards the end of the operation, the temperature was raised to 180'. In the case of the cEsium salt, the platinum crucible, with its contents covered with a small and low inverted glass funnel, was placed in a glass vessel blown of suitable size at the neck to receive it, which was then fitted with a luted stopper perforated by the two tubes for the delivery and exit of the current of dried air. The temperature of the crucible was then cautiously slightly raised, the current of air under reduced pressure being maintained, and the operation continued, with care to avoid projection of the liquid, until dryness was atltained; any slight amount projected was retained by the inverted funnel. After apparent dryness had been attained, the temperature was raised, eventually to 200O. The platinum crucible was subsequently removed, heated to redness, Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. transferred to a desiccator, and weighed when cold, the weight being again taken after a second heating. The minute quantity of salt projected upon the funnel was of course determined, and added to the content of the crucible. The results obtained were as follows : Potassium ,YeZenate.-1 2.049 2 grams of a solution saturated at 12O contained 6.4482 grams of K2Se0,. Hence 100 grams of water at 12" dissolve 115-0 grams of potassium selenate. Rubidium SeZenate.-8*6805 grams of a solution saturated at 12O contained 5.3368 grams of Rb2Se0,. Hence 100 grilms of water at 12O dissolve 158 9 grams of rubidium selenate. Cccsiuna SeZenate.-9.6226 grams of a solution saturated at 12" contained 6.8321 grams Cs,SeO,. Hence 100 grams of water at 12O dissolve 244.8 grains of czsium selenate. Data concerning the density of the heavy solutions produced by dissolving considerable quantities of these salts in water were subse- View Article Online

NORMAT, SELENATES OF POTASSIUM, RUBIDIUM, AND C'IESIIJM 8.5 I

quently obtained in connection with the determinations of the refraction constants of the dissolved salts. The details mill be given under that head, but the final results are tabulated here, as of most interest in the present connection.

Densities of Solutions of the three Xelenates at 20°, conapcirecc! with Tt'atei. nt 4".

Salt. C once 11 t rnt i o 11. Density. K,SeO, ...... 35.76 1.3591 ,, ...... 41.79 1.4385 ,, ...... 50.00 1.5590 Rb,SeO, ...... 40.60 1.4685 ,, ...... 47.07 1.5806 Cs,SeO, ...... 45.94 1.5541 ,, ...... 53.43 1.7432

The following is the most interesting conclusion to be drawn from the determinations of solubility. TJLe normal selenates of potassium, rubidium, and ccesium dissolve to a progressire extent in zunter, 100 grams of tJLat liquid at 12' dissolving 115 grams of potassium selenate, 150 grams of rubidium selennte, and no less tJLan 245 granzs of the ccesium salt. 5"Jbe progression is thus in tJLe order of the atomic weigJLts of t?Le alkali metals respectively contained, and becomes more rapid as the atontic weiglht rises. This conclusion is parallel to that arrived at as regards the sulphates, whose solubilities in 100 c. c. of water at the ordinary temperature were found to be: for K2S0,, 10 grams; for Rb,SO,, 44 grams; and forCs,SO,,

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. 163 grams. The much greater solubility of the selenates is clearly evident from these numbers.

G 0 N I0bl ET R I C A L 1 N V E S T I G AT I0N. The normal selenates of potassium, rubidium, and cssium crystallise, like the sulphates, with which they are closely though not absolutely isomorphous, in the rhombic system. The closeness of the isomorphism with the sulphates will be apparent from a comparison of the angular values, subsequently given, with those of the sulphates given in the former memoir, the differences being only a very few minutes. The order of the differences is indeed similar to that between the three members of either group. The same planes are common to all, more or less differently developed, however, upon the different salts. Hence the same spherical projection as was given for the sulphates is applicable to the selenates, tilt: vwy slight angular View Article Online

852 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAT,, STUDY OF

differences between the various members of either group being inappre- ciable in a spherical projection drawn upon this scale. It is given in Fig. 1. Two more forms appear on this spherical projection than were shown by the sulphates, namely m = {lO2>&-Fm, and s = {l32)+63. These two new forms have been observed upon potassium selenate. The pseudo-hexagonal nature of the symmetry is a characteristic of the selenates equally with the sulphates, the angles ap, pp’, and p’b being approximately, and as regards pp’ exactly, 30’.

FIG.1. Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11.

The goniometrical measurements have been carried out precisely as for the sulphates. The difficulty introduced by the deliquescence will be discnssed in connection with the particular salts.

Pota.ssium Selenate, K,SeO,. The gilezltest of all the difficulties met with in connection with this investigation has been to obtain suitable crystals of the potassium salt for the goniometrical and optical work. It is almost invariably deposited in large groups of much striated and distorted crystals, most View Article Online

KORMAT, SELENATES OF POTASSIUM, RUBIDIUM, AND CBSITTM. 853 frequently irregular triplets, and it is only very rarely indeed that small single individuals, with plane faces of the required high degree of perfection, are obtained, either in vessels exposed to the air at the ordinary pressure or over oil of vitriol in vacuo. Upwards of a hundred different crops have been grown under every conceivable modification of conditions, and it has only been by these repeated attempts during twelve months that an adequate number of such desirable crystals have been obtained for the purposes of this work. Twenty such crystals were employed in the goniometrical measurements, which were carried out on particularly dry days, when the deliquescence of the salt proved to be sufficiently slow to permit of the measurements with each crystal being completed. The very large number of crystals measured were employed because of the unusual simplicity, that is, paucity of faces, of the perfect single individuals which alone were suitable for measurement. Hence the completion of the work with each occupied much less time than in the cases of the more complex crystals of the rubidium and czsium salts, and the deliquescence of this salt being the least rapid of all three, there was greater chance of completing the measurements with each crystal on a clear day when a northerly or north-easterly wind was blowing. In only a few cases was the Fuess desiccating chamber used on the crystal stage of the goniometer during the actual measurements, but this accessory proved very useful to enable a prolonged preliminary scrut- iny of the crystal to be made in order to become familiar with the disposition of the faces. The results of the observations, measurements, and calculations are given below and in the accompanying table. No angular values were accepted which involved any ambiguous signal images. Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. The ratio of the nzorphological axes, calculated from the goniometrical measurements, is : a :b : c = 0.5731 : 1 : 0.7319. Thefornzs o6sewed were the following : a= {lOO)m Po0 ; b = (010)Cm i;m ; c = (001)oP; p= {llO}m P; q = {Oll)Pm;_g'= {OZl)ZPm ; nz= {zoa)gPm; o= (111)P; o'= { 112)iP; s = { 132)$P3. The perfectly developed plane-faced small individual crystals, which are alone suitable for the trustworthy measurement of the morphological angles, and which are so exceedingly difficult to obtain, are unfortunately of a most simple character. They exhibit, as a rule, no other forms than the brachypinacoid 6{010), parallei to which they are usually tabular, the primary prism, p{110), and the two brachydomes q(O11) and ~'(021). Numerous beautifully perfect crystals of this type have been examined, of a size varying from one to three View Article Online

854 TUTTON : COMPARATIVE CRYSTAT,LOGRAPI-IICAL STUDY OF

millimetres in their longest dimensions, but only on one such crystal were minute faces of the pyramidal forms 0{111)and o’(112) observed sufficiently perfect in character to afford trustworthy reflections. Occasionally, a single crystal of this nature shows a face of the very unusual (as regards this series of salts) macrodome m(103); the re- flectipns afforded by such macrodome faces observed were excellent. The chief characteristic of these perfect individual crystals is the relatively large development of the brschypinacoid bf010.) The basal- plane c{001), and the macropinacoid cc{lOO), are only represented by narrow strips or lines, if present at all, as shown in Fig. 2. Fig. 3 shows a crystal with the rarer faces developed.

FIG. 2. FIG. 3.

The crystal depmits of this salt usually obtained consist of large, distorted, and greatly striated prismatic growths, either of a radiating or intergrown character, quite useless for goniometrical or optical purposes, particularly for a purpose such as that of the present investigation, where a question of a few minutes of arc is involved. The crude prisms shorn usually large but distorted faces of the primary Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. prism, and very much striated and indented brachypinacoid faces, tailing off with curvature or vicinal formations into the successive brachydomal end forms. The striations are parallel to the edge 62. Moreover, repeated twinning in triplets is largely characteristic, and such irregular triplets frequently grow to very large size. Their nature has been carefully studied, and they prove to be similar in character to the well-known pseudo-hexagonal triplets of potassium sulphate. Two characteristic triplets are represented in Figs. 4 and 5. The twin plane of the form shown in Fig. 4 is almost perpendicular to the 21 faces, as in the case of potassium sulphate, and the resultant triplet resembles a hexagonal prism with pyramidal termi nations. The faces of the prism are all p faces, and the end forms o and 0’. But owing to the angle pap between the p faces of each of the three crystals being 59” 38’ instead of exactly 60°, the faces of the triplet are not truly plane, each face being made up of parts of y faces of two individual twinned crystals. Whenever a well- View Article Online

NORMAT, SELENATES OF POTASSIUM, RUBIDIUM, AND CESIUM. 855

formed triplet less striated than usual is examined, the dividing edge of the two parts is distinctly visible. Usually, however, the triplets are so deeply striated and distorted that they may easily be mistaken for much striated, truly hexagonal prisms. The type shown in Fig. 5 is very characteristic of numerous crops. It may be considered as a triplet on the same twin plane as the last, but with the interpenetra- tion of the three individuals less complete, none of them passing through the structure far enough to be continued on the opposite side. It exhibits the faces of the brachypinacoid b and of the brachydomes q and p‘ in re-entrant angles, and these faces are usually very much serrated parallel to the edge bq. This form of triplet may also be regarded as twinned parallel to the p faces ; for if two individuals be laid parallel, with a p face of the first in contact with the 11 face of the second, and if rotation be then effected round the normal

FIG. 4. FIG. 5.

to this plane as axis, and if next a third individual be brought similarly in contact with the adjacent p face of the first and finally Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. rotated, in the same manner as the second, a triplet of precisely this character would be produced. Usually there is more or less inter- penetration, the b faces in the re-entrant angles being narrower than the p faces. In addition to the end forms wz(102), o(lll), 0’{112), and in the re- entrant angles p(O11) and q’{021}, a new pyramidal form, ~(1321,was observed; it was located as being situated at the intersection of the two zones [mo’b] and [psq], but the reflections were too distorted to afford trustworthy measurements. The angles marked with an asterisk were the two most frequently and satisfactorily determined, which were consequently employed as the two basal angles required in order to calculate all the rest. The relative simplicity of perfect individual crystals of this salt, compared with that of the analogous rubidium and czesium salts, is strikingly shown by the smaller number of angles of which it was found possible to obtain trustworthy measurements. View Article Online

8.56 TUTTON : COMPARATIVE CRPSTALLOGRAPHICAL STUDY OF

MorlAotogiccd Anggles of Potnssiunz Selenate, K,SeO,. - I I No. of Me an C a1CII Dif- Angle. mexsnre- Linii t s. - ments. I observed. lated. ference.

_____ ~-

C6p = 100:110 29” 46‘ 29” 49’ 1’ fpp’ = 110: 130 - 30 0 p’b = 130:OlO - 30 *ll pb = 110:010 60 11 pp = 11O:llO 59 26-59 45 59 36 59 3s 2

cq‘” =‘001 : 012 , - 20 6 p’l‘q = 012 : 011 i - - 16 6 cq = 001 :011 1 36 13-3G IS 36 16 36 12 4 011 : 021 19 20-19 33 19 26 19 25 2 :$I’ 1 021 :031 - 9 51 q”b = 031 : 010 -_ 24 *29 0’6 = 021:OlO 34 10-34 32 34 20 p6 = 011:OlO 53 37-53 52 53 45 53 45 2 = 100: 111 - 44 9 {:; {:; = 111:Oll - 45 51 clot = 100 : 112 - 59 3 { O’f = 112 : 012 - 30 57 G5 42-65 46 65 44 65 43 1 - 48 34 bo’ = 010: 112 - 72 51 0‘111. = 112 : 102 - 17 9 co’ = O01:112 36 25 36 21 4 0’0 = 112:111 -- 19 26 19 28 2 co = 001 :111 55 46-55 52 55 49 55 49 0 op = 111:llO 34 8-34 14 34 11 34 11 0 Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. -- 65 17 - - 48 57 65 38-65 56 65 48 65 46 2 - 72 34 - - 34 31 72 46-72 58 72 52 72 55 3 - 44 14 - 45 33 olq’ = 1x2 : 051 - 45 46 2%‘ = 021 :iso - 44 27 pro’ = 130 : 112 - 59 6 = ll2:ogl - 61 36 qp’ = 011 :130 - 59 18 c7n = 001:102 32 26-32 42 32 31 32 34 3

Total number of measurements, 270. View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CiESIUM. 857

12ubicliunz Se Zenate, Rb,SeO,. Rubidium selenate, unlike the potassium salt, is readily obtained in excellent single crystals, frequently of large size, by the evaporation of concentrated solutions over oil of vitriol under reduced pressure. In dry weather, it is also possibIe to obtain them by evaporation of such solutions in the open air, but in a moist atmosphere crystallisation rarely occurs. Twelve small crystals of the most perfect character were selected from several distinct crops and employed in the goniometrical measurements. Particularly dry weather mas utilised for the purpose, and it was found possible, in most cases, to complete the measurements before the deleterious effects of deliquescence upon the brilliancy of the signal images became apparent. The desiccating chamber of the goniometer was used mainly for the same purpose as in the case of the potassium salt, that of preliminary examination. All angular values involved in the slightest doubt, owing to ambiguity of the reflections from the crystal faces concerned, were discarded.

FIG. 6.

The following are the results of the observations, measurements, and Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. calculations of the morphological constants. The ratio of the morpJhoZogical axes, calculated from the measured angles, is : a : 6 : c : = 0.5708 : 1 : 0,7386. Theforms observed include all those given on the spherical projection except o”(332). They are : a=(100)mkm; ~={oIo)“aim; c=(001)o~; p,;{110)m.~’; p’=(130)mP3; p=(Oll)Po~;g’=(O21}2?~; g = (031)3Fo0; p”’ = (01 2 ] ; 0 = { 11 1 >P ; 0’ = { 1 12) +P. The most typical habit of rubidium selenate crystals is represented in Fig, 6. It is somewhat similar to the form of rubidium sulphate exhibited in Fig. 6 (p. 639) of the sulphate memoir. It is characterised by the predominance of the brachydome q’(021), and the relative unimportance of the basal plane ~(001)and the brachypinacoid View Article Online

858 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

b(010). The macropinacoid a(100) is the most important of the three primary rectangular rhombic forms, and is almost invariably a prominent face. The primary prism p(ll0) is also very prominent, much more so than the prism p’(130), which is very frequently absent altogether. The primary brachydome p(O11) is another well represented form, although not usually nearly so extensively developed as g’. The primary pyramid o{ 11 1) is usually well developed, while the pyramid o’(112) is generally smaller when present at all. The brachydomal form p”(031) was frequently observed on the crystals of this salt, and afforded excellent reflections. The other, less common, brachydome, g”’(O1a}, was less frequently discovered, and was then only very narrow, yielding poor images in the zone [cpb] but satisfactory ones in thezone [a0’4’7. Occasionally, cry st nls were found with broad brachypina- coidal b faces.

iMocl*phoZogicccZ Angles of Rubidium Selenate, Rb2Se0,.

No. of Mean Calcu- Dif- Angle. measure- Limits. lated. ference. ments. observed. ___ np = 100:110 40 29” 35’--29” 53’ 29” 43’ 29” 43‘ 0’ 8 29 52-30 7 29 58 30 0 2 p‘b = 130:OlO 9 30 11 -30 23 30 17 30 17 0 pb = 11O:OlO 32 60 11-60 25 60 17 60 17 0 cp = 001:011 16 36 29 36 27 2 qp‘ = 011 :021 12 19 16--19 38 19 26 19 27 1 q’f = 021 :031 6 9 29- 9 59 9 47 9 49 2

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. p”b = 031 : 010 8 24 8-21 28 24 16 24 17 1 iplb = 021:OlO 13 34 0-34 15 34 4 34 6 2 = 100 : 111 51 43 41-43 59 43 51 3t = 111 :011 59 45 56-46 21 46 9 46 9 0 - 100:112 5 58 39-58 55 58 48 58 45 3 112 : 012 3 31 11--21 18 31 16 31 15 1 bo = 010:l~l 39 65 32-65 57 65 43 65 42 1 {oo = 111:lll 17 48 25-488 48 48 34 48 36 2 bo’ = (110 : 112 3 72 42-72 59 72 49 72 47 2 o’o’ = 112 : 1i2 1 - 34 21 34 26 5

GO’ = 001 :112 4 36 34-36 48 36 39 36 41 2 0’0 = 112:111 10 19 20-19 38 19 27 19 27 0 co = 001 :111 37 55 59-56 19 56 8 56+ 8 0 iop = 111:llO 65 33 42-34 8 33 52 7 64 51-65 8 64 58 65 1 3 6 49 8-49 20 49 16 49 13 3 plp = 021 :110 11 65 38-65 54 65 47 65 46 1 View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 859

MotyAo log iccc I Angles of Rubidium Se lenate, Rb,SeO,-cmt inued. ~- No. of Angle. Calcu- Dif- lated. ference.

- ___ -- PO' = 110:1i2 1 - 1 72 22 72 19 3 O'P. = 1i2:oii 1 - I 34 50 34 49 1 gp = oil :iio 6 72 45-72 50 72 48 72 52 4 $0 = 130:111 1 43 59 44 1 2 00' = 111 :112 1 45 42 45 39 3 0;~;= 1i2 :o(~i 1 46 0 45 59 1 dp = 0% :130 1 44 22 44 21 1 p'o' = 130 :112 1 - 58 54 58 51 3 0'9 = 112:011 2 61 58-62 0 61 59 621, 2 qp = 011: 130 1 - 59 11 598! 3 pp" = 110 : 031 -8 63 0-63 17 63 10 6381 2 Total number of measurements, 486. It will be seen that the calculated and observed angles agree most mtisfactorily. The angles marked with an asterisk, being considered the two determined with the closest approximation to certainty, were employed as basal angles for the purposes of calculation. The great majority of the measured angles lay much closer to the mean values than is indicated by the limiting values, which latter were usually more or less outlying values.

Cmsiwm Selenate, Cs,SeO,.

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. Owing to the extreme solubility of cEesium selenate, solutions require to be evaporated in vacuo to a highly concentrated state before crystallisation occurs. When all disturbing influences are avoided, the crystals grow to very large size, and are usually bounded by excellent faces. It ig also occasionally possible to obtain crystals by evaporation in the open air in prolonged dry weather, but in a moist state r€ the atmosphere it is quite impossible. The crystals deliquesce with considerable rapidity in ordinary moist air, dissolving eventually entirely in the attracted moisture. A large number, between twenty and thirty, freshly-grown individuals were employed in the measurements, all of as perfect a character as possible when first attached to the crystal holder of tho goniometer. This large number was found necessary, inasmuch as it was only found possible to measure a few zones on each before the influence of deliquescence began to be manifest. By selecting dry weather for the measurements, and choosing different zones on different crystals, however, the usual large number View Article Online

860 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

of measurements were eventually obtained. The use of the desiccating apparatus provided with the Fuess goniometer did not appear to be of much advantage with this salt, inasmuch as the crystal proved to be almost as excellent a desiccating agent as the calcium chloride employed therein. It was, however, used when the atmosphere was not so dry as it usually was on the days chosen, the air within it having been dried for some hours previously, and also largely for the purpose of preliminary examination of the crystal. The following are the results of the morphological investigation. The ratio of the morphologicccl axes, calculated from the angular measurements, is : a : b : c = 0*5’700: 1 :0,7424. Theforms observed include all those marked on the spherical projection except ~”(031)and 0”{332}. They are as follows : cc={lOO)a,l’x,; b=(010}30+rm; c(OO1)oP; p={llO}KJJz’; p’={l30}~1>3;q={Oll)Pc~; q’=(021}2P~; p”’=(O12)~Po0 ; o’={lll)P; 0’=(112)4P. PIG. 7.

The crystals of msium selenate resemble those of the sulphate in usually exhibiting a much larger development of the basal plane ~(001) than the potassium and rubidium salts. They are frequently more or

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. less tabular in this direction. The two pinacoids are usually also well developed forms, but so considerably bevelled and truncated by the domal, prismatic, and pyramidal forms as to be frequentlly much reduced. Fig. 7 represents a typical crystal. The brachydome, p’(021), is not infrequently more extensively developed than shown in the figure. Of the two prisms, ~’(130)is the only one usually present, the occurrence of the primary prism, p( 1 lo}, being comparatively rare ; this is just the reverseof the case with potassium and rubidium selenates, and is a characteristic feature of the salt. The faces of the primary pyramid, o(l1l}, are generally well represented, those of the pyramid o’( 11 2) being less common and usually smaller. The brachydomal form, q”’{012), was frequently observed, and occasionally afforded excellent reflections. The results of the measurements are given in the accompanying table : View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 861 Morp?~ologicaZAngles of Casium Selenate, Cs,SeO,. - No. of Mean Cdcu- Dif- Angle. mensure- Limits. ference. ments. observed. lated. . = 100:-110 3 29" 35'-29" 44' 29" 40' 29" 41' 1' = 110 :130 3 29 58-29 59 29 59 30 0 1 30 59 31-59 50 59 41 59 41 0 p'b = 130:OlO 32 30 8-30 26 30 19 30 19 0 (eq"' = 001 : 012 8 20 18-20 27 20 23 20 22 1 dyq = 012:Oll 5 16 10-16 18 16 14 16 13 1 {"a = 001:011 32 36 22-36 49 36 35 36 36 1 = 011 :021 32 19 16-19 40 19 28 19 26 2 = 001 :021 40 55 53-56 15 56 3 56 2 1 = 021 :010 49 33 40-34 8 33 57 33 58 1 = 100:111 63 43 36-43 54 43 43 * {:; {:; = 111 :011 62 46 7-46 35 46 17 46 17 0 ao' = 100 :112 1 58 37 58 35 2 = 112:012 1 31 23 31 26 2 Jbo = 010:111 69 65 31-65 47 65 39 65 40 1 LOO = i1i:iii 31 48 29-48 53 48 41 48 40 1 = 010: 112 2 72 39-72 47 72 43 72 43 0 = 112: 112 - - - 34 34 co' = 001 : 112 11 36 30-37 4 36 50 36 52 2 0'0 = 112:111 9 19 14-19 40 19 24 19 26 2 co = 001 :111 70 56 7-56 32 56 18 56 *18 0 op = 111:llO 57 33 33-33 49 33 42 PO = 1io:iii 2 64 51 -65 0 64 56 64 55 1 opl = iii :02i 14 49 12-49 37 49 22 49 20 2 qp = 021 :iio 2 65 44-65 51 65 47 65 45 2 Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. 1 72 8 72 12 4 o'q = lZ2:gl 1 - 34 51 34 58 7 qp = 011 :110 2 72 46-72 57 72 51 72 50 1 = 130:111 17 43 38-44 o 43 51 43 64 3 = 111 : 112 1 - 45 50 45 43 7 o'q' = 112: 02_1 1 - 46 11 46 6 5 = 111 :021 18 91 44-91 58 91 50 91 49 1 = 021 :i3o 16 44 12-44 26 44 17 44 17 0

$0' = 130 : 112 1 58 34 ti8 42 8 o'q = 112:Oll 1 - 62 17 62 16 1 qp' = 011 :i$o 6 58 57-59 6 59 2 59 2 -0 Total number of measurements, 693. The angles marked with an asterisk were the basal angles used in the calculations, on account of their numerous repetitions and particularly trustworthy character. It will be observed that, in all cases where more than one measurement was made, the observed and VOL. LXXI. 3M View Article Online

S62 TTJTTON : COMPARATIVE CRYSTALLOGR.APHICAL STUDY OF

calculated values approximate very closely. All values of an un- trustworthy nature, owing to lack of definition of the signal images involved, were rejected.

Comparative Table of JfoquJLologicul- Angles of tJLe- Three S’elenates.

Angle. Pot assi uni Differ- Rubidium Differ- Cesium selen ate. ence. sele nate. ence. selenate.

ap = 1OO:llO 29” 49’ -6 29” 43’ -2 29” 41’ = 100:130 59 49 -6 59 43 -2 59 41 = 110:130 30 0 0 30 0 0 30 0 yD = 110:01a 60 11 +6 60 17 +2 60 19 p’b = 130:OlO 30 11 +6 30 17 +2 30 19 cp’” = 001 : 012 20 6 + 10 20 16 +G 20 22 g”’p = 012 : 011 16 6 $5 16 11 +2 16 13 cy = 001:011 36 12 f 15 36 27 +9 36 36 q‘l’ = 011 : 021 19 28 -1 19 27 -1 19 26 p’q” = 021 :031 9 51 -2 9 49 -2 9 47 py6 031 :010 24 29 - 12 24 17 -6 24 11 pb = 021 :010 34 20 - 14 34 6 -8 33 58 = 100:111 44 9 - 18 43 51 -8 43 43 45 51 $18 46 9 +8 46 17 ao’ = 100 :J12 59 3 - 18 58 45 - 10 58 35 = 112:112 61 54 + 36 62 30 + 20 62 50 = 01O:lJl 65 43 -1 65 42 -2 65 40 48 34 $2 48 36 +4 48 40 /by‘ = 010 :112 72 51 -4 72 47 -4 72 43 (00 = 112:112 34 18 +8 34 26 +8 34 34

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. co‘ = 001 : 112 36 21 t 20 36 41 + 11 36 52 0’0 = 112:111 19 28 -1 19 27 -1 19 26 co = 001 :111 55 49 + 19 56 8 + 10 56 18 or)” = 111 : 332 9 49 -3 9 46 -2 9 44 o”p = 332 : 110 24 22 - 16 24 6 -8 23 58 I op = 111:llO 34 11 - 19 33 52 - 10 33 42 po = l~0:l~l65 17 - 16 65 1 -6 64 55 op’ = 1g: 0_21 48 57 + 16 49 13 +7 49 20 q’p = 021 :110 65 L6 0 65 46 -1 65 45

I”” = 1JO : 112 72 34 - 15 i2 19 -- 7 72 12 34 31 + 18 34 49 3.9 34 58 p = 011 :no 72 55 -3 72 52 -2 72 50

$0 = 130 :111 44 14 - 13 44 1 -7 43 54 00’ = 111 : 1g 45 33 +6 45 39 +4 45 43 o’p’ = q2: 021 45 46 + 13 45 59 +7 46 6 q’p’ == 021 : 130 44 27 -6 44 21 -4 44 17

~2’0’ = 130 :112 59 6 -15 58 51 -9 58 42 lA2 :g 61 36 + 25 62 1 + 15 62 16 pp’ = 011 :130 59 18 -- 10 59 8 -6 59 2 View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CESIUM. 863

Although only traces of the form 0’’ were observed, no measurements being obtained of its position, the calculated values of the angles which it makes with the neighbouring o and p faces are included in the table, for comparison with the analogous values for the sulphafes.

Comparison of the Morphologicccl Angles cyr the three Selenates. The values of analogous angles on the crystals of the three salts are set forth side by side in the accompanying table. The following facts are apparent : The values of the morphological angles of the crptals of the rubidium salt are without exception intermediate between those of the analogous nngles on the crystccls of the potussium und cc~siumsults respectively. The dixerences between corresponding ccngles of the tltree salts are very smull, rarely attaining 20 minutes of urc for the passage from the potussium to the rubidium snlt, or front rubidium to ccEsium selenate, tend neveq* reuching a degree foy tlbe replccenaent of potussium by ccesium. The intermediccte position assumed by the rubidium salt with respect to the morphologicccl ccngles is somewhut newer to that of the ccesiunz salt thccn to that occupied by potussium selenccte. It is evident, therefore, thcct chccnge of cctomic weight of the alkali metul is accompunied by cc corresponding chccnge in the magnitudes of the morpho- Zogicccl angles of the crystccls, the progyessive chccnge of the former from potassium thvough rubidium to ccesium, 01’ vice vewGc, being invariably ccccompccnied by a sintilarly progressive change in the Zatter. The morpho- logiccel angles are, therefore, a function of the cctomic weight of the alJcccli metnl, the function being such as produces a diminishing efect as the

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. utoniic weight rises highev and higher. A similar progressive change in the morphological angles according to atomic weight was observed, it will be remembered, in the sulphate series. As regards the actual difference between the angles of analogous salts of the two series, such difference rarely exceeds 20’, and is usually much less.

Comparison of the Morphological Axial Ratios. The axial ratios found for the three salts were as under : For K,SeO ,...... a : b : c= 0.5731 : 1 : 0.7319 ,, Rb,SeO, ...... a : 6 : c = 0.5708 : 1 : 0,7386 ,, Cs,SeO,...... cc : b : c = 0.5700 : 1 : 0.7424 The following facts are displayed : The rno~phologicalaxial ratios for rubidium selenate Gcre intermediate between thosefor potassium uncl c~siumselenates respectively. There is thus 3M2 View Article Online

864 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

a progression in the axial ratios corresponding to the progression in, the atomic weight of the metal contained. me change is grecctest for the ratio of c to b, and is likewise greater when potassium is replaced by rubidium than when the Iutter is repluced by ccesium, tht is, becomes less marked us the cctornic weight rises. Precisely similar relationships were established for the sulphates. It is interesting to observe that the replacement of sulphur by selenium is accompanied by only a very slight change in the ratio a/b, while c/b diminishes almost exactly by 0.0100.

Cornpmrison of the Predominating Habits. The three selenates are characterised broadly by the same differences of habit as were observed with regard to the sulphates. The brachypinacoid b(O1O) is characteristic of the potassium salt, the basal plane ~(001)of the cssium salt, and the domal forms, bevelling both, of the rubidium salt. The crystals of potassium selenate are tabular parallel to b, those of cssium selenate tabular parallel to c, and those of the rubidium salt are prismatic parallel to the intermediate brachydomal forms. There is here observed a distinct progression in the zone [bq'c],from b, through q', to c. If the development of the macropinacoid a(100) is considered, it is extremely narrow, frequently a line, and often altogether absent, in the potassium salt ; it is a prominent form in the rubidium salt, and it forms the broad end of the modified rectangular block in the cssium salt, also a progressive development. Again considering the basal plane, it is either very narrow or absent in potassium selenate, usually present, and better developed, in the rubidium salt, and is the predominating form of cssium Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. selenate. Hence, from whatever point of view the development of the primary faces is regarded, it is clear that The habits genemlly assumed by the crystals of the three selenates respectively are considerably diferent in character, and the darerences exhibited are of a distinctly progressive clwracter, following the progressive change in the atomic weight of the metal.

Cleavage. As stated by Topsiie and Christiansen (loc. cit.), the crystals of potassium selenate cleave parallel to the brachypinacoid b(010) and the basal plane ~(001). These directions are identical with the cleavage planes of the three sulphates. This fact was confirmed on individual crystals during the course of this work, and it appears that the cleavage in the former direction is the more perfect of the two. View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND C&SIlJM. 865

Rubidium selenate likewise afforded a good cleavage parallel to the brachypinacoid, and a less perfect one parallel to the basal plane. Identical results followed the investigation of the cEsium salt for cleavage. Hence The directions of cleavage cure identiccd fey the three selenates, and they cwe also identiccd with those of the ccnalogous su2plutes. They are parallel respectively to the bmchypinacoid and the basal plcme, and the clecmage along the former is the moTe peqfect one.

INVESTIGATIONOF VOLUME RELATIONSHIPS. Relative Densities. On account of the highly hygroscopic nature of the selenates of potassium, rubidium, and czesium, precautions of an unusually thorough character were essential in order that the salts should be obtained

FIG. 8. Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11.

absolutely anhydrous in the fine state of subdivision required for the determination of the densities, From the moment that the dry crystals, which had been previously stored in closed bottles within a desiccator, were reduced to powder until the dried powder was sahly transferred to the pyknometer and the latter sealed by means of its spring-pressed cap, the whole of the manipulative operations which involved the exposure of the salt to the air were performed in a specially constructed air-tight desiccating chamber, within which the author was able to perform all such operations from outside by inserting his hands in caoutchouc gloves provided for the purpose in two adjacent sides of the chamber. As this arrangement has proved highly succetjsful in View Article Online

86 6 TUTTON : COMPAItATIVE CRYSTALLOGRAPHICAL STUDY OF

the case of the present investigation, it may be of use to other investi- gators to briefly describe the form of apparatus which the author finds most convenient. It is shown in Fig. S. It consists of a box of hard mahogany, about one cubic foot in size. The top of the box and one of the sides are fitted with relatively large and hermetically sealed-in windows, a and b, and a second side adjacent to that containing the fixed window is fitted with a removable one of similar size, c, which acts as the means of opening the chamber in order to place the required articles within it. The glass is hermetically sealed in the frame, and upon the back of the latter, and also on the margin of the rectangular aperture in the side of the box against which the frame fits, is cemented a thick pad of caoutchouc, The closing is effected by the ten bolts shown in the figure, and the caoutchouc pads render it very efficient. The bolting is a very simple matter, as the bolts themselves are fixed in the side of the box and the corresponding holes in the frame fit easily over them, the pressure produced by screwing on the winged nuts being ample to effect hermetic sealing. The two remaining sides, d and e, are fitted with circular apertures through which are inserted the caoutchouc gloves. Caoutchouc operating gloves lined with silk or other fine fabric answer best, as the lining is not only warmer to the hands but preserves the gloves from tearing when stretching; the wrists of the gloves should also be fairly long. The sealing of the glove apertures is very simply effected by first cementing a caoutchouc annular pad round each aperture, on the outside of the box, then stretching the hose-like wrist of the glove well round a hard mahogany curtain-pole ring slightly larger than the wrist and sufficiently large to enable the hand to pass easily through it; these rings are then in each case clamped firmly against the pad round the aperture in the Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. box side by means of a hard wooden plate,f, pierced by a similar aperture and padded with caoutchouc on the inner side, and bolts similar to those used for the removable window. By this device, the gloves can readily be renewed at any time. In order that the apparatns may also be employed for working in an indifferent atmosphere with substances attacked by the ordinary atmospheric gases, two short brass tubuli, g, are fitted diagonally opposite each other in two non-adjacent sides of the chamber, for the entrance and exit of the indifferent gas with which the box is to be filled. These tubuli are closed by tightly fitting caoutchouc stoppers when not required. In order to use the apparatus, it is only necessary to place all the required articles inside over-night, along with one or two crystallising dishes containing concentrated sulphuric acid to dry the atmosphere inside during the night, and then next morning to insert the: hand through the annular apertures into the gloves, and proceed with the operation. In order to prepare for the determination of the densities of the View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CESIUM. 867

salts under consideration, there were placed in the desiccating chamber over-night, or, usually for greater security, several days previously : the two closed bottles containing two crops of the crystals of the particular salt under investigation ; an agate mortar and pestle ; a sieve of platinum gauze; one double and one single sheet of note paper, the larger one laid over the greater part of the floor of the chamber to retain projected particles of the expensive salt, and the smaller one folded in two and then opened again, for use in transferring the POW- der, after sifting, to the receptacle for it; and, lastly, besides a few subsidiary accessories found useful and the desiccating vitriol, two glass tubes about six inches long and three-quarters wide each having a bulb of about 60 C.C. capacity blown at one end and fitted at the other open end with a solid stopper of caoutchouc. After allowing adequate time for the drying of the air in the chamber, the POW- dering and sifting were carried out within the chamber with the gloved hands as above described, and the powder from each crop of crystals was transferred to a separate bulb and the latter then closed with its stopper. Only sufficient salt was placed in each bulb for one determination, each determination being thus carried out independently with a separate crop of crystals, but two such independent determinations were conveniently prepared for at the same time. The salt was thus obtained in a state of fine division, free from any attracted moisture, but it still retained the usual traces of mother liquor which had been enclosed in cavities. To remove this, it was heated to 150' in a current of dry air for 10 hours. This was effected by means of the following arrangement. One of the two bulbs con- t'aining the powder (the other being mcanwhile stored in a desiccator), was suspended in an air bath heated to 150". The closed caoutchouc Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. stopper was replaced by a doubly bored cork, through one hole of which passed a tube reaching to just within the bulb, and bent above to con- nect to a series of pumice and sulphuricacid drying tubes, terminating in speed-measuring bulbs partly filled with sulphuric acid, to control the rapidity of the air current. The second hole of the cork carried an exit tube leading to another drying tube and a Woulfe's bottle act- ing as safety reservoir and a water pump. The current of air was maintained at a very slow rate, to ensure effectual drying in its passage through the U-tubes. The contents of the salt bulb were frequently agitated, in order to expose fresh surfaces. The thus effectively dried salt was then removed from the bath, stoppered with its closed stopper, and stored in a desiccator while its fellow was similarly treated. The pair were then placed in the special desiccating chamber along with the two pylinometers with their metal spring stands, the materials for transferring, and the desiccating vitriol ; the agate mortar was again included to enable a final crushing View Article Online

868 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAT, STUDY OF

to be given to the salt, which may have slightly caked during the heating. The transference of the contents of each bulb to a particular pyknometer was subsequently, after the interval of a day or two, effected by manipulating in the chamber as before. Thus the salt was obtained in the pyknometers without having once been in contact with ordinary moist air. The chamber was then opened and the pyknometers were removed to a desiccator until ready for weighing. The weighing of the pyknometers containing the salt, the coveriug of the latter with carbon tetrachloride and then the elimination of the included air, the complete filling with carbon tetrachloride

and raising to a fixed temperature, ZOO, together with the subsequent weighing, were all carried out precisely as described in the communication concerning the sulphates (Zoc. cit., p. 650) for thb capped pyknometer method. The liquid employed was, as stated, carbon tetrachloride, and this liquid has been found as admirably suitable for the selenates as for the sulphates, there being no trace of action. Four determinations were made for each selenate with salt derived from different crops of crystals. The results are given in the accompanying table. The density of the carbon tetrachloride, and the weights of that liquid contained by the two pyknometers when full at 20°, were re-determined immediately before commencing the series, and the whole twelve determinations were then carried out in succession without any intervals other than those necessary for the dryiDg operations previously described. Relative density of Potassium SeZennte at 20'. Weight of salt employed. Sp. gr. at 20'14".

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. 5.2188 grams 3.0655 6.0426 ,, 3.0646 5.3471 ., 3.0651 5.3227 ,, 34x75

Mean 3.0657

IZeZcdive density of Bubidi?m ,YeZenctte cct 20'. Weight of salt employed. Sp. gr. at 20"/4". 5.9151 grams 3 *s957 6.4452 ,, 3.9007 4.6954 ,, 3.s907 5.0515 ,, 3*'301s -. - Mean 3.8995 View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CXSIUM 560

Relative density of CGES~Z~WLSeleizccte at 30'. 1Jrc igh t of salt employ ed . sp. 6'. at 20"/4". 6.2565 grams 4.452 1 4.7692 ,, 4.4564 8.5228 ,, 4.4484 95198 ,, 4.4543 Mean 4.4528 It will be observed that relatively large quantities of the salts were employed, and this fact, coupled with the great precautions employed in preventing access of moisture to the powder used in the determinations, render it highly probable that the mean numbers now given are very near the truth. Such slight differences as there may be between the results afforded by different crops of the same salt, above 0.001, which is the maximum error attributable to the operations, are un- doubtedly due to minutmedifferences in the densities of the crystals belonging to such crops. Pettersson determined the densities of these three salts in the course of his well-known investigations between the years 1872 and 1876 (Nova Actn rZ. SOC.Scient. l+snlie&s, series 3, 1873 and 1876). In the first of his two communications, he gives the results of his determinations of the densities of potassium and rubidium selenates. Quantities of potassium selenate varying from 1.2610 to 3.6442 grams at temperatures ranging from lSo to 21' yielded values from 3.074 to 3.077. Amounts of rubidium selenate ranging from 1.4954 to 1.7698 grams afforded densities varying from 3.896 to 3.943. In his second memoir, Pettersson gives similar data for cmium Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. selenate ; two quantities of 2.3455 and 2,0025 grams respectively, at 15.2" and 15*5O, gave densities of 4.31 and 4.34. From the fact that these values are slightly lower than the author's, and those for rubidium selenate slightly higher, it would appear as if Pettersson's specimen of the caesium salt contained an appreciable amount of rubidium, and his rubidium selenate more than a trace of cmium. The density of potassium selenate given by Topsae is 3.052. The author's value for this salt is thus about the mean of the values of Pettersson and Topsije respectively. In addition to the determinations with carbon tetrachloride, another series were carried out with turpentine, as in the case of the sulphates, with the view of subsequently determining the density at 60" with that liquid in order to ascertain whether expansion occurred with rise of temperature. As shown in the memoir concerning the sulphates (Zoc. cit., p. 654), such a mode of determining the expansion does not afford sufficiently accurate values of the coefficients of cubical expan- View Article Online

870 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

sion for the purpose of instituting comparisons between the three salt,% but if no action occurs between the liquid and the salt, it affords ample evidence of the expansion or contraction of the latter by change of temperature, knowledge which is particularly desirable in connection with the effect of temperature on the molecular optical constants. The question of the accurate determination of the coefficients of expansioniwill therefore be deferred to a future communication, which will embrace also the expansion of the sulphates. Turpentine was found to exert an appreciable solvent action upon the selenates, more pronounced as the temperature rises, rendering the results for the densities too high. Hence they are discarded. They show, however, quite clearly that the salts expand with rise of temperature, but the amount of expansion indicated is not so great as would be the case if there were no action between liquid and salt wcelerating with rise of temperature. The fact that expansion is proved is, however, ample for the purposes of the consideration of the iuolecular optical constants.

The molecular volumes M/cl of the three selenates, calculated from the densities at 20”, are as given in the accompanying table : 220 s For K,SeO, ...... il/jcZ = -~ = 72.03 3.0657 Diff erencc 8.30

313.2 - 80.32 ,, Eb,SeC),.. ilqd = ~ - 3.8‘395 Difference 11.35

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. ,, Cs,SeO, ..... ,ll/d=-- 408*2- 91.67 4.4528 The molecular volumes found by Pettersson were 71.91, 79.97, and 94.6. The first agrees fairly with the value now given, but itiis evident tliat the value for rubidium selenate is somewhat too low and that for the caesium salt considerably too high, owing to the errors in the determination of the densities of those two salts, due probably to admixture of caesium with the rubidium salt, and rubidium and possibly potassium with the cmium salt. The differences given by Pettersson are likewise correspondingly incorrect, 8.06 and 14.63. I’ettersson’s results for the corresponding sulphntes were previously shown to be incorrect to an almost identical extent, from the same cause. Hence the difference observed by Pettersson between corresponding sulphates and selenates agrees with that observed by the author, namely 6.6, and it is satisfactory to be able to contirm l’ettersson’s statement that the replscemont of sulphur by selenium in these salts View Article Online

NORiMA.1, S CLENATES OE POTASSIUM, RUUIDIUM, AND CIESIUM. 871

is accompanied by an increase of molecular volume to about this extent. The molecular volumes found by the author for the sulphates (loc. cit., p. 656) were 65.33, 73 77, and 85.17, and the differences between these values and those of the corresponding selenates now given are respectively 6.69, 6-55, and 650. Although the mean difference is 6.6, it is quite clear that the differencediminishes as the atomic weight of the metal rises, the replacement of sulphur by selenium being ap- preciably less and less effective in increasing the volume in the cases of the heavier rubidium and still heavier cssium molecules than in th case of the potassium salt. The difference between the volumes of the sulphates of potussillin and rubidium was found to be €5.44,and that between the volumes of rubidium and czesium sulphates 11.40. The closeness of these numbers to those now found for the selenates, S.30 and 11.35, is very striking. The very slight disparity, however, is doubtless real, and of significance, for the replacement of a lighter by a heavier alkali metal is seen to be productive of a slightly less change in the heavier selenates than in the lighter sulphates, a result analogous to that just indicated for the replacement of sulphur by selenium in the cases of the molecules containing respectively heavier and lighter atoms of the diffwent alkali metals. The two results thus unite in proving that the heavier the initial molecule the slighter is the change of volume produced by a given replacement. This important fact may be most clearly illustrated by the rubidium salts. Thus, if we consider rubidium sulphate, a certain increase of volume is produced by replacing the sulphur by selenium, but the amount of increase is not so great as when the sulphur in the lighter potassium sulphate is replaced by selenium ; and if we con- Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. sider rubidium selenate, a definite increase of volume is produced by replacing the rubidium by caesium, but the amount of increase is not so large as if the rubidium in the lighter rubidium sulphate had been replaced by caesium. These differences in amounts of increase are very small, the order of the numbers being of the same character and the differences only occurring in the decimal fractions ; they are, however, real, and not due to experimental error. The main results of the investigation, as regards the densities and molecular volumes of the salts, may be summiwised as follows. The relative density ccnd molecular volume increuse tohen a lighter is replaced by (6 heavier alkuli naetcd, and the vulwes for rzcbidizcm selenchte ure consequently interniediute bet ween the vnlues f or the selemtes of potussiurn and cesium respectively. The increase in density is yreibter when potussiui~&is repkuced by rubidium than when the Iutter is repluced by ccesiu~~~,UIZJ the increme in wolecular volume is, on t7~econtrary, greatest when rubidium is yeplaced View Article Online

872 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

6y cesium, being 11.35, compared witlb 8.3 wJben potassium is replaced by rubidium. TJLese rules are identical witJL tJiose observed zn tibe case of tJt,e szc1;riJmtes of the same three metals, and the parallelism 6etween tJhe two series as regards densitg and molecular volume is very close. It is observed tJLat the replacement of sulpJmr 6y selenium is accompanied by an increase of molecular volume of 6.5 to 6.7, tJLe minimum amount corresponding to tJt,is change in cesium sulpJLate cmd tJLe maximum to tJLe same in potassium sulpl~ate,the amount for the rubidium salt being intermediate ; the efect of the replacement consequently diminishes as fh2 weigJ6t of the initial molecule increuses. It is also observed tJLat the daferences between the molecular volumes of the three selenutes, altJLough rouyJdy tJAe same, are, in ?*edit?/,sliyJLtly less tJian the co~responclingdzree.rences, 1 I *4ccnd 8-44)for the lighter sukhates. Hence a comparison of the results for the two series indicates that although there is a very close parallelism between them, yet that tJt,e efect of the replacement of either the metallic or acidforming element is percepiibly less in tJbe case of a JLeavieY tlmn in the case of CL lighter initial molecule. T?~~ougJ~outall these cJLnnges, even with respect to the minute dzflerences just refewecl to, tJLe rubidium salt invuriably 6eJLaves in an inteymediate manner. Distance Wcctios. The relative distances apart in the three rectangular directions of the centres of contiguous structural units, which, in all probability, are the chemical molecules R,SeO,, as shown in the previous communication (Zoc. cit., p. 524) have been calculated, as in the case of the cor- Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. responding sulphates, from the formulte :

The results are as follows :

x :\I, :w K,SeO,...... 3*1854 : 5.5581 : 4.0680 Diff., 96’7 Diff., 1959 Diff., 1819 Rb,SeO, . . .3.2821 : 5.7540 : 4.2499 Diff., 1412 Diff., 2519 Diff., 2089 Cs,SeO, . . .3*4233 : 6.0059 : 4.4588 Total diff., 2379 Total diff., 4478 Totaldiff., 3908

They may also be expressed more simply, by dividing out by +h for K,SeO,, as follows. The values for the potassium salt are then identical with the morphological axial ratios. View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM AND CESIUM. 873

X :+ :w K,SeO,...... 0-5731 1 0.731 9 Diff., 178 Diff., 352 Diff., 32'7 Rb,Se04 ...0.5909 1.0352 0.7646 Diff., 250 Diff., 454 Diff., 376 Cs,Se04 . . .0.6159 1.0806 0.8022 Total diff., 428 Total diff., 806 Total diff., 703

The following statement embodies the facts indicated by these ratios. The replacement of potassium by rubidium, and this alkali metal in turn by ccesium, is accompanied in each case by a considerable increuse in the separation of the centres of contiguous units of the homogeneous crystal structure, along the directions of each of the morphological ccxes. An increase in tlhe atomic weight of the alkali metal is tILusproductive of an extension of the structui-e in erery direction, and the increase becomes relatively greater, the determining injuence being a growing one, as the atomic weight rises. Hence the ultimate parts of the structure of the

rubidium salt occupy positions yelatively intermediate hetween those foil potassium and cGesium selenates, but these intermediate positions are sonzewhat closer to those foi- the former than to those for the latter salt. The extension is greatest along the b axis, and much the least along the a axzs. If the initial relative lengths of the axes are considered in making the comparison, it will be found that the extension along the c axis is considerably the greatest in proportion to its length, that along the cc axis being still the least. Similar conclusions were derived from the investigation of the Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. sulphates. A conapurison of the first series of ratios with those for th,e sulphcctes indicates that an extension of volume in id thee directions occurs, as was to be expected, when sulphur is iqeplacecl by selenium.

OPTICALINVEST I GA TI ON. The optical portion of the investigation was carried out on precisely similar lines as in the case of the sulphates. The whole of the section plates and prisms employed were prepared, as described in the memoir concerning those salts, by means of the author's accurate grinding goniometer, and all the measurements were carried out with the aid of the spectroscopic momchromatic light apparatus likewise therein referred to. The author now exclusively uses the electric arc as source of light, which enables slits of the most extreme fineness to be employed, the monochromatic yellow light equivalent to that of sodium, for instance, being so truly monochromatic as to consist of rays whose View Article Online

874 TUTTOK : COMPARATIVE CRYSTAT,T,OGRhPHICAT1 STITDY OF

wave-lengths are within those of the two D lines. The steady Brockie- Pel1 projection lamp is used, mounted in a lantern of the best con- struction, provided with a triple-movement adjusting table for the lamp, a matter of very great convenience for centering purposes, as it is of considerable moment in determining refractive indices to maintain the light accurately concentrated by the condensers upon the slit of the monochromatic illuminator. The Brockie-Pel1 lamp, being self- centering, does this roughly itself, but it is of great advantage to be able to achieve it exactly. In order to avoid having to rise and approach the lantern, to do this during observations, a pulley gear has been arranged along an adjustable bar ; the bands pass at the lantern end of t,he bar round the three large milled heads of the adjusting table, which have been grooved for the purpose, and, after being di- verted by suitable guiding pulleys, pass round three similar grooved milled heads at the other end of the bar, adjustable in slots for the purpose of tightening the bands and arranged compactly over each other upon n small base-block which can be arranged to rest upon the supporting table of the observing instrument. The bar is pivoted at the lantern end on the base of the lantern, and the guiding pulleys are so arranged that the rota;tion of the bar about the pivot makes no difference to the tightness of the bands. Hence the terminal block of pulleys at the other end of the bar may be placed anywhere conveniently within reach, and thus the electric arc may be accurately centred by the observer while observing, thus enabling him to actually see when the maximum illumination is attained. The whole arrangement, as used in connection with the most recent and accurate Fuess spectrometer (No. l~),is sh own in the accompany- i ng ill ustration. Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. The crystals of the normal selenates of potassium, rubidium, and caesium resemble those of the corresponding sulphates in the feebleness of their double refraction, which necessitates the use of very thick section-plates in order to obtain adequately well defined interFerence figures in convergent polarised light. Particularly is this so in the case of czesium selenate, which has been found to be one of the most interesting of substances from an optical point of view, and from a cause of especial importance with regard to the object of this inves- t iga tion. Opticul Propeyties of Potussium Xelenate. The pkccne of the optic ccxes (optic binormals of Fletcher) is the macropinacoid n{100). The &*st mediccn line is the morphological axis c. The double refrccction is positive. In these respects, the selenate of potassium resembles the sulphate. The amount of double refraction is likewise small, as in the case of the sulphate, a plate perpendicular to View Article Online

NORMAT, SELENATES OF POTASSIUM, RUBIDIUM, AND CESIUM. 8’7s Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. View Article Online

876 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

either median line requiring to be at least 2 millimetres thick in order to afford sharp interference figures. The disposition of the axes of the optical indicatrix of Fletcher, or of its polar reciprocal, the optical velocity ellipsoid referred to in the memoir concerning the sulpbates, is consequently as follows. The direction of the morphological axis b is that of the minimum axis a of the optical indicatrix, and that of the maximum axis 8 of the optical velocity ellipsoid. The direction of the morphological axis cc is that of the intermediate axes p and l~ of the optical indicatrix and optical velocity ellipsoid respectively. The direction of the morphological axis c is that of the maximum axis y of the optical indicatrix and that of the minimum axis g of the optical velocity ellipsoid. Determinatio.lzs of Refv*actiue Indices.--The rapidity of deliquescence of the crystals of potassium selenate is not such as to interfere seriously with the preparation of prisms or section-plates, provided the first ground surface is protected from the moist atmosphere during the grinding of the second by being buried in the wax which cements the crystal to the holder of the grinding goniometer. As soon as both surfaces were finished, in order to be preserved unattacked, they required to be furnished with miniature cover-glasses, cemented by a solution of hard balsam in benzene, the drying of which was allowed to occur in a large desiccator, The micro-cover glasses employed had been carefully selected, being such as afforded single images of the signal slit ; they were subsequently cut up into small fragments of the approximate size of the ground surfaces, which varied from 1 to 3 millimetres in width. Eight 60" prisms were employed, ground upon four excel- Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. lent crystals of different crops, two complementary ones upon each, so orientated as to furnish together all three refractive indices. The crystals used were in all cases single individuals of the most perfect development, selected from the product of some scores of crops. The results of the measurements are set forth in the accompanying table. The intermediate index p, corrected to a vacuum (the correction being +0.0004), is accurately expressed for any wave-length A, as far as F, by the formula :

491 699 + 77 344 000 ooo+ p=1.5250+- - .... A2 A4

The u indices are also closely expressed by the formula if the constant 1.5250 is diminished by 0-0038, and the y indices if the constant is increased by 0.005'7. The p values being thus nearer to the a than to the y indices, the statement that the double refraction is of positive View Article Online

NORMAL SEIrENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 877

Determinations of Refractive Indices of Potassium Xelenata. I Mean ! Nature of Crystal 1, Crystal 2, Crystal 3, Crystal 4, Index* I light. refractive 2 prisms. 2 prisms. 2 prisms. 2 prisms. index. . . _____- Li 1 -5328 1'5318 1.5318 1-6316 1-5320 I C I 1'5334 1.5323 1'5323 1.5320 1-5325 VibraLnsJ Na I 1.5357 1.5353 1 -5352 1 -6347 1.5352 parallel TI 1 1.5389 1 *5382 1'5382 1 '5378 1'5383 to axis b. F , 1-5428 1-5421 1,5420 1'5414 1'5421 , G 1 1.5487 1'5476 1'5475 1 -5472 1.5478 I I Li 1,5363 1-5358 1.5356 1 '5350 1'5367 i 1.5368 1 '5363 1-5361 1'5355 1-5362 Vibrations Na ; 1.5397 1 '5391 1 $386 1.5384 1-5390 parallel T1 1'5428 1 '5422 1-5418 1.6414 1.5421 to axis a. F ! 1.5467 1 '5464 1.5456 1.5451 1'5460 GI1.5522 1 '5520 1.5514 1'5511 1-5517 Li 1.5420 1 -5412 1-5412 1 '5406 1-5413 C 1'5426 1'5417 1 '5417 1-5410 1-54i8 Vibrations Na 1'5454 1 5446 1'5444 1.5439 1.5446 parallel T1 1'5486 1.5479 1'5476 1 '54 69 1'5478 to axis c. F 1 '5526 1 -5518 1.5517 1-5510 1-5518 G 1-5584 1'5578 1,5576 1.5567 1-5576

sign is confirmed. The total relatively small difference of 0~0093-0*0099 between the minimum and maximum values for any wave-length, may be taken as the measure of the feebleness of the double refraction. Topsae and Christiansen give the following indices : a. B. Y. C ...... 15323 1-5373 1.5422 D ...... 1.5353 1.5402 15450 Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. F ...... 15417 1.5475 1.5533 It will be observed that the agreement is fairly close for the a and y values, but that considerable difference is apparent between the author's p values and those of Topsije and Christiansen. That the latter are incorrect, is evident from the fact that they are slightly nearer to the y than to the a values, a relation which neither corresponds to the indu- bitable positive double refraction, as proved by the quarter undulation mica test, nor to the disposition of the first median line, which is parallel to the 7 direction of vibration and not to the a, nor does it agree with an optic axial. angle of 76O, an angular value in which both the author and the Danish observers concur. Alteration of Refraction hy Increase of Tempe~*atwe.-!l!ha refraction determinations of two of the prisms were repeated at looo, and the results are afforded in the accompanying table. The most recent and altogether admirable form of heating apparatus supplied by Fuess, with the large spectrometer No. IA, was employed. VOL. LXXI. 3N View Article Online

878 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

Refmctive htlices of Potassizcm Selenmte tit 100' C. Diminution from values at ordinary Index. Natnre of Light. Indices at 100". temperature. Li 1.5276 0.0044 ie 1.5381 44 a 1-5310 42 Vibrations parallel 1.5341 42 to axis 6. 1.5378 43 1.5440 38 Li 15307 50 15312 50 P J 1.5340 50 Vibrations parallel T1 1.5371 50 to axis a. IF 1*5411 49 1.5469 4s Li 1.5353 60 ic 1.5358 60 Y 1.5387 59 Vibrations parallel 1.5418 60 to axis c. IF 1.5458 60 1.5520 56

The effect of this rise of temperature, from 16-19' to 100°, is thus observed to be a diminution in refractive power, the amount of lower- ing of the indices being 0.0038 to 0.0060, varying slightly according to the wave-length, and more considerably according to the direction Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. of vibration. The amount of diminution increases with the actual value of the refractive index, in other words, with the length of the axis of the optical indicatrix, being much the most considerable for the direction of the major axis A, which is identical with the morphological axis G. Further, the double refraction diminishes, from 0,0096 at the ordinary temperature to 0.007'7 at 100". The order of the indices and the sign of double refraction are unaltered. Axes of the Optical Indicatrix ccnd of the Opticccl Velocity ElZipsoid.-- A concise expression of the optical character in different directions is afforded by optical ratios of the nature of those given and fully described in the author's previous memoirs, termed axial ratios of tho optical velocity ellipsoid. For the sake of uniformity and facility of comparison, similarly calculated axial ratios of the same reference ellipsoid are now given for the selenates. As, however, the recent

important conception of Fletcher of the 'I optical indicatrix " is at once simple and fully adequate to express mathematically all the corn- View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CrESJUM. 8'79

plex optical phenomena of crystals, the optical indicatrix will first be employed as the reference ellipsoid. This ellipsoicl is the polar reciprocal of the optical velocity ellipsoid. The axes of the optical indicatrix may be correctly considered as expressed by the refractive indices themselves, but for facility of comparison of t,he relative lengths of the axes it is convenient to take the length of one axis as unity, by dividing out by the corresponding value of the refractive index for that direction. The order of the morphological axes mill therefore be taken, and the value for t'he b axis considered as unity. The axes of the indicatrix are a, p, and y, corresponding to the refractive indices, and necessarily ub>t. The values for potassium selenate are as follows :

Axes of Optical Inclicchtrix. 8. a. y. At the ordinary temperature, a : 6 : c= 1.0025 : 1 : 1*0061 At loo", CG : b : C= 1.0020 : 1 : 1.0050 Axes of Optical Velocity Ellipsoid. Lr. x. t. At the ordinary temperature, CL : 6 : ~=0*99'75: 1 : 0.9939 At loo", cc : b : c=On99t30 : 1 : 0.9950 The much greater effect produced by rise of temperature upon the refraction along the direction c, and the simultaneous diminution iu double refraction, are clearly shown by these ratios. Molecular Optical Constants.-These constants have been calculated from the foregoing refraction data and the densities, with the aid of both the Lorenz and the Gladstone and Dale formult-e. They are as Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. follows :

Morphologienl axis. a. b. c. 9L2 - 1 c 0*1018 0.1012 0.1027 Specific:refraction, = 11 ...... (92 + 2)d G 0.1042 0.1036 0.1052 n2- 1 M- J C 22.48 22.35 22.67 Molecular refraction, r. - - 111 ...... nJi-2 d \ G 23.02 22.88 23.23 Specific dispersion, iyG - iyC ...... 0.0024 0.0024 0.0025 Molecular dispersion, ma - lire ...... 054 0.53 0.55 n-1 Molecular refraction, -M ...... C 38.65 38-38 39.05 d

Meusurements of Optic Azicd Angle.-Four pairs of section-plates mere prepared with the aid of the grinding goniometer. They were of considerable thickness, averaging about 2 millimetres, in order to afford well-defined interference figures. In the cases of the first and second pairs, the sections perpendicular to the first and second median lines 3~2 View Article Online

880 TUTTON : COSIPARATIVE CRYSTALLOGRAPHICAL STUDY OF

respectively were ground upon one and the same crystal in each case ; the individual sections of the third and fourth pairs were ground upon different crystals, but those of each pair belonged to the same crop. Section 2 did not permit of n satisfactory determination of the extremely large angle in air, being narrow in the direction concerned. The crystals employed in every case were single, mell-formed individuals, the selected product of very numerous crops.

Detewnincction of Appcwent Angle in Ail. of Potassizena Selen te. Nature of Light. Section 1. Section 3. Section 4. Mean 2E. Li 145' 20' 145" 25' 144" 50' 145" 12' C 145 40 145 45 145 3 145 29 Na 14'7 10 147 18 146 10 146 53 T1 149 25 149 0 147 23 148 36 F 151 10 151 0 148 30 150 13

i NO. of No. of Observed section 0 bser ve d Calculated Mean of values of perp. 2nd values of values 2Va. 2Ha. median 2Ho. 2Va. liue. line.

70" 20' 1CL 93" 31' 70 27 2n 93 25 76" 46' Li 70 40 3t6 93 7 70 14 4 TL 93 32

70 16 In 93 24 Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. 70 2L 2(1. 93 18 76 c: i0 35 3n 92 56 47 TO 10 4n 93 27 70 4 In 93 4 70 15 2fL 92 54 Na 76 50 TO 22 3CL 92 32 69 55 4n 92 59 69 43 In 92 30 70 0 2n 92 21 TI 70 5 3n 92 3 76 53 69 40 4n 92 25

69 24 la 92 0 GD 45 2CL 91 45 76 F 69 45 3a 91 25 57 69 25 4n 91 57

The measurements of 2Hn and 2Ho were carried out in monobromo- naphthalene, and those for each pair of sections were completed at the View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CZSIUM. 881

same sitting, using the same cell of liquid, so as to avoid error on account of any possible difference of refraction of the latter during the two series. The sign of double refraction indicated by the quarter undulation mica plate was positive, a result confirmative of that derived from the refraction measurements. Efect of Rise of (renaperature on the Optic Axial Angle.-Owing to the extremely large angle, this is difficult to determine, but repeated observations at 120" with the best of the sections perpendicular to the first median line indicated that any change of 2E is extremely slight, the actual measurements indicating in the mean a slight increase not exceeding 2" for 100" rise of temperature. This is in accordance with the refraction observations at loo", the indications of which are most clearly shown by the axial ratios of the optical indicatrix; the altered values of those axial ratios for 100" bear the same relations to each other as do the original values for the ordinary temperature, 61 being to 50 approximately as 25 to 20, and it is, of course, these relations, the relative distances of the intermediate from the extreme values, which determine the amount of separation of the optic axes.

Opt icccl F~opei*tiesof Rubicliuna Se Zenate.

The plccne of the optic axes (optic binormals of Fletcher) is the macropinacoid chL(100). The Jimt nzetlin?~line is the morphological axis c. The double ?*ejkctionis positive. It is thus optically similar to potassium selenate. The double refraction, as in the case of the latter salt, is very feeble, and consequently section-plates perpendicular to the median lines require to be thick in order to afford well-defined interference Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. figures in convergent polarised light. A thickness of about 3 millimetres is most suitable. The disposition of the axes of the optical indicatrix, or of the optical velocity ellipsoid, is therefore as follows : The direction of the morphological axis b is that of the minimum axis a of the optical indicatrix, and that of the maximum axis B of the optical velocity ellipsoid. The direction of the morphological axis a is that of the intermediate axis p of the optical indicatrix, and that of the intermediate axis b. of the optical velocity ellipsoid. The direction of the morphological axis c is that of the maximum axis y of the optical indicatrix, and that of the minimum axis t of the optical velocity ellipsoid. Detemainutions of Refrccctive Indices.-The rapidity of deliquescence was not found to be sufficiently great to offer much difficulty in tho preparation of suitable prisms, By taking the following precautions, excellent ones were eventually obtained. These precautions consisted in View Article Online

882 TUTTON : CONPBRATIVE CRYSTALLOGRAPHICAL STUDY OF

the liberal use of oil upon the grinding disc during the actual operation of grinding, the exercise of such celerity as was compatible with accuracy during the adjustment of the crystal upon the grinding instrument, the removal of the wax from the crystal entirely by immersion in benzene after cutting off from the holder the end part of the wax in which the crystal mas embedded, the painting of the first ground surface with balsam solution or the embedding of it in the wax while grinding the second surface, and the immediate cementing of thin, parallel sided, and truly plane cover-glasses to the surfaces after their final cleansing in benzene. They were allowed to stand some hours in desiccators before being employed for the determinations of refractive index, until the cementing balsam solution had sufficiently hardened to be incapable of permitting movement of the cover-glasses during the observations. Eight 60' prisms were employed, ground out of six different crystals, a complementary pair of prisms being ground upon the first and fifth of these crystals, and one prism upon each of the rest. It is naturally more dificult with deliquescent substances to prepare two prisms upon the same crystal, and this was only achieved in the two instances specified, the natural reference faces of the crystal being found deteriorated after completion of the measurements in the other cases. Every prism was ground so as to afford directly two indices, as usual throughout this work. The results of the measurements are given in the accompanying table :

Detesminations of Refrnctive Indices of 122cbidiuna Xelenctte.

_PI1. Nature Mean 2rystnl1, rrysta12, Crystal 3, kystal4, 2rystal5, Crjstal6, Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. Indcx. of ofractive prisms. L prism. 1 1 prism. 1 p ri sni . Zprisins. 1 prisiii. 1i gh t. Z , index. _____ .~ ~- I I---- Li 1'5488 - 1'54i9 - 1'5479 1.5483 1.5482 (1 1.5492 - 1.5485 - 1.5485 1.5487 1.5487 Vibrations NR 1.5519 - 1*5512 - 1-5512 15517 1'5515 parallel T1 1.5550 - 1*5542 - 1.5546 1.5551 1'5547 toaxis b. li' 1.5590 - 1'5580 - 1.5585 1.5589 1'5586 i G 1.5651 - 1.5638 - 1.5644 1.5649 1'5646 r Li 1.5505 1.5506 1.5497 1-5509 1'5495 1.5506 1*5504 P C 1.5512 1'5610 1.5503 1.5514 1*5503 1.5510 1'5509 Vibratioiis Na 1.5541 1'5539 1'5529 1'5542 1.5532 1.5536 1.5537 parallel I T1 1-55i4 1.5572 1.5562 1'5574 15565 15570 1-5570 toaxis TC. I F 1.5612 1-5612 1.5598 1'5614 1.5605 1.5610 1.5609 G 1.5673 - 1'5655 1.5672 1-5665 1.5670 1.5668 Li 1'5551 1.5550 - 1,5551 1'5545 - 1.5549 Y' C 1.5556 1'5554 - 1.555'7 1.5550 - 1'5554 Vibrations ' Na 1-5583 1'5579 - 1.5585 1'5580 - 1'5582 parallel ' T1 1.5618 1.5612 L 1-5616 1-5614 - 1'5615 to axis c. I F 1.5658 1'5650 L 1*5658 1.5653 - 1'5655 G 1.5717 - - 1.5717 1.5712 - -1'5715 View Article Online

NORMAL SELENATES OE' POTASSIUM, RUBIDIUM, AKD CIESIUM. $83

The intermediate index of refraction p, corrected to a vacuum, is accurately expressed for any wave-length as far nearly as F of the spectrum by the formula : 319 $80 2 917 500 000 000 p= 1.5416 +-- + 3- ..... A2 x4 The a indices are likewise closely reprclduced by the formula if the constant 1.5416 is diminished by 0.0022, and the y indices if the constant is increased by 0.0045, The fact that the former is the smaller difference determines that the double refraction is, as stated at the head of the paragraph concerning t h 3 optics of this salt, positive ; and the relatively slight difference of 0~0067-0*0069 between the extreme values, for any specific wave-length affords the rueawre of the feebleness of the double refraction. Alteration of Bejraction by Increase of Tenaperature.-Two additional prisms were prepared from the two halves of one and the same crystal, which was broken for the purpose; it was considered inadvisable to use the crystal unbroken, as the first heating of a crystal invariably very slightly alters the optical properties, and so the prism employed second, which would have been subjected to a previous heating, would not be likely to afford the most trustworthy results. The two prisms were provided with cover-glasses, to preserve the surfaces from the moisture of the atmosphere, but the minimum of balsam solution mas employed in their cementing. If this precaution is taken, it is found possible to maintain the prisms at 100' for an ample duration of time to enable trustworthy determinations of minimum deviation and the prism angle to be obtained for five wave-lengths ; repeated successive determinations of the prism angle under the circumstances show no Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. change. One prism afforded the indices a and p, and the other /3 and y. The two values of /3 thus obtained for 100' were practically identical. The following table exhibits the results of the determinations at 100" :

12efVmtive Indices OJ IZzcbidizcm Xelenate at 100". Diiiiinutioii froni Nature of Iudices at values at ordiimy Index. light . 100". temperature. i Li 1-5450 0,0032 a 'c 1.5455 32 Vibrations parallel Na 1*5478 37 to axis b. 1 T1 1 *5509 3s IF 1.5546 40 View Article Online

884 TUTTON : COMPARATIVE CRYSTALI,OGllhPHIChT, 8'L'Ul)Y Oh'

Diminution from N;ltLu.c of Iiitlices at values at ordinary Index. ligli t. 100". temperatures. P Li 1.5460 35 Vibrations parallel 1.5474 35 to axis a. 1.5497 40 1.5527 43 15564 45

) Li 1.5501 4s Y Ic 1.5506 48 Vibrations parallel ' Na 1.5529 53 to axis c. 1 T1 1,5560 55 JF 1.5597 5s The result of heating the crystals of rubidium selenate from the ordinary temperature (16-1 9") to 100" is thus seen to be a diminution of refractive power to the extent of 0.0032 to 0.0058 according to tho index and wave-length chosen for comparison. It will also be observed that the amount of diminution increases as the wave-length diminishes, and also increases at a rapidly growing rate from a to y, that is, as the absolute value of the refractive index increases, or, in other words, as the length of the axis of tlie optical indicatrix increases, the change being far away the largest for y, corresponding to vibrations parallel to the axis c. Further, the double refraction also diminishes, the difference between a and y for Na, light being 0,0067 at the ordinary temperature, and only 0.0051 at 100"; and finally, the dispersion slightly diminishes, the amount between Li and F being 0*0104-0~0106 at the ordinary temperature, and only 0 0095-04096 at 100". The order and disposition of the indices and the sign of the double refraction Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. remain unaltered.

A.ces of Optical Iqadicatrix aid of Opticnl Velocity EZlipsoitl. The relative lengths of the axes of the optical indicatrix, arranged in the order of the morphological axes, with whose directions they coincide, and calcuktted as described for the potassium salt, taking the length along the axis b as unity, are as follows. Pay At the ordinary temperature., . ' .. cc : b : c = 1.0014 : 1 : 1.0043. At 100" ...... C6 : b : C= 1.0012 : 1 : 1,0033. The axes of the optical velocity ellipsoid, the polar reciprocal of the indicatrix, are as under : b It r At the ordinary temperature...... CL : b : c = Oa'J986 : 1 : 0.9957. At 100" ...... CL : b : c = 0.9988 : 1 : 0.9967. The much greater effect of rise of temperature upon the refraction View Article Online

NORMAL SELENATES OF POTASSIUIU, 1tU f;IDIUM> AND CA3SIUM. 885

along the direction of the morphological axis c, and the diminution in double refraction brought about by the same cause, are very clearly exhibited by these ratios. NoleouZcw Opticcd Constants.-The following are the values of these constants, derived, by use of the formuls of Lorenz, and of Gladstone arid Dale, from the refractive indices and the densities previously given in this communication : Morphological axis. a. 6. C. 122 - 1 C O.OS19 0.0816 0,0825 Specific refraction, -y,- = it ...... (12' + 2)d iG 0.0838 0.0835 0.0844 ' pa2 - i 41 C 25.64 25.55 25.83 Molecular = 111 ...... n +2'2 r G 26.25 26.16 26-42 Specific dispersion, ITG-iYc ...... 0.0019 0.0019 0.0019 Molecular dispersion, ))tG--i)rc ...... 0.61 0.6 1 0 59 32 - 1 Molecular refraction, __ '11 ...... C 44.28 44.10 44.64 d 3feasurements of Optic Axial A12gle.-Four pairs of section-plates wcre employed in these measurements, all of them being of considerable thickness, varying from 2 to 3 millimetres; this is imperative in order to obtain well-defined interference figures showing very small inner rings round narrow and sharp hyperbolic axial brushes, on account of the very feeble double refraction. Each pair of sections were prepared on one and the same crystal, so that the calculated 2Va derived from each particular pair represents the actual true optic axial angle in that individual crystal. The results derived from the four crystals proved to be very close to each other. The plates perpendicular to the first median line were in every case prepared by

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. grinding two surfaces parallel to the basal plane c(001}, and those perpendicular to the second median line were in some cases formed by the natural faces of the brachypinacoid h(010), arid in others were prepared by grinding two surfaces parallel to the direction of the brachypinacoid. Large crystals had naturally to be selected for the purpose, and fortunately suitable crystals with large development parallel to the brachypinacoid, and yet of ample thickness, are readily obtainable. Crystal 1, crystals 2 and 3, and crystal 4, belonged to thee distinct crops.

Determination of Apparent Angle in Air of Eubidiunz. Eeelewate. Nature of light. Section 1. Section 2. Section 3. Section 4. Mean 2E. Li 120' 10 121' 26' 119' 15' 120' 50' 120" 25' C 120 20 121 32 119 25 130 58 120 34 Na 121 3 122 0 120 11 122 5 121 20 T1 121 48 122 23 121 0 123 0 122 3 P 122 40 122 50 121 50 123 55 122 49 View Article Online

Nature of Mean light. perp. 1st j valncs of pcrp. 2nd 2Va. inedian 2Ha. 1 iiiediaii 2Va. liiic

I ~ ___ ~ ' 64" 31' In 1 101" 14' 63 53 2tc I Li ' 100 48 68"56' j 63 48 ! 3t6 1 101 3

I 63 63 ~ 4rL 101 33 I I 64 30 1(6 101 9 63 48 2 (6 100 41 c 63 -1-1 3n 100 55 68 55 63 49 4n 101 27

64 20 1cc 100 55 63 30 2n 100 12 x

The immersion liquid employed for the measurements of 2Ha and 2Ho was monobrornonaphtlialene. The two series of measurements Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. in this liquid were carried out in immediate succession, at the same sitting, using the same cell of liquid, in order to avoid all errors due to a difference in the refractive power of the liquid on the two occasions. The dispersion indicated by the calculated values of 2Va for the vnrious wave-lengths will be observed to be very slight, so slight, indeed, that a small error in the determinations of 2Ha and 2Ho would be capable of reversing the sign. The fact that all four crystals indicate the same order of dispersion is probably sufficient proof of its accuracy. In order, however, to confirm it, search was made for a liquid possessing the same refractive power as the crystal, in order that, by immersion therein, the optic axes might be observed separated at their true angle, and the true dispersion might thus be actually demonstrated. Rectified oil of anis answers the purpose admirably, its refractive index for sodium light being 1.554, almost exactly the mean refractive index of the crystal for light of the same nature. Section 4 was, therefore, immersed in pure oil of anis, and examined View Article Online

NORMALSELEN~TES OF POTASSIUM, HUUIDIURI, AND CBSIUM. SS7

first in white light, when it was observed that the beautifully defined hyperbolic brushes were faintly, but distinctly, coloured red inside and blue outside, indicating a larger true angle of the optic axes for red than for blue, and that the difference for the two ends of the spectrum wvas but small. Subsequent careful measurements for the bright C and T1 light afforded 68" 36' as the angle for C, and 68" 32' for TI, exactly the same order and amount of dispersion as is indicated by the mean of the calculated values. That the indications thus obtained are trustworthy is apparent from the fact that the angle observed in the oil of anis is very near the calculated true value, differing from the latter only by 20' ; for this proves the nearness of the refractive powers of the oil and the crystal, and consequently also the suitability of the former for ZL confirmatory experiment of this description. Hence there can be no doubt that the order of dispersion is p>v. The positive nature of the double refraction was confirmed by the quarter undulation mica test. Bfect of Rise of Tenzpewcture on the Optic Axial Angle.-Section 4 was heated to 115", and the apparent angle in air measured at this temperature. It mas found to be 128" 10' for sodium light, the dispersion for other wave-lengths remaining about the same as at the ordinary temperature. Hence 2E increases 6" for 100" rise of temperature. After cooling again to the ordinary temperature, the angle was found to be 129" 25', 01" 20' larger than when previously measured at the ordinary temperature, indicating that the heating to so high a temperature as 115" induces a permanent increase of the optic axial angle. The above fact, that the optic axial angle becomes greater as the temperature of the crystal is raised, is in complete conformity with the results of the measurements of the refraction at 100"; for the Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. greater reducing effect upon the c axis, referred to in the consideration of those results, has the effect of causing a relatively closer approximation of the refraction for this axis, which corresponds to the y index, to that for the a axis, corresponding to the p value, and while in- sufficient to bring these two indices, y and p, as near together as the a and p values, beyond which a change of sign of the double refraction would occur, together with a reversal of the median lines, it is still sufficient to materially increase the optic axial angle, as is actually found to be the case.

Optical Properties of Cmsium Xelenate.

Cmium selenate is a salt whose crystals are endowed with exceptionally interesting optical properties. So different is it in many respects from its rubidium and potassium analogues, that it would appear at first to possess little in common with them. The plana of its optic axes View Article Online

888 TUTTON : COMPAKATIV~ CliYSTALLOGRAPHICAL STUDY OF

is different, namely, tlie basal plane c{OOl}. The first median Zinc is the morphological axis 6. The double yef?*action, moreover, is reversed, being negative ; and it is so extraordinarily feeble that a section-plate no less than six inillimetres thick is necessary in order to generate well-defined interference figures in convergent polarised light. The arrangement of the axes of the optical indicatrix and of thc optical velocity ellipsoid, at the ordinary temperature, is as follows : The direction of the morphological axis b is that of the minimum axis a of the optical indicatrix, and that of the maximum axis a of the optical velocity ellipsoid. The direction of the morphological axis c is that of the intermediate axis p of the optical indicatrix, and that of the intermediate axis b- of the optical velocity ellipsoid. The direction of the morphological axis a is that of the maximum axis y of the optical indicatrix, and that of the minimum axis c of the optical velocity ellipsoid. Detevnzinutions of Refractive Indices.-The crystals of casium selenate are so extremely deliquescent that somewhat formidable difficulties were experienced in manipulating them. Unless efficiently protected by a coating of balsam or varnish, a freshly made prism will rapidly proceed to dissolve in moisture attracted from the atmosphere, and become replaced upon the crystal holder by a globule of solution. By taking advantage, however, of a prolonged spell of fine, dry, cold weather, and employing all the precautions enumerated in the description of the work on rubidium selenate, using somewhat larger crystah than usual, which can readily be grown of great perfection, nine suitable prisms were eventually obtained. The ground surfaces were in each case protected by cover-glasses cemented with the minimum Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. quantity of balsam, and the whole of the exposed part of the crystal was coated with balsam solution, which was allowed to harden by placing the crystal, cemented by hard optician's wax upon its holder, for 24 hours within a spacious desiccator. It was then found possible to carry out the measurements of the prism angle and minimum deviation with the usual deliberation, no deliquescence being possible for at least the space of an hour after removal from the desiccator. The results of the measurements are given in the accompanying table. Crystal 5 was a large individual which was broken into two halves, and a prism ground upon each half, in complementary directions. It was only found possible to grind one prism on each of the other crystals, which were unsuitable for breaking, being as broad as long, on account of deterioration of the natural reference faces by deliquescence ; but the results given in the first three columns were obtained with pairs of siinilar crystals selected from the same crop, and marked 1 and la, 2 and 2a, and 3 and 3a, as the refraction of View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUJI. 880 Detemzinnt ions of RQJwctiae- Indices - of- Ccesizcm - Seleuate. - Nature Crys tds Crystals Crystals Mean Yrystal4, 3ystal I;, Index, of 1 and la, ? and 2a, 1 and 3cc, efractive 1 prism. 1 prisms. light. 2 prisms. 1 prisms ? prisms. iudex.

Li 1.5951 1'5949 1.5945 - 1.5953 1-5950 a C i *5956 1*5954 1.5953 - 1.5958 1-5955 Vibration: Na 1-5988 1.5987 1.5987 - 1'5993 1'5989 parallel T1 1'602G 1'6022 1*6022 __ 1.6028 1-6024 to axis b. F 1.6071 1'6068 1.6067 - 1-6073 1'6070 G 1.6139 1'6135 1.6135 - 1'6141 1.6 138 Li 1.5958 15959 1-5957 1'5965 1.5962 1'5960 s C 1'5963 1'5964 1.5962 1'5970 1.5967 1.5965 Vibration: Na 1.5997 1.5995 1.5995 1.6005 1.6003 1'5999 parallel TI 1*6033 1.6030 1-6031 1'6038 1'6038 1.6034 to axis c. F 1.6080 1'6076 1.6076 1.6083 1.6083 1'6080 G 1.6148 1.6145 1-6144 1'6151 1'6150 1-6148 Li 1.5961 1'5963 1-5961 1.5968 1.5967 1.5964 Y C 1.5966 1.5968 1'5966 1'5973 1.5972 1.5969 Vibration; Na 1'6001 1.5998 1.5999 1.6008 1.6007 1.6003 parallel T1 1.6038 1%035 1'6034 1'6043 1.6042 1'6038 to axis a. F 1$084 1.6081 1.6079 1'6088 1.6087 1'6084 G 1.615 2 1'6149 1-6147 1.6156 1.6155 1'6152

crystals belonging to one and the same crop is usually found to be practically identical, as was actually observed in the cases now under consideration. 'Upon investigating the prisms, it was at once seen that the character of the refraction was quite different to that of the two salts previously described. The directions of the p and y vibrations are reversed, and Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. the two images of the signal afforded by each of the prisms lay extraordinarily near to each other, indicating extremely feeble double refraction. Great care was taken to be quite certain of the orientation of each of the prisms during its preparation, two or three zones on the crystal being first measured on the goniometer of the grinding instrument in order to know exactly the orientation of the various faces, this being essential as several of the zones are so similar, and czsium selenate grows in numerous kinds of habits besides the prevailing one, often with the faces of secondary importance unduly developed. Hence there is absolutely no doubt about the peculiar disposition of the axes of the optical ellipsoid. The directions of vibration of four of the prisms were parallel to the axes a and c, and each of these prisms exhibited a pair of images so close together as to overlap and appear by non-polarised monochromatic light as a single image rather broader. than usual. Upon introducing the nicol in front of the eye-piece, it was observed that the apparent single image was indeed two images, View Article Online

800 TUTTON : COMPARATIVE CRYSTAT,LOGRAPHICAL STUDY OF

one extinguishing at 0' and the other at 90" of the nicol. The separation of the two was only between two and three minutes of arc. Moreover, the separation of the images afforded by prisms whose directions of vibration mere parallel to the axes b and c was only between four and five minutes of arc, and that of the images corresponding to vibrations along a and 6, the most widely different in velocity, affording respectively the indices y and a, less than eight minutes. This maximum difference corresponds, as is shown by the table, to a difference between a and y of only 0.0014, ari exceptionally small amount of donble refraction. The intermediate index of refraction /3, corrected to a vacuum, is accurately represented by the following formula from the extreme red to beyond T1 green. 792 056 3514 600 000 000 /3= 1.5804 -I- -- - 7 h4 + '.''..

The a indices are equally well re-produced by the formula if the constant 1.5804 is diminished by 0.0010, and the y indices if it is increased by 0*0004, The fact of the latter number being the smaller of the two is in accordance with the previous statement that the sign of double refraction of caesium selenate is negative, differing in this respect from its rubidium and potassium analogues. Alteration of Refraction by Increase of Ternperature.--Four excellent prisms were employed for determinations of refraction at loo", and extremely interesting and exceptional phenomena observed with them. Two afforded vibrations parallel to the axes a and c, one gave vibrations parallel a and b, and another vibrations parallel b and c. Those whose directions were parallel to a and c, the images corresponding Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. to which two directions are only separated by 2-3' at the ordinary temperature, showed a gradually increasing separation of the two images as the temperature rose. At 55O, it was 6', and at 100" it had attained S', corresponding to a difference of 0.0014 in refractive index. At the same time, the minimum deviation of each had largely decreased. The one affording vibrations parallel a and 6 showed a slight decrease in the amount of separation of the two images at 100" compared with their relations at the ordinary temperature ; both images, however, moved considerably nearer to the direction of the incident ray. The phenomena exhibited by the cther prism whose vibration directions were parallel 6 and c were particularly interesting. At the ordinary temperature, the separation of the two images was 4-45', and the image corresponding to b was nearest the incident ray, in accordance with the fact of its affording the a index. Upon heating, the images moved bodily nearer to the incident ray and also at the same time nearer to each other, until at 80-90" they coincided with each other; View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CiESICM. 891

on continuing the heating, they passed each other, and at 100" they were separated by about 2', with the image corresponding to c now nearest to the direction of the incident ray. Hence the effect of raising the temperature! to 100" is to diminish the general refraction and to reverse the relations of the refractions along the b and c axes; at an intermediate temperature in the neighbowhood of 90", the refractions along these two axes are identical, or, in other words, at this temperature the crystal exhibits uniaxial optical properties. The actual values of the refractive indices at 100' derived from the four prisms are given in the accompanying table :

Refractive Indices of Cesium SeZencc5e at 100'. Diminution from values along same Index. Nature of liglit Indices at 100". axis at ord. temp. Li 1.5899 0.0061 a 1.5904 61 Vibrations parallel to 1.5939 60 axis c. 1.59'75 59 1.6021 59 1.5901 49 P 1.5906 49 Vibrations pzrnllel to 1,5941 48 axis 6. 1.5977 47 1a6023 47 ,Li 1,5913 51 1.5915 51 VibrationsY parallel to ICNa 1.5953 50 cc. Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. axis I T1 1-5989 49 iF 1m6035 49

The actual amount of the diminution of refraction on raising the temperature of the crystal to 100" is thus seen to vary from 0.0047 to 0.0061, according to the axis and wave-length chosen for comparison, The amount of the diminution, however, does not vary as the absolute value of the refractive index, as in the potassium and rubidium salts, the order of the morphological axes to which the indices correspond being different in the caesium salt; but, which is a most significant fact, the amount varies precisely similarly to what it does in those salts with respect to the morphological axes, and therefore with respect to the axes of the optical indicatrix with which the former axes are, in accordance with rhombic symmetry, identical. The least amount corresponds to the axis b, the next, and not very much larger amount, corresponds to the axis cc, while the largest, and by much the largest, amount corresponds to the axis c. It is precisely this more rapid rate View Article Online

892 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

of change in the velocity of vibration along the c axis which produces its remarkable approach to the value along the b axis, the fact of its attaining equdity with the latter, and its subsequent overstepping thereof with the interesting attendant consequences. One of these consequences is that the negative double refraction of the ordinary temperature becomes reversed to positive, the a and p values being closest at 100". As regards the amount of the double refraction, this attains a minimum at the temperature (90") at which the crystal becomes pseudo-uniaxial, being then less by about 0*0002 than at the ordinary temperature; beyond this, it again increases owing to the rapidly diminishing c index, until at 100" it is again equal to the amount of double refraction at the ordinary temperature, 0.0014, beyond which it mill doubtless continue to increase.

Axes of Optical Indicatrix and of Optical VeZocitp ElZipsoid. The axes of the optical indicatrix, calculated as described for the potassium salt, are given below : rap At the ordinary temperature ...... a : b : c = 1.0009 : 1 : 1.0006, rBa At 100'...... a : 6 : G = 1*0008 : 1 : 0.9999. The axes of the optical velocity ellipsoid arc as under : t u Ir At the ordinary temperature ...... a : b : c = 0.9991 : 1 : 0,9994. c xr ;I At 100"...... : b : C= 0.9992 : 1 : 1*0001.

These ratios show very clearly the reversal of the a and ,8 indices Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. and of the 8 and Xr velocities by raising the temperature to loo", and the numerical extent of the change both as regards this phenomenon and as regards the double refraction ; the relatively much larger change along the axis c is strikingly apparent. ,Voleculccr Optical Constants.-These constants are given in the accom- p.Lnying table :

Morphological axis. CC z, C n2 - 1 C 0.0765 0.0761 0.0765 Specific refraction, ~ - =n ...... (n2+ 2)d G 0.07S4 0.0783 0.0784 n2 - I .M- 31.19 31.23 Molecular refraction, - ~ - ---in .. { E i;::: n2+2 d 31.96 32.00

Specific dispersion, irG - iic ...... 0~0019 0.0019 oooo19 Molecular dispersion, inc - inc...... 0.77 0.77 0.77 n- 1 Molecular refraction, A1 ...... C 54.76 54.63 54.72 d View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND C~SIUN. 893

Measurements of Optic Axial Angle.-Five pairs of excellent sections were prepared and employed in these measurements. The term section- plate would be here n misnomer, as +hetwo parallel surfaces require to be at least half a centimetre apart, owing to the exceptionally feeble double refraction, and the best of the sections perpendicular to the first median line were from 6 millimetres to 1 centimetre thick; this is, of course, a very considerable thickness compared with the largest size of crystals obtained from solutions of so extensively soluble and expensive a salt. The two pairsof surfaceswere in the first three cases prepared upon the same crystal, and in the other two upoii crystals belonging to the same crop. The measurements of 2E, 2Ha, and 2Ho were invariably carried out at the same sitting, to avoid changes in the refraction of the immersion liquid. The optic axial angle mas found to differ slightly in the different crystals derived from different crops, a property likewise exhibited by several of the sulphates previously described which showed very slight double refraction, n 17ery minute dif-ference in refraction in different directions in such cases determining relatively large differences of optic axial angle. The apparent angle in air is very large, and difficult to measure properly with such thick sections, whose breadth also requires to be very considerable. Trustworthy results were only furnished by the three crystals 3, 4, and 5. The results are given in the first of the accompanying tables.

Determination of Appa~entAngle in Air of CQS~UNLSdenate.

3. 4. Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. Nature of light. Soctiou Sectioii Section 5, Mean 2E. Li 134" 10' 138" 50' 131" 50' 135" 0' C 133 50 138 30 131 30 134 40 Na 131 40 136 10 129 0 132 20 T1 130 0 134 30 127 35 130 40 F 128 0 132 0 125 10 128 20

The dispersion is seen to be somewhat considerable. The hyperbolic brushes were fairly sharp in monochromatic light with the thick sections employed, but in white light they consisted of broad rainbow bands, and the figures, which showed several complete rings round the axes, were very beautiful. The results of the measurements of 2Ha and 2Ho in bromonaphthalene are given in the second of the tables. The figures were very sharply defined and readily measured in monochromatic light. The negativc; sign of double refraction was confirmed by the indications afforded or) the introduction of a quarter undulation mica plate, when observing VOL. LXXI. 3 0 View Article Online

804 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

the figures given by several of the sections perpendicular to the first median line.

Deternaiizntioia of true Optic i1:cictZ Angle of Cmsium Selenate.

No. of No. of Nature of section Observed section 0bserved 2alcuIated perp. 1st values of Mean light. imp. 2nd values of values of 2 Va. median 2Ha. median 2Ho. 2Va. line. line.

68" 35' 1n 100" 10' 72" 36' 71 50 2cC 99 16 75 11 Li 69 30 3 CL 101 34 72 41 73" 29' 74 55 4n 103 30 75 30 68 37 5a 103 10 71 28 68 25 la 100 22 72 24 I $1 30 2a 99 42 C 69 8 3CL 101 43 72 22 73 7 $4 28 4n 103 57 68 0 5n 103 20 70 58 67 30 In 101 25 69 46 2a 100 29 Na 67 45 3CL 102 44 71 I 71 49 $3 5 4n 105 3 73 66 25 5n 104 26 69 2846 ,( 66 44 In 102 15 70 28 68 15 2n 101 23 71 53 T1 65 37 3n 103 31 69 70 18 71 20 4n 106 21 72 8 64 40 5n 105 30 67 4812 65 ti0 In 102 54 1

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. 66 30 2n 102 20 F 64 0 3a 104 30 67 40 68 58 70 40 4a 107 30 71 18 62 40 5a 106 30 65 58

E'ect of Rise of Temperutzcre on the Optic Axiul Angle.-The effect of raising the temperature to 100' upon the refraction phenomena, consisting in the movement of the intermediate refractive index p from its proximity to the y index, through the middle position, nearer and nearer to the a index, the accompanying change of the negative double refraction to positive, the attainment about 90" of identity of p and a, and the subsequent transposition of these indices, the old a index being passed and henceforth becoming p, and the index which was hitherto p then becoming a, indicated that exceptionally interesting interference phenomena in convergent polarised light might be expected to follow the same t'hermal change. Before testing any of the sections practically, the exact nature of the phenomena to be anticipated were View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM; AND CBSIUM. 895

thought out. On examining a section perpendicular to the first median line, the optic axial angle ought to increase, until it exceeds 90" and passes altogether out of the field of view. If the angle in air is being observed, as usual with the air-bath heating apparatus, it is already very large and the hyperbole should proceed at once to pass out of the field of the instrument. If, then, a section perpendicular to the second median line (that is, second at the ordinary temperature) be examined under rising temperature, the hyperbolae at first ought to be invisible, but as the temperature rises they should soon make their appearance at the margin of the field, and then continue to approach along the horizontal diameter, nearer and nearer until about 90" they should coalesce in the centre and produce the uniaxial cross, surrounded by circular rings. Beyond this temperature, they should separate in the vertical plane, and eventually, past loo", pass out of this field of view and become visible through a pair of c(OO1] faces. Upon proceeding to test these theoretical deductions from the refraction observations practically, they were found to be precisely in accordance with fact. On examining sections perpendicular to b, the first median line, and consequently parallel to the 6(010} faces, the hyperbolic brushes are observed almost immediately on warming to pass right out of the field and become invisible, the last angle measurable verging on 160". Sections 2a, 3a, 4n, and 5a respect,ively, parallel to a(100i and therefore perpendicular to a, at ordinary temperature the second median line, were then slowly heated and the phenomena carefully followed. They were suspended just as when observing 2Wo in bromonaphthalene, the air-bath heating apparatus replacing the cell of the latter. Very soon, below 50°, the hyperbolic brushes became visible, at the extremities of the horizontal diameter. At 65", they were sepa- Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. rated 89" for C and 98' for T1. They then approached each other as the temperature rose, until at about 85" they were separated 18' for C and -32" for T1. At 90°, they were crossed for C, the cross being surrounded by several circular rings. The cross was then produced for the other wave-lengths in succession, until about 96" it was produced for F. At looo, the brushes were separated for all colours in the vertical plane, very widely for the red. Beyond this, and under 150", they passed along the vertical diameter out of the field altogether. The temperatures given me corrected for the slight conduction of heat from the crystal by the platinum holder, which, being in firm contact with the adjusting apparatus from which it is suspended, introduces a considerable error. The correction was determined by replacing the crystal in the holder by the bulb of a small thermometer, and repeating the heating; it amounted to as much as 10". The indications of the two bath thermometers were thus about 10" higher than the real temperature which the crystal attained. The corrected temperatures agree with 302 View Article Online

896 TUrTON: CONPARATIVE C!RYSTALLOGRAPH[CAL STUDY OF

those of the corresponding refraction phenomena. The temperature error in determining the latter with the aid of the most recent form of Puess apparatus, in which the crystal is supported on a glass holder, is extremely slight. During the heating of the sections perpendicular to what is ordinarily the second median line, and somewhere between 60' and SO', the sign of double refraction was tested by the quarter undulation mica, the two nicols being placed respectively horizontal and vertical for the moment ; the fact of the double refraction being now positive was proved by the indications afforded. After the conclusion of these experiments, the author was fortunate in obtaining a magnificent crystal of cmsium selenate of more considerable thickness parallel to the axis a, and of correspondingly greater dimensions in the other directions. When the cb{100} faces were ground down further than they were developed, so as to form two broad terminating parallel surfaces, and thus obtain a large field in spite of the great thickness, the section-plate thus obtained was no less than 12 millimetres thick, and of about the same breadth. With this fine section, the interference figures obtained on heating to 50-150" were beautifully clear and well-defined, showing numerous rings, the inner ones small. At 82" (corrected), the following angles were observed :

2E for Cs,SeO, at 82" with section 12 mm. thick parallel cc{ loo}. Li ...... 30" 35' C ...... 31 25 Na ...... 37 25 T1 ...... 43 30 F ...... 51 3 On allowing the temperature to rise further very slowly, the uniaxial Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. cross was observed to be produced for the various wave-lengths at the following temperatures (corrected) : Temperature for procinctioii of cross. Li ...... 92O c ...... 93 Na ...... 94'6 T1 ...... 96 P ...... 98 After the production of the cross for each of the wave-lengths in turn, the arms opened and again formed hyperbolic brushes separated vertically over each other, as with the other similar sections, and at 150' the optic axes, even for the extreme blue, had passed out of the field at the extremities of the vertical diameter. Finally, a similarly suitable section-plate parallel to the third axial plane c(001) was obtained, and observed at higher temperatures. No View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CA3SIUM. 8:jy

trace of an interference fignre was observed up to 150". When this temperature, however, was passed lemniscates began to make their appearance, and as the temperature still rose, eventually rings became visible, first one, and then another and another, until at 280' four rings were visible round each optic axis, and the axes themselves, represented by their hyperbolic brushes, were clearly visible on the margin of the field, and exhibited slow movement towards the centre of the field with each further increment of temperature. Moreover, the double refraction was now again negative. The observations were not continued further, on account of the great risk to the valuable polariscope. Hence within this range of 280' of temperature czsium selenatts exhibits the remarkable phenomenon that its first median line is parallel to each of the three axes of the optical indicatrix in turn, first B, then CC, and finally c ; moreover, the plane of the optic axes crosses from c{OOl) to b{010},the crossing at about 95" being of course accompanied by the temporary assumption of uniaxial optical properties ; and further, the double refraction changes, first from negative to positive, and then again from positive to negative. The actual angular change in the mutual positions of the optic axes between these limits of temperature is over 200". From theoretical considerations, based upon the relative rates of change in the optical velocity along the three axial directions, it is possible to follow the changes to still higher temperatures. The fore- cast derived from such considerations with regard to temperatures actually experimented with was so completely verified by the phenomena actually observed, that there can be little doubt as to the accuracy of such deductions concerning higher temperatures below the melting point. The rapid changes between the ordinary temperature Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. and 100" were due to the swift approach of the intermediate value along c to that along the minimum b, owing to the much greater diminution of refraction along the former direction, and the subsequent transposition by c passing behind b, and becoming the minunium, the extent of movement being measured by the change of 0.0007 in the c axial ratio OP the optical indicatrix Meanwhile, the maximum a had approached b only to the extent 04001. At 2SOo, the mininiuin c, now the direction of the first median line, is about O.pOl2 removed from the intermediate b, and the latter 0.0006 from the maximum c. If the change goes on continuously, at 400" to 500" higher, that is, about 750°, the uniaxial cross and circular rings should again be produced round c as optic axis, the difference between cc and b being reduced to zero, and c being far removed from them. Above this temperature, the plane of the optic axes will change to ccr\lOO), and thus the optic axes will have occupied all three principal planes of the optical indicatrix in turn. It is evident, however, that the changes become slower View Article Online

898 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

as the temperature rises, after once the most rapidly altering c value has passed that corresponding to b, that is, after the first crossing of the optic axial planes and production, for the first time, of the uniaxial interference figure ; the second simulation of uuiaxial properties must occur at a relatively much higher temperature, being due to the relatively more slowly approaching a and b values.

Conapa&on of the Optiaul Froperties. The optical orientation of the three salts is as follows :

For K,%eO,, a is p, b is a, c is y. ,, Rb,SeO,, a is p, b is a, c is y. ,, Cs,SeO,, a is y, b is a, c is p. The plane of the optic axes is also identical for the first two salts, a(100}, and different, namely c{OOl), for the cmesium salt. The first median line for potassium and rubidium selenates is also identical, being the axis c, and is the axis 6 for the caesium salt. The double refraction is positive for the potassium and rubidium salts, and negative in the case of czesium selenate. It will thus be observed that there is a reversal of the optical character between the rubidium and caesium salts. It will be shown in the sequel that this is the consequence of a progression in the optical constants, following the progression in the atomic weight of the metal present in the salt. The comparison of the optical constants will at once be proceeded with.

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. Refractive Indices.-The accompanying table exhibits the refractive indices of the three salts, arranged so that those for the same morphological axial direction are side by side. It is clear from an inspection of this table that- The refractive indices of rubidium selenate are intermediate between those of the selenates of potassium and ccesiunz, and neayest to those of the potassium salt. This fact is equally true if the indices for the higher temperature of 100' are compared. Perhaps the best mode of obtaining a definite idea of the relationship may be obtained by choosing the wave-length of sodium light, and taking the- mean of all three indices of each salt for that wave-length. The numbers obtained are as follows : Mean refraction for sodium light of K,SeO,. .. . .,15396

99 9, Rb,SeO, .. .1-5545Difference 14'

9, 7, Cs,SeO, ...1.599'7 I) 452 View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CESIUM. 899

Comparative Yuble of Refractive Indices of the three Selemtes.

Axial direction. Nature of Rb,SeO,. Cs,Se04. light. Ii,SeO,.

f Li 1.5357 1-5504 15964 C 1.5362 1'5509 1'5969 Na 1-5390 1.5537 1-6003 a T1 1*5421 1*5570 1.6038 F 1.5460 1.5609 1.6084

\ G 1.5517 1-5668 1.6152 Li 1.5320 1'5482 1'5950 c 1*5325 1*5487 1.5955 1'5515 15989 b Na 15352 T1 1-5383 1.5547 1.6024 F 15421 1-5586 1.6070 G 1'5478 1'5646 1.6138 Li 1.5413 1*5549 1.5960 C 1*5418 1.5554 1.5965 Na, 15446 15582 1*5999 T1 1.5478 1$61 5 1*6034 F 1,5518 1.5655 1*6080 G 1.5576 1.5715 1'6148

The result is almost the same if the p indices for sodium light are compared, the differences being 147 and 462. The relationship of the differences is thus about 1 : 3. The following further conclusion concerning the refractive indices may therefore be appended. An increase in refraction is observed to accompany an increase in the atomic weight of the alhd metal, and increase 6econzes relativelp Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. this greater as thatomic weight rises, than in mere aritlmzetical proportion to that rise. The amounts of incrense in the refractive indices, due respectively to the repkacement of potussiurn by rubidium, and of t?Le latter b? cmsiurn, are appoxirncctely as 1 : 3. AxialReZationships of the Optical Indicatrix or Optical Velocity Elli$soid. -The difference of refractive power along different directions in the crystals of the three salts is very clearly indicated by the relative values of the three rectangular axes of the optical indicatrix, or its polar reciprocal the optical velocity ellipsoid previously defined. These axial values are compared below, the order in which they are taken being that of the morphological axes a, 6, c, with whose directions they are identical (see next page). These ratios exhibit one aspect of the change produced by changing the metal, namely, the effect of the total change upon the mutual reln- tions of the values along the three axial directions, that along the 6 axis being considered as unity, But they do not express the total View Article Online

900 TUTTON : COMPARATlVE C!RYSTAT,LOQRAPIIICAL STUDY OF

Ases of optical Ascs of optical vc~lnritg Salt. iiidicatris. cllipsoicl. nbc n 7, c Pay h X t I<,SeO,...... :.0025 : 1 : 1.0061 0.9975 : 1 : 0.9939 B 11 a y 1s b 11 (I t 18 Rb,SeO, ...... 1 a0014 : 1 : 1.0043 0*998G : 1 : 0.9957 y5 UP37 t I, rr ll 37 Cs,SeO, ...... 1*0009: 1 : 1.oooci 0.9991 : 1 : 0.9994

change itself. This, however, can be done either by means of the refractive indices for sone particular tvave-length, or, more clearly, by a similar set of ratios to the above in which the values for the rubidium and czsiuni salts are not obtained, as they are above, by comparing the value along each axis with the value along the b axis of that same salt, but by comparing the values along the three directions of all three salts with the 6 ~al~efor potassium selenate. The ratios thus obtained are set forth below :

Asea of optical Axes of optical velocity Salt. indicatrix. el lipsoid. I(,SeO, ...... 1.0025 : 1 : 1.0061 0,9975 : 1 : 0.9939 95 106 89 94 105 86 Rb,SeO, ...... 1.0120 : 1.0106 : 1.0150 0.9881 : 0.9895 :0.9853 804 309 271 285 293 257 Cs,SeO, ...... 1.0424 : 1.0415 : 1.0421 0.9593 :0.9602 : 0.9596

From these latter ratios. the following facts are apparent. The axid values of the olitical iizdicatrix, or of its polar reciprocal tJLe Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. opticcil tdocity ellipsoid, j’oi- rtcbidiunz selenate we internaediate between those for potnssiicin ant1 cesium seleqzates, whatever direction in the crp- tuZs is chosen for compct&on, the d?$ere?aces between d7fei*e?zt salts being Zcirge coiizpvetl with, the ihyerence between the two extvenze values for dierent direciions iyb the crystals of an9 salt. TVie.ePa a~~cclogousdii-ections a!re compared, it is observed tliut the vcclues for the s*ubidiunz salt lie nearer to the potassium tlmn to the cmiurn salt in the propo~tio12of 1 : 3. IIence ?jr the optical iiaclicui!i.ices (fthe three sn7ts zuwe constructed about the sume origirz, using rectccngular arin2 co-oidinates, the incliccttrix for the ccesium salt would contain zuitliin it tlmt for tlie rubidium salt, and this agccin would conteiin tlie indicatrix of the potassium salt, and the two latter ellipsoidccl surfaces zvou,ld be separated by only oi~e-thirdthe distcciace wl~icliwould sepccrccte the two former outer suifccces. Similarly as regards the optical velocitg ellipsoids, ezcept that in this cue the outer suqface zvozclcl be that for tlie potctssiuna snlt, and the ellilisoid corresponding to cesium selenccte would be the innerntost. View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CflSIURI. 901

It will be advisable at this point to compare the refractive indices of the selenates with those of the sulphates, to ascertain the effect of replacing sulphur by selenium. This can most concisely be clone by comparing the mean values of a11 three indices of each of the sulphates for sodium light with the analogous values just given for the selenates. For the sulphates they are as under :

K,SO,. .. ,. . 1.4952 Difference from K,SeO,...... 0.0444 RTSO, ... 1.5136 Difference 18* ), l%b,SeO, ... 0.0401, 499 " Cs,S04 ... 1.5635 " >? ), Cs,SeO, ... 0.0362 It will be observed that The replucement of szdpJmr by selenium in this series of salts is CLCCOI)~L- pccniecl bg an incrense in tJLe qsefriiet ive index, SUCJL inci*e(cse, hoxiever, tliminisleing in ccnaount CLS the ~10miczoei;jlLt of the nlkrili nretnl, mrl therefore the weight of the initicil iizolecde, rises. The wJrnctioia dntioizs observed between !lie tlwee selenntes ccre 2wecisely pnrallel to tllose found foi. the sudpliates. The increase in the dimensions of the opticnl indicutiix, or tlie cor9.e- spo?a&?ag decreccse of the optical velocity e&$soid, bi-ozrght about by I'E- pltccing the alkali metal by one of IuigJLer cbtontic weiglit, occurs to necwly the same extent along the directions of the 7~3o?phOlOgiCdaxes a ccnc2 b, htto CL less extent alony the axis c. The effect of this last fact upon the mutual relations of the opticnl nxinl values of each particular salt is most clearly shown by the first series of ratios, from which the following additional factsare apparent. The mplncenzent of one cilkccli metal by mother of hiyhei- ntornic weight is cicconqmnied by a cliiiziiazct ion of the double reJrnction, zohich cilq*entlyin the potccssien~i sdt is vem~feeble. Ilence tAe ccwium salt exhibits this Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. poperty in ccn exceptioiznlly feeble mciianer, tlie ii$eyence betweew tlt e two extreme cczic~lvalues of the iladicntrix OT velocitp ellipsoid being less t7~m one iia cc thousrcncl ;ci sectionylute cc cehmeti'e tliick is coiasepueiitly r~yuivedto exhibit c.5 well-deJined in,ter.fei*eiLceji&.we in stro?ylyconceigeiit pol a yised l igh t. In this coqavergence of the thee cixinl vci Zzces towcmls unity, the In.og?-ess of the c valzte is m2ccI~ more rccpid ~h~inti'int of tlie otlier two, ccn @ect upon the nzuturil relations of the vcilues for myone scilt wlbicJL is clue to the less total chn?zge in the dinzeirsions of either ellipsoid dong this direc- lion previously renaccrked. For the ellipsoicl is rapidly growing or diminishing with the rise of atoiuic weight (according as the indicatrix or its polar reciprocal is chosen for observation) along the directions cc and b and less rapidly along c, but the latter direction is, in the initial ellipsoid (for the potassium salt), already that of the maximum axis ; hence a slower growth along this direction allows the more rapidly movingyvitlues for the other axes to more or less overtake it, thus View Article Online

902 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

bringing all three nearer to equality. As, however, the other two more nearly equally rapidly, their re!ation is but slighty altered, whereas their rapid movement compared with c brings about a considemblc change in therelation of tlie latter to them. The more rccpicl appoacl~towards unit9 of the axial vcclue along the ueTtical axis c, has the interesting efect, on attuining the cmium salt, of reversing the sign of the double refraction from positive to negatire. The c value is no longer that of the maximum or minimum axis (according to the ellipsoid used) but that of the intermediate axis, having reached equality with the a value and then overstepped it; and this new intermediate axis c is nearer in value to that along a, whereas, previously, the axis a, which had been the intermediate one, bad most nearly approximated in value to 6.

FIG. 10. Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11.

All these clmnges tire strictly progressive in character, tlie ru6idium salt invariably occupying an intermediate position. The yeversal of the sign of the double rpfraction in the cesium salt, and its renzarku6Zy feeble dis- pluy of double refraction, are both necessary consequences of this progression in the opticalproFerties uccompanyiny the progression in the atomic weight of the rnetul contccined in the salt. The interesting facts regarding the refraction just summarised are graphically presented by the curves given in Pig. 10. The continuous curves are derived by taking the axial values of the optical indicatrix, according to the first series of ratios, as abscissae, and the atomic weight of the alkali metal as ordinates. The line corresponding to the unit axis b is of course a straight one. The convergence towards the caesium salt is very striking, as is also the intersection of the a and c curves View Article Online

KORMAL SELENATES OF POTASSIUM, ItURIDIUM, AND CESIUM. 903

followed by the inevitable reversal of their relative positions. The same facts are shown by the dotted curves, which correspond to the second series of ratios. The true relations of the refractions are here clearly exhibited for the different salts, as also the feebleness of the double refraction, expressed by the closeness of the curves to each other. So close do they approximate at the caesium salt, that they appear almost to coincide, and it is difficult to follow the crossing of the c curve, which the first series exhibit so well. The two series of curves corresponding to the optical velocity ellipsoid are precisely similar, only laterally inverted. An exact idea of their relations is obtained by holding the page up to the light and looking through it from behind. It will be most convenient next to compare the optic axial angle phenomena of the three salts for the ordinary temperature. Optic Axial Angle Phenomencc.-The refraction relationships are necessarily followed by corresponding ones with respect to the interference phenomena in convergent polarised light. In the case of potassium selenate, the direction c is that of the maximum axis of the indicatrix and b that of the minimum, while the intermediate a is somewhat nearer to b than to c. Hence c is the first median line and a(100) the plane of the optic axes (binormals); corresponding to the relative position of the intermediate value a, the optic axial angle is 76O 50' (Na). On arriving at the rubidium salt, owing to the nature of the curves, or, in other words, the specific character of the progression for this series of selenates, the conditions are similar, except that the double refraction is weaker, the values being all closer together and thicker sections being required to afford equally well-defined interference figures, and the relative position of the inter- rnediat.0 a value is somewhat nearer still to b, causing the optic axial Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. angle to be slightly less, namely, 68O 53' (Na). Between the rubidium and cssium salts, however, occurs the interesting intersection of the a and c curves, so that an entirely new order prevails in the cssium salt. The c axis is now the intermediate one, and in value is nearest to the new maximum a. Hence the first median line becomes 6, the plane of the optic axes c{OOl), and the sign of double refraction is reversed to negative. The relative value of the intermediate axis c determines that the optic axial angle is 71" 49' (Na). But it must be remembered that this is not directly comparable with the values of the angle for the other two salts, because the optic axes are now separated in another plane, at right angles to that which contained them in the two other salts. The optic axial angles themselves, therefore, cannot be compared as regards their angular magnitude, but they are precisely such as would nccturullp follow from tlte pogressive developent cf the optical indicatrix, which has been shown to ncconzpnp the progression in the atomic weight of the three alktcli metals. View Article Online

904 TUTTON : COMI’ARATIVE CRYSTALLOGRAPHlCAL STUDY OF

It will now be expedient to compare these relationships with those observed in the sulphate series. The three italicised rules following from a comparison of the optical axial ratios of the selenates, regarding the intermediate position of the rubidium salt, the enclosure of the rubidium and cmium ellipsoids within that for the potassium salt, and the slower increase of the dimensions of the indicatrix along the axis c, are equally applicable to the sulphate series. There are naturally, however, specific differences between the two series, due to the retardation of the advance in the optical properties on replacing sulphur by selenium offered by the increasing weight of the initial molecule, as pointed out in the comparison of the refractive indices, the effect of such replacement being less marked when it occurs in the heavy czsiiim salt than in the lighter potassium salt, the rubidium salt offering an intermediate resistance to the change. The most striking evidences of this are afforded by the fact that the differences between the second series of ratios expressing the total change are less in the selenate than in the sulpliate series, and by the interesting fact that the crossing of the c and a curves occurs in the sulphate series just before the rubidium salt is reached, whereas in the selenate series this does not happen until just anterior to the attainment of the cesium salt. Hence the particularly interesting member of the sulphate series is the rubidium salt, with its phenomenally low double refraction, and the great sensi- tiveness of its optic axial angle to change of wave-length and temperature together with its accompanying manifestation of crossed axial plane dispersion, whereas in the selenate series it is the cmium salt which is particularly interesting in this respect. Moreover, the specific

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. diffeyence between the two series due to the specific difference between sulphuy and selenium, and the different effect of replacing one by the other in molecules of different initial weight, is apparent in the different nature of the curves corresponding to the first series of ratios, those for the sulphate series being practically straight lines, and those for the a and c axes of the selenates exhibiting greater curvature convex to each other. It may therefore be stated that The optical ps-opevtiesof the se lenate se?*iesexhibit marked specific dvyev- ences from those of the sulphccte series, owing to the progressivelp dzfe’erent efect of replcccing sulphur bp selenium in tJbe three sulplmtes of p*ogves- sively [email protected] naoleculuq*u:eights, but the whole of the relationships of these opticcd pi-opeytiesexhibited by the tlwee sults of each sepsies weof a precisel9 pccrdlel nature, being functions in euch cuse of the atomic weight of the rcllculi metul which they contuin. Comparison of the Opticccl Properties at IIigher Tempemtures. - It is important to ascertain that the above conclusions are not, View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUhf, AND CABIUM. 905

impaired by making the comparison at temperatures other than the ordinary. On comparing the s.efiuctive indices at looo, t?Aose for the mbiclium salt cxre still obsewed to be intermediate throughout. Their relations may be concisely expressed by taking the mean of all three indices of each salt for Na light, as was done for the ordinary temperature. The numbers thus 0btaine.d are as follows :

For 100". For orJ. temp. For K,SeO, ...... , . .1.5346 Diff., 155 1'5396 Diff., 149 ,, Etb,SeO, ...... 1.5501 DiE,, 413 1.5545 Die., 452 ,, Cs,SeO, ...... 1.5944 1.5997

The relations of the dtyerences for the two chemical changes are thus substantially the scme as at the oidinary tenaperature, namely, 1 : 3. The following further facts are observed on instituting the comparison between the results for the three salfs at the ordinary tern perature and for loo', details of the comparison for each individual, salt having been discussed under that salt. A diminution in ref7~cctivepower accompccnies a rise of ten2pewitui.e of the crystals of all three sdenates. The amount of diminution is much the greatest for each salt along the diyeclion of the mos.phologica2 axis c ; the minimunz diminution corresponds to the direction of the axis b, c~ndthe intermediate amount along the axis a is nearest to the latter. T?Ae clou0le r~fractionalso diminis?bes on raising the tempemture, but owing to an interesting result of the nzo9.e vapid change cclong the diyection of the axis c, t?k only occurs in the case of the c~~siunzsalt until a certain nzinimum is attained, about go", cftev wlzicl~the double refraction again increases. The greater diminution along c, and the decrease of double refraction Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. in the cases of the potassium and rubidium salts, are very clearly seen from a comparison of the axial ratios of either of the optical ellipsoids. Those for the optical indicatrix are set out side by side below : 1I,SeO4. Rb,Se04. Cs,SeO,. n b c nb C nb c Bay Pa Y Ya B For ord. temp. 1.0025 : 1 : 1.0061 1.0014 : 1 : 1'0043 1.0009 : 1 : 1.0006 Bay Pa Y YB a For 100' ...... 1.0020: 1 ;1.0050 1.0012 : 1 : 1.0033 1.0008 : 1 : 0.9939 Diff. ... 5 11 2 10 1 7 Another interesting observation is that the difference between tIie values of each specific ratio for the two temperatures in the case of the rubidium salt is intermediate between the corresponding differences for the potassium and czsium salts, the change being progressive even to this point of detail. It will be convenient next to consider the relation, if such there be, View Article Online

906 TUTTON : COMPAltATIVE CRYSTALLOGltAI'IIIC'dL STUDY OF

of the highly interesting and unique optical behaviour of caesium selenate on raising the temperature, to the rule of progression according to the atomic weight of the metal which has been found to hold with respect to the optical constants. It has already been shown, and it is quite evident from the Greek letters above the ratios just given, that, even at the ordinary temperature, a remarkable change has been effected in the cmium salt by the rule of progression, and that not only are the p and y directions reversed, but the ratios are become reduced nearly to unity, expressive of the great diminution of double refraction. It is also evident that the rapid reduction of the c values by rise of temperature, 11 and 10 for the potassium and rubidium salts, is large compared with the total difference of either ratio of the cssium salt from unity, and that if it continues to proceed at anything like the same rate, the c value of the cssium salt must actually attain unity and even pass below it and become a decimal number. It is seen that in practice it indeed becomes reduced by 7, and thus the +0*0006 entirely disappears, and becomes - 0.0001, the ratio being now a decimal, 0.9999. This result gives rise of necessity to another exchange of directions, this time between a and p, and, moreover, as the new p is nearer in value to the new a, whereas the former /3 was nearest to y, another change of the sign of double refraction occurs; and as two further consequences the first median line must become changed from the direction of b to that of a, and the plane of the optic axes (binormals) from c(OO1) to b(010). Hence it is clear that the interference phenomena which are manifested by caesium selenate crystals in convergent polarised light, on becoming heated, are due, in the first place, to the optical Conditions which obtain in the caesium salt at the ordinary temperature, as the direct Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. result of the progressive change in the nature OF the optical indicatrix, which has been shown to accompany the replacement of one alkali metal by another, following the order of the progression in atomic weight ; and, in the second place, to the progressive modification of those conditions brought about by rise of temperature, which likewise follows the order of progression of the atomic weights of the alkali metals, For the reduction of the c ratio from 1.0006 through unity to 0,9999 in accordance with the second progression, of necessity means that on heating the crystal, after the change of thc first median line from b to a, the optic axial angle about CG narrows and narrows until, when unity of the b and c values is attained, the separation of the axes becomes zero, and the crystal simulates uniaxial optical properties, its interference figure consisting of a rectangular cross and circular rings ; beyoncl this occurs the change of the optic axial plane, the axes again separating, but in the plane perpendicular to that which formerly contained them. Owing to the presence of dispersion, the production of View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 907

the uniaxial figure naturally occurs at slightly different temperatures for the different wave-lengths. The further phenomena observed on continuing to raise the temperature, described in connection with the cssium salt, are equally the direct result of the continuous changes in the nature of the optical indicatrix, due to the same cause. The following general statement may therefore be made concerning the unique optical phenomena presented by crystals of cssium selenate on raising their temperature. The ciystals of cesium selenate exhibit 7,enaurkable optical phenomena on ?*aisingtheir temperature. A 60" prism whose vibration directions ure pnrallel to b and c afovds, ut the ordinary temperature, two clearlysepccratecl refmcted images of the signal-slit corresponding respectively to the a and p indices; on heatiPLg the prism, the inzagss approach each other, ubout 90" they coincide, as the crystuls were uniaxial, after which tJLeg again sepavate und pass on the other side of each other, us p and a. On examining a section-plate perpendiculur to the jyst median line b in convergent polarised light at the ordinury temperature, the intepfeyence jipre shows the optic axial brushes widely separated at the margin of thejeld iiz the plane c(QOl), the double refraction being negative. On heating the plate, the sepayation increases, and the brushes disappear fi*om the jeld. On following up the phenomena with a section-plate yerpendiculur to a, the second mediun line at the ordinar9 temperuture, the axiccl brushes soon mnke their ccppearunce cgter the heat is upplied, the double refraction being now positive ; they then approach each other as the temperature further rises, until, at temperatures vccrying for the diferent wave lengths from 92' to 98", the9 unite in the centre of the Jield to produce t?Le uniaxial cross and circular rings. Beyond this temperature, they ugain sepurute, but in the plane b(010) ut right ungles to that which formerly contained Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. them, und continue to do so until they disappear out of thisfield. On still following tAe efect with a section-plate perpendicular to the third axiul direction c, the axial brushes are seen to muke their appearance at 280" surrounded by nunzerous rings and lenaniscates, the sign of double refrnctive being once more negative; and as the heuting is continued as far as is possible with safety to the polariscope, the uxes are observed to approach nenver and nearer the centre of the field. Calculations show that the uniaxial cross and rings would again be produced about 75Q0, after which the axes would sepavate in t?Le third plane a(10Q); the changes, however, are less rapid at these highev temperutures. Moreover, uTl the section-plates require to be at least a centimetre thick, on account of the extreme weakness of the double refraction. The whole of these unique optical phenomena exhibited by the ciystals oj ccesiunt selenate are the direct consequence of the progressive changes in the nature of the opticul indicatrix of tAis series of isomorphow salls. both at the ordinary tenzpei*atureand upon raising the temperature, which View Article Online

908 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

are observed to cccconzpany the progressive clhaqe in the atomic weights of the three alkali metcds. Molecular Optical Constccnts.-The specific rind molecular refractions and dispersions of the three salts', calculated by the Lorenz formula+ and the molecular refractions according to the formula of Gladstone and Dale, are compared in the accompanying tables. The following facts are observed. I'he whole of the nzoleculw optical constants of rubidium selenate aise internaediate in value between those of potccssium ancl ccesiunz selenates. The nzoleculay .refraction and dispersion increase with the cctonaic weight oJ the contaiyied nteta2, such increase becoming greater as the atomic weight rises, the dzzerences between the constants for the rubidium and cwsiuna sslts behy greater tlmn those between the values fb?. t?Le potassium and ?.ubidiunt salts. The speci'jic reJ;rmtion decreases as the atomic weight rises, c~7zc.l the decrease is greater when rubidium yeplaces potccsse'una thnwhen rubitlium is replaced by cesiunz. These rules are equally vcdid whether the constants are calculated by the fomzulw of Lorenx or of Gladstone awl Bale, ancl, as rcgards the refiaction, to$,a fever may be the wave-lengthfor wl~ichthe conzparison is instituted. Noreovey, (1,s both density and YefrcLctive power have been proved to diiiziizis'h oiL misiny the tempercbture, these coiwlrtsions concerning tlde njoleczclar optical constants ctre i?i!dependextof the temperature.

n2- 1 Specific refraction -~-___ = ir. (n2+ 2)d Pol- lea$ c (Ha). For ray IIy near G. I 1 ,- cc b C n b c KE(,SeO,...... 0.101S 0.1013 0*1027 0.1042 0,1036 0.1052 196 202 204 20 1 208 Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. 199 Rb,Se04 . .O*OS19 0 -0S16 0.0825 0.0838 0.0835 0.0844 64 52 60 54 52 60 Cs,SeO, ...0.0765 0.0764 0.0765 0.0784 0,0783 0.0784 n2-1.M- Molecular refraction - in. ?++2 d For ray C (ZIa). For ray IIy near G. ,\- -- __-_.- ____ rc b c cc b C I<,SeO, ...... 22.48 22-35 22.67 23-02 22.88 23.22 3.16 3.20 3'16 3.23 3.28 3.20 Rb,SeO, ...... 25-64 25.55 25.83 26.25 26.16 26.42 5 .tjl 5.64 5 .'I0 5 .t7 6 *SO 5 *58 Cs;,SeO, ...... 31-25 31.19 31.23 32.03 31.96 32.00

Specific dispersion, i?c:- lie. Molecnlar dispersion, lira - nrc. cc b c n b C K,SeO, . . .0.0024 0.0024 0.0025 0.54 0.53 0.55 Rb,SeO, ...0.0019 0.0019 0.0011, 0.61 0.61 0.59 Cs,SeO, ...0.0019 0*0010 0.0019 0.77 0.77 0.77 View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 909

n-1 Molecular refraction (Gladstone) ~ M for ray C. d CC b C K,SeO,...... 38.65 38.38 39.05 5 *63 5 -72 5 *59 Rb,SeO,...... 44*28 44.10 44.64 10’48 10.53 10.08 Cs,SeO,...... 54*76 54-63 54.72

It will be expedient next to compare the molecular optical constants of the selenates with those of the sulphates, given in the former communication, in order to ascertain the effect of replacing sulphur by selenium. On instituting the comparison, the following facts are observed. The replacement of sulp/~u~by selenium in this series of salts is ccccom- panied by an increase in molecular refraction of 3.4-3.8 Lorenx units or 6-2-7.2 Gladstone units for the ray C, accoi-ding to the direction in the cvystal chosen for comparison. The relations of the three sults of euch group as regards molecular refruction are identical, the actual dzferences being, however, very slightly greater in the selenate group than in the sulplmte group. Por the replacement of potassium 69 rubidium, u meun increase for the ray C of 3-13 Lorenx units or 6.52 Gladstone units occurs in the sulphate group, and of 3.17 Lorenx or 5.65 Gladstone units in the selenate group. For the replucemerat of rubidium by ccesium, un increuse is observed of 5.46 Lorenx or 10.04 Gladstone units in the sulpliute group and 5.55 Lorenx or 10.36 Gladstone units in the selenate group. The spectjic refraction of the ccesium sults of the two groups ispracti- cully idsnticccl, but potassium selenate shows a considerable Tqeduction of specijc refraction compared with potassium suZp?mte, and rubidium

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. selenate u less reduction. The dzfleerences of specific refi-action in the selenate series are thus smaller, but theiy relations are the same, the values f.7- the rubidium salts of both series being nearer to those for the c~sium than to those for the potassium salts. The spec$% and molecular dispersions of the selenates are Iuigher than those of the suZphates, but the relations between the calues for the thee sults of each group are the same.

Determinutions of Refiaction Equivulents in Solution. It was considered very desirable, in view of the interesting facts described in a previous memoir (Trans., 1896, 69, 502) regarding the similarity of the refraction equivalents of the sulphates in the crystallised state, and in the state of solution as observed by Dr. Gladstone that a careful determination of the values of the selenates for the state of solution should be made. With Dr. Gladstone’s concurrence, these determinations have been carried out by the author, The solutions VOL. LXXI. 3P View Article Online

910 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

employed were highly concentrated, in order to avoid the slightly disturbing effect of large dilution which has been observed by Dr. Gladstone to occur in the cases of numerous salts which he has investigated (Trans., 1895, 87, 831). In the case of potassium selenate, which has been shown earlier in this memoir to be much more soluble than the sulphate of the same metal, and whose saturated solution contains 53.5 per cent, of the salt, two of the solutions contained as much as 50 per cent. The densities of the solutions were determined by means of the same 10 C.C. pyknometers as were employed for the determinations in the solid state, and were carried out for 20° in the same manner, the salt having been rendered perfectly anhydrous previous to solution by the same means as are described in connection with those determinations. The refractive indices of the solutions were determined by means of a small hollow 60' prism holding about a cubic centimetre, whose two side plates were cut from micro-cover-glasses selected from a large number on account of their perfect planeness, as tested by the perfection of the reflection of the signal-slit obtained from each surface. The large Fuess No. 1~goniometer was employed as spectrometer. The refractive index of the distilled water employed was first determined, after recent boiling and subsequent cooling to the ordinary temperature 16-1 So, and the following values, practically identical with those of Dr. Gladstone, obtained : Li, 1.3313 ; C, 1.3317 ;Na, 1.3334 ; T1,1.3353 ; F, 1.3379 ;G, 1,3407. The molecular refractions were calculated in Gladstone and Dale n-1 units for the wave-length C, the specific refraction - being first d calculated by means of the usual formula, Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11.

and then multiplied by the molecular weight of the salt, The results are given in the accompanying table. It will be observed that the concordance of the results with these highly concentrated solutions is very satisfactory,the differences between the values for the same salt being so slight that any noteworthy effect of difference of concentration is absent. Consequently, the mean value for each salt may be fairly taken as representing the molecular refraction of that salt in the dissolved state, and this is given in the last column but two. In the last column but one is given the mean value of the molecular refraction for the crystallised condition, obtained by taking the mean of the three values for each salt given at the end of the last section, corresponding to the three refractive indices of the crystals The last column contains the differences between the figures in the two View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 911 - Deternainatiow- - of Molecular - Refraction in Xolzctim.- - - I Mole Per- Iensity culai Mean cent- 6. R. M can Weight Weight of Refractive ~ ige of '2;If ?all u. R. Differ- Salt. of Of oliition index of slt in 111 if cry- ence. Bdt. water. at solution. of::' solu- 1 solu- BtSlP. tion. 80"/4". tion. -

K2SeO4 7'4811 13.4364 35'76 1 '3591

9) 7.9016 11-0075 41'79 lS43€5

39 '83 38 -69 + 1*14

Y, 11 '3803 11.41 79 49'92 1*5578

\G 1-4116 ILi 1.3988 C 1.3992 12.4584 ~ Na 1.4015 J, 12.4600 50*00 1'5590 T1 1'4038 39.73 F 1'4067 1.4105 \G - Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. Rl),ScO 9.4685 13$51 1 40'60 1'4688 F 1.3820 G 1-3860 44-81 44'34 + 0.47 Li 1.3E42 C 1'3846 Va 1.3865 99 10.5352 11.8460 45-07 1'5806 44'91 T1 1-3888 F 1.3915 G 1.3954

Li 1-3810 C 1.3814 Ca,SeO 11'4459 13.4690 Na 1-3837 54.3e 45'94 1'5841 TI 1-3857 F 1.3882 ,G 1.3919 54-40 54T0 - 0'30 fLi 1'3935

12'7636 11'1240 53'43 Na 1.3962 54.44 39 1-7432 !"IT1 1'3g3gi1'3984 - - - - - View Article Online

912 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAL STUDY OF

previous columns, the signs indicating the order of change on effecting solution. The result of the comparison of the values for the two states is interesting. It is observed that- The moleculcw refraction of enclb of the tTwe3 scclts for tibe state of solution in water is uppli*oxinzately,but not exactly, the same as the ozeun of the three vuluesfor the crystal. TVhela potassium selenute is dissolved in watey its refraction equivalent rises by 1.14 ;in the case of rubidium selenccte u less rise of 0.47 is observed ; while for ccesium selenate there is no longer a rise but a decrease, to the extent of 0.30. These slight dzflerences, due to change of state in the three cases, thus exhibit a reyular progression, varying directly as the spec@ refractive energy and inversely as the atomic weight of the alkali metal contailzed in the salt. On reference to the table at the end of the last section, the specific refraction will be observed to decrease as the atomic weight of the metal increases. The third and fourth determinations of the value for the potassium salt in solution mere made in order to further confirm the somewhat large difference of over one unit in the case of that salt, with quite independent material; for the sake of greater certainty, the material for the third determination was directly derived from the original stock by recrystallisation. The results in both cases are observed to be amply confirmatory. Hence there can be no doubt that the mean value for solution, 39-83, is correct. The author’s value for the crystals is also practically identical with that obtained by calculation from the measurements of Topsije and Christiansen, so that the result is confirmed on every side. The determinations for this salt in solution are less liable to error than those for potassium sulphate, on ac- Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. count of its far greater solubility and the consequent greater concentration of 50 per cent. possible, it being only possible to make a 10 per cent. solution of potassium snlphate. It mill doubtless be interesting to compare these results with those obtained for the sulphates, the observations for solution of K,SO, being due to Kanonnikoff, and those for Rb,SO, and Cs2S0, to Gladstone and Hibbert (loc. cit., 838), the author’s material having been used in the cases of the two latter salts. The actual figures for the state of solution are those given in the memoir of Gladstone and Hibbert for the strongest solutions employed, which were nearly saturated, and they refer to the wave-length C. Molecular refraction Meau hi. R. Salt. of crystals. of crystals. 11. R. for solution. Difference. K,SO, .. 32-22, 32.14,32*39 32.25 32.15 (Kanonnikoff) - 0.10 Rb,SO, . . . 37.83,37.74, 37.75 37.77 38.39 (Gladstone) + 0.62 Cs,SO, . . . 48.05,47*89,47 50 47.81 47.57 (Gladstone) - 0.24 View Article Online

NORMAL SELENATES OF' POTASSIUM, RUBIDIUM, AND CXSIUM. 013

It is evident that the sulphates of rubidium and caesium exhibit similar differences to the selenates, the rubidium salt showing a positive difference and the czsium salt a negative one. The values for solution of these two salts are strictly comparable with those for the crystals, as the determinations were made with material lent to Dr. Gladstone for the purpose by the author from the stock employed in the crystallographical work. The relations of the two values for the potassium salt, however, are not similar to t'hose of potassium selenate in the two states, and it consequently became a matter of importance to revise Kanonnikoff's value for solution. Dr. Gladstone has published two values for potassium sulphate in the dissolved state, in his first paper (Phil. Tmns., 1870, 160, 15) t,he number 33.11, and in the 1895 memoir (p. 840) the value 32-41 for D. In order to decide the question, two totally independent determinations of the refraction equivalent of potassium sulphate in thestste of solution were made, with the same pure material as was employed for the crystallographical investigation. The powdered salt was thoroughly dried in the same manner as the selenates, and the determinations were carried out in exactly the same way as those of the latter salts. The two independent solutions happened to be of exactly the same degree of concentration, being nearly saturated solutions, and the identity of the results proves the trustworthy character of the determinations. The numerical details are given in the accompanying table :

Deternainations of Molecu Zur Refimtion of Dissolved Poiassium SuZplute.

~ ~~

Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. - - Per- Mole- RSca11 Weight centage Density M. B. Mean Weight of solu- Refractive :ular re- 0f.Salt RI. R. Dif- of ~ ofsnlt of salt. tion at index of fraction in water. 1 in solu- solution. )f salt iu )f crys- ference. 20°J4'. solu- tals. ~ ~ tion. olu tion. tion.

... - .- Li 1.3421 C 1.3425 Na 1.3143 33.39 1.3714 14'2444 1 8.78 1'0705 1 i 1'1 1.3483 1 I? 1.3486 ,G 1.3516 33.39 32.25 ~ f1.14 Li 1.3421 C: 1'3425 1,9024 19.7552 8 78 33'39

I View Article Online

914 TUTTON : COMPARATIVE CRYSTALLOGRAPHICAT, STUDY OF The value 32.15 given by Kanonnikoff (J p~.Chenz., 1885, [ii], 31, 321) is thus found to be too low, and the value obtained by Dr. Glad- stone in 1870 is doubtless nearer the truth than his later value, being nearly identical with the number now presented, which is precisely as much higher, 1-14, than the mean value for the crystals, a3 w-as observed in the case of the analogous potassium selenate. This result thus renders the parallelism between the two groups of salts, the sulphates and the selenates, complete. It is extremely interesting that this should be so, for it indicates that the generalisation advanced in 1868 by Dr. Gladstone (F~oc.Roy. ~OOC.,1868) that '' the refraction equivalent of ,z solution is the sum of the refraction equivalents of the solvent and of the substance dissolved " is substantially correct, slight differences due to change of state being sometimes on one side and sometimes on the other. This important fact, that the differences occur on both sides of zero, was overlooked in a recent memoir by Pope (Trans., 1896,69, 1535), which tlie author has felt called upon to criticise (Trans., 1897, '71, 235). The results further confirm Dr. Gladstone's conclusion that in salts of low specific refractive energy the difference due to change of state is a negative one, the czesium salts having tlie lowest specific refractive energy ; and also his remark that there are clear indications of some connection between the amount of this change and the amount of the specific refractive energy and the combining proportions of the consti- tuent elements present. It has been shown in the previous section that the specific refraction is an inverse function of the at,omic weight of the metal contained in the salt, and thus one is inevitably led to conclude that this interesting order of the differences, arising on changing the state of the salt, is Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. primarily due to the variation in the nature of the atoms, which is inter- preted by the change in their fundamental const tnt, atomic weight.

SUMMARYOF CONCLUSIONS. The main results of the investigation are contained in the following summary. 1. The normal selenates of potassium, rubidium, and cwsium dissolve to a progressive extent in water; the progression in solubility follows the order of the atomic weights of the three respective metals contained, and becomes more rapid as the atomic weight rises. They are much more soluble than the corresponding sulphatee.

JlOlplL ozogy . 2. The values of the morpi'mlogical angles of the crystals of rubidium selenate are without exception intermediate between those of the View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 915

analogous angles on the crystals of the selenates of potassium and cmium, and somewhat nearest to those of the latter salt. Hence progressive change in the atomic weight of the alkali metal is accompanied by a progressive change in the magnitudes of the morphological angles. The latter are, therefore, a function of the atomic weight of the metal present, and the function is such as produces a diminishing effect as the atomic weight rises. The replacement of sulphur in the corresponding sulphates by selenium produces only a slight alteration in the angles of the crystals, rarely exceeding 20’. 3. The morphologicaZ axial ratios for rubidium selenate are intermediate between those for potassium and czesium selenates. There is thus a progression in the axial ratios corresponding to the progression in the atomic weight of the metal contained. The change becomes less marked as the atomic weight rises. It is greatest for the ratio c/b. The replacement of sulphur in the analogous sulphates by selenium is accompanied by only slight change of the ratio a/b, but a more considerable change in the ratio c/b. 4. The habits generally assumed by the crystals of the three selenates respectively exhibit a progressive development of the primary forms, following the progressive change in the atomic weight of the metal. While inducing certain slight specific changes of form, the replacement of sulphur by selenium does not affect the general habit. 5. The directions of cleavage are identical for both selenates and siilphates. Yo lwme. 6. The relative density and molecular volume increase when a lighter is replaced by a heavier alkali metal. The increase in density is Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. greater when potassium is replaced by rubidium than when the latter is replaced by czesium, and the increase in molecular volume is, on the contrary, greatest when rubidium is replaced by czesium. The replacement of sulphur in the sulphates by selenium is accompanied by an increase of molecular volume of 6.5 to 6.7, the amount varying within these limits inversely as the weight of the initial molecule. The differences between the molecular volumes of the three selenates, 8.3 and 11.35, are slightly less than the corresponding differences, 8.44 and 11.4, for the lighter sulphates. Hence the effect of the replacement of either the metallic or acid-forming element is perceptibly less in the case of a heavier than in the case of a lighter initial molecule. Throughout all these changes the rubidium salt invariably behaves in an intermediate manner, corresponding to the intermediate position of the metal as regards atomic weight. 7. The replacement of potassium by rubidium, and of the latter by caesiiim, is accompanied in each case by a considerable increase in the View Article Online

916 TUTIlON : CONPARBTIVE CRYSTALLOGRAPHICAL STUDY OF

separation of the centres of contiguous units of tJAe homogeneous crystal structure (in all probability the chemical molecules), along the directions of each of the morphological axes. This influence of the nature of the alkali metal in producing extension of the structure becomes relatively greater as the atomic weight rises, the intermediate relative positions of the ultimate parts of the structure of the rubidium salt being somewhat closer to those for the potassium than to those for the caesium salt, An extension of volume in all directions also accompanies the replacement of sulphur in the corresponding sulphates by selenium. Optics. 8. The refractive indices of rubidium selenate are intermediate between those of the selenates of potassium and caesium, and nearest to those of the potassium salt,. This is equally true whether the comparison be made at the ordinary temperature or 100". An increase in refraction is observed to accompany an increase in the atomic weight of the alkali metal, and this increase becomes relatively greater as the atomic weight rises than in mere simple proportion to that rise. The replacement, of sulphur in the analogous sulphates by selenium is also accompanied by an increase of refractive index, and such increase diminishes in amount as the weight of the initial molecule increases. 9. The axial values of tJLe optical indicatrix or of its polar reciprocal, the optical velocity ellipsoid, for rubidium selenate are intermediate between those for potassium and czesium selenates; so that if the optical indicatrices of the three salts were constructed about the same origin, using rectangular axial co-ordinates, the closed ellipsoidal figure for the cssium salt would contain within it that for the rubidium salt, and this again would contain the indicatrix of the potassium salt,, Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. separated from it by only one-third the distance which would separate the two former outer surfaces ;the reverse order obtains for the optical velocity ellipsoid. The increase in the dimensions of the optical indicatrix, or the corresponding decrease of the optical velocity ellipsoid, which accompanies the rise of atomic weight of the metal, occurs to a less extent along the direction of the axis c than dong the two other axial direct ions. 10. The replacement of one alkali metal by another of higher atomic weight is accompanied by a diminution of the double refraction ; this property is already feeble in the potassium salt, and is so weak in the cesium salt that the difference between the maximum and minimum axes of the indicatrix is less than one in a thousand. A section-plate over 1 centimetre thick is consequently necessary to furnish a well- defined interference figure in convergent polarised light. In the convergence of the axial values towards unity, the c value proceeds much more rapidly than the others. View Article Online

NORNAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 917

11. The more rapid approach towards unity of the c axial value has the effect OIJ attaining the cmium salt of reversing the sign of the double refraction from positive, its nature for the potassium and rubidium salts, to negative; for nor; only do two of the values, c and a, actually attain equality, but slightly overstep it, and pass each other, and are on opposite sides to their former relative position when the cesium salt is reached. Hence this interesting fact is a direct result of the rule of progression of the optical constant's which accompanies the progression in the atomic weight of the alkali metal. 12. The optic axiul angles are precisely such as would naturally follow from the progressive developmcnt of the optical indicatrix, which has been shown to accompany the progressive change in the atomic weight of the metal present, the observed change of direction of the acute bisectrix from the axis c to the axis b, and the optic axial plane from the macropinacoid to the basal plane, when the czsium salt is attained, being the inevitable result of the continuity of that progression. 13. The opticul properties of the selenates exhibit marked specific differences from those of the sulphutes, owing to the progressively diEerent effect of replacing sulphur by selenium in the three sulphates of progressively different molecular weights, but the whole of the relationships of these optical properties exhibited by the three salts of each group are of a precisely parallel nature, being functions in each case of the atomic weight of the alkali metal which they contain. 14. The refractive indices, the axial ratios of the optical indicatrix, and the double refraction of the crystals OF all three selenates diminish on raising their tenq2evatzcre. The amounts of diminution of the two latter observe the inverse order of progression of the atomic weights of Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. the three metals. The amounts of diminution of the two former are much the greatest for each salt along the axis c, and an interesting result of this is that diminution of the double refraction only occurs, in the case of ciesium selenate, until a certain minimum is obtained, about 90') the c value at this temperature becoming identical with, and subsequently crossing, the b value, after which the double refraction again increases. A 60" prism of cEsium selenate whose vibration directions are parallel to 6 and c, affords at the ordinary temperature two clearly separated refracted images of the signal-slit of the spectrometer, corresponding to the a and p indices; on heating the prism, the images approach each other, about 90" they coincide as if the crystal were uniaxial, after which they again separate on the other side of one another, as p and a. 15. As a necessary consequence of the foregoing, the crystals of ca~iumselenats exhibit unique interference phenomena in convergent polarised light when their temperature is raised. On slowly heating a View Article Online

918 TUTTON : COMPARATIVE CRYSTALCOGRAPHICAL STUDY OF'

section, a centimetre thick, perpendicular to the first median line bythe optic brushes visible at the margin of the field separate still further and disappear out of the field. On following the phenomena by heating a section perpendicular to the second median line a, the brushes soon appear, the double refraction being now changed from negative to positive, and approach each other until they coalesce to produce the uniaxial cross and circles, at temperatures varying for the different wave-lengths from 92' to 9So, after which they separate in the plane at right angles to their former one, and eventually disappear again out of the field. With a third plate perpendicular to the axis c, the brushes make their appearance at about 280°, the sign of double refraction being again reversed to negative, and approach each other as the temperature still rises to the maximum which can be employed with safety to the polariscope. Theoretical considerations show that the uniaxial figure mould again be produced about 750°, after which the brushes would separate in a third plane. 16. The whole of the molecular optical constants of rubidium selenate are intermediate between those of potassium and caesium selenates. The molecular refraction and dispersion increase, and at a growing rate, with the atomic weight of the metal, the increase being greater when caesium replaces rubidium than when the latter replaces potassium. The specific refraction decreases as the atomic weight rises, and the decrease is the greater when potassium is replaced by rubidium. These rules are independent of temperature. The replacement of sulphur in the analogous sulphates by selenium is accompanied by an increase in molecular refraction of 3.4-3.8 Lorenz units, or 6.2-7.2 Gladstone units, according to the direction chosen €or comparison. The relations of the three salts of each group as regards molecular Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. refraction are identical, but the actual differences are slightly greater in the selenate group than in the sulphate group. The same applies to the relations of the specific refractions, but the actual differences are smaller in the selenate group. The specific and molecular dispersions of the selenates are higher than those of the sulphates, but the relations between the values for the different members of each group are the same. 17. The molecular refraction of each of the three selenates for the state of solution in water is approximately, but not exactly, the same as the mean of the three values for the crystal. When potassium selenate is dissolved in water, its refraction equivalent rises by 3.8 per cent. ;in the case of rubidium sulphate, a less rise of 1.0 per cent. is observed ; while for caesium selenate there is no longer a rise but a decrease, to the extent of 0.5 per cent. These slight differences, due to change of state, thus exhibit a progression, varying directly as the specific refractive energy and inversely as the atomic weight of the alkali metal con- View Article Online

NORMAL SELENATES OF POTASSIUM, RUBIDIUM, AND CBSIUM. 919

taiaed in the salt. Precisely similar differences for the two states are also observed in the sulphate group. 18. The whole of the morphological and physical characters of the crystals of the three selenates have now been shown to follow the same order of progression, either direct or inverse, as the atomic weights of the three respective alkali metals which they contain. The intermediate member as regards atomic waight, rubidium selenate, has been found to occupy an intermediate position with respect to every property. For certain constants, such as the morphological angles and axial ratios, the density and the specific constants which involve it, this intermediate position is nearest to that of the cssium siilt ; while for others, such as the refractive indices and the axial ratios of the optical indicatrix, and invariably, which is of the greatest significance, for the molecular physical constants, the values for the rubidium salt are nearest to those for the potassiuni salt. Hence the replacement of rubidium by czesium is accompanied by a greater change in the nature of the molecule than the replacement of potassium by rubidium, such change being manifested by t'he greater modification of the morphological and physical characters of the homogeneous structure, the crystal, to which by its regular repetition the molecule gives rise; the effect produced by the heavier cmium atoms, compared with that due to the atoms of rubidium, is thus greater thizn in mere simple proportion to the numerical value of the increase in atomic weight. These conclusions are independent of the temperature, and the remarkable optical phenomena exhibited by the crystals of caesium selenate, on raising their temperature, have been shown to be the direct result of the continuity of the progression according to the foregoing rules. Hence the final conclusion with regard to these seleiiates is that- Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11. The whole of the nzorphological and physical properties of the crptals of the r?Lonzbic nornml selenates of potassium, rubidium, and c~sium exhibit pogi*essivevariations which follo~the ordele of progression of the atomic weights of the cclkuli metals which the salts contain ;the variations are, therefore, functions of the atomic weight of the cclkali metal, and ustmllg fuiictions which inootve higher powem of the atomic weight thun the Jirst. A precisely similar conclusion was derived from the investigation of the sulphates of the same three metals. The results of the two investigations are strikingly similar in every particular. A more or less specific character is imparted to each of the two groups by the fact that the replacement of sulphur by selenium does not produce an equal effect upon all three sulphates, but becomes less powerful in modifying the crystallographical characters as the weight of the initial molecule increases, that is, as the atomic weight of alkali metal present in the original sulphate increases. Hence the results for the two View Article Online

920 THORPE : THE SO-CALLED HYDRATES groups are not represented by two parallel lines, but by two converging lines. While each group i hus exhibits specific idiosyncrasies, the mutual relations of the members of the one group are identical with those of the members of the other. Moreover, the effect of replacing the lighter atoms of sulphur by the heavier atoms of selenium is invariably to produce variation in the morphological and physical properties of a similar character to that produced when a lighter is replaced by a heavier alkali metal. Hence the results of these two investigations, as well as those derived from the investigation of the double sulph:ites, whose details were presented to the Society last year (Trans., 1896, 69, 344), agree with the assumption that- Fhe characters of the crystals of isomoq3hous series mefunctions of the cctonzic weigJLt of the interchangeable elements, belonying to the same family groq, which give rise to the series. That is to say- The difleT*ence in the nature of the elements of the same family group which is ntanifestecl in thei?. regulccdy vai-ying atomic weigh, is also expressed in the sinzilady q*ejuZui*vnriaction of the cl~ccricctemof tke crystals of c6n isonzo~~phousseries of scdts of which these elements are the inter- clmzgeable constituents. Published on 01 January 1897. Downloaded by Temple University 26/10/2014 05:07:11.

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