Bull. Hist. Chem., VOLUME 25, Number 2 (2000) 103 STEFANIE HOROVITZ, ELLEN GLEDITSCH, ADA HITCHINS, AND THE DISCOVERY OF ISOTOPES Mrln . nrCnh nd Gffr W. nrCnh, Sr Wlfrd Grnfl Cll In th ntf dvr pr, n tnd t f r f 860 (4. h ht r nltd th n th "Grt " nd nr th rrhr h th v tht th fndtn f htr dpndd d th tl dvr r ntrbtd nfntl pn th n vl f th t ht f h l t th dvr. h frt dttn f plr a nt—nd n th tblt f th lnt th l xpl h brvtn d b rd lv. t tdnt, ln rnll, bt t hr prvr, h frt r n th fçd f trdtnl h Anthn h, h rvd th bl rz fr th tr th th dvr f rdtv trnfr dvr (. In th frt dd f th 20th ntr, tn (. h vr p n th d n th l f ttrbtn t th lbbnh rrhr h r dntfd b n lnd t tht f th prnt. hd nfnt fft f hdn th ntrbtn f r xpl, thr dd t thr I, t n ntt, fr f r bl t br thrh thr II, t rdthr, t thr , nd th "l ln" nd ttn rntn pr r n. At th t, h f th p blvd t rhr. b n nd n lnt. It MC nd At n n r hr n n n 0 h prvdd th nxt p n th pzzl th tt pld tv thh brdnt rl (2 (th th ttnt tht (6: th xptn f Mr Cr nd Mtnr. r Our experiments trnl ndt tht rdthr xpl, Ernt thrfrd frt rrh tnt ntrl nprbl fr thr b hl pr n—rrt r. W hv rprtd lhr . n hr rr, nldn th dvr f th rl f Expl f hll nnprbl pr (nd rp th rdtv t (. In th ppr, th f ll b f rdtv lnt bn t lt vr rp n th ntrbtn f thr n t th rl r dl. thr thn th br phr rd l n tp: Stfn rvtz, Elln Gldth, nd Ad nt hll nnprbl, Sdd td tht thn. But first, it is necessary to rv th rnd th tr tp b ntrdd (. r tht d th dvr f th xtn f tp pbl. It th jnSdd Grp plnt tht prvdd th nxt tp n th pzzl (8. In rd Background tv d, l f n prtl rltd n ttp hft t th lft n th prd tbl th n pn rntd ntt tnd t frt tht, fr th rl n t ht l f fr nt, hl l f bt htr f htr, t ht f pr prtl rltd n ntp hft t th rht th prtn. r xpl, th vl f th t ht nlbl hn n t ht. Strtn th th f lnt th pr f f th Krlrh Cn t f rn nd thr, Sdd l 104 Bull. Hist. Chem., VOLUME 25, Number 2 (2000) lated the expected atomic weights of the lead produced jn n rlzd tht th t [f pr t from their respective radioactive decay as very differ- ht dtrntn] rrd h r tht ent from the 207.2 of 'normal' lead (9). To chemists, nl th rlt f rnzd xprt ld b dl finding samples of lead that had 'abnormal' atomic ptd. Sdd prtd n h ffrt, nl t hv weight would be a confirmation of the existence of iso- h rlt vd ptll n rtr. topes and proof of the group displacement law. In par- h world's leading expert on the measurement of ticular, lead from the decay of uranium-238 was pre- atomic weights was Theodore William Richards of dicted to have an atomic weight of about 206 while that Harvard (13), but almost as high in estimation were his from the decay of thorium-232 was expected to have an two former students, Gregory Paul Baxter, also of Harvard, and Otto Hönigschmid at the Radium Institut in Vienna (14). Stfn rvtz Hönigschmid had the talent and also the opportu- nity, for the major source of radioactive ores at the time was the mine at St. Joachimstal in Austria (15). Much of his work was accomplished with his research student, Stefanie Horovitz. Horovitz was born in Warsaw on April 17, 1887, her family moving to Vienna about 1890 (16). She graduated from the University of Vienna in 1914 with a doctorate in organic chemistry, although she seems to have started as a research worker at the Radium Institute of Vienna with Hönigschmid in late 1913. Hönigschmid was actually affiliated with the Technical University of Prague from 1911 to 1918, but he maintained research facilities in Vienna. Horovitz's initial task was the time-consuming separation of lead from the residues of the radioactive ores after the radium had been extracted. This was fol- lowed by the demanding gravimetric procedures to the Élln Gldth, rdtn pht, 02. (. Krnnn nearest hundred thousandth of a gram. The first report nd A. C. pp by Hönigschmid and Horovitz provided a value of 206.736 for the atomic weight of lead from the St. Joachimstal mine, compared to 207.190 for 'normal' lead atomic weight of about 208. It is the contributions of (17). Such a significant difference from a respected three women scientists to the discovery of lead with 'ab- analytical laboratory was the first definitive evidence normal' atomic weights that will be the focus of this that atomic weights were not necessarily invariant. As study (10). a result of its importance, this paper by Hönigschmid To accomplish this task, researchers needed lead- and Horovitz was chosen by Henry Leicester as one of containing samples from uranium or thorium ores. In the crucial publications in chemistry in the first half of addition, for the results to be accepted among the scien- the twentieth century (18). The two researchers were tific community, the researchers themselves had to have dedicated to their work, as is apparent in a letter from credibility in the field of the determination of atomic Hönigschmid to Lise Meitner (19): weights to high precision. Although Soddy and his col- ... Miss Horovitz and I rd l l. On th laborator, Henry Hyman, and Maurice Curie, nephew btfl Snd r tll ttn n th lbrtr of Marie Curie, both reported atomic weights of radio- t 6 l. active-origin lead that were significantly different from Subsequently, Hönigschmid and Horovitz analyzed new that of normal lead (11), neither of these reports was samples from St. Joachimstal as well as samples from sufficiently reliable in the eyes of analytical chemists. two other mines: pitchblende from German East Africa; As Badash has commented (12): and bröggerite from Norway. These results were even Bull. Hist. Chem., VOLUME 25, Number 2 (2000) 105 more convincing, giving values as low as 206.046 (20). firmly fixed ideas concerning the chemical elements A difference of over one mass unit could not be explained and their mutual relations, as well as the nature of by experimental error. There clearly were significant atoms. differences in the atomic weight of lead, depending upon His first report described lead samples with abnormally source. low atomic weights, the lowest value of 206.40 coming from a sample of uraninite from North Carolina (26). Hönigschmid and Horovitz made a second contri- This particular lead sample was noted in the data table bution to the isotope story. Boltwood had claimed the as being provided by Gleditsch. This was Dr. Ellen discovery of another radioactive element, ionium (21). Gleditsch, who at the time was working with Bertram Most chemists accepted the existence of this element, it Boltwood at Yale. In the paper, Richards stated that this even being assigned a symbol, Io. However, it was the "most valuable" of atomic weight the samples had and spectro- been supplied by scopic analyses Gleditsch as lead performed by chloride; thus Hönigschmid Gleditsch played an and Horovitz that active role in the showed ionium discovery process to be no more by performing the than an isotope extraction of a pure of thorium, mak- lead salt from the ing thorium only uranium ore. In a the second ele- subsequent paper ment for which (27) Richards re- isotopic behavior ported atomic had been proven weights of 206.12 at that time (22). and 206.08 for lead These were the from uranium ore last publications samples obtained of Horovitz. from Norway. He Many years later, added that these two Horovitz's fate samples "of especial was discussed in Gleditsch, with her assistants Ernst Fyn (left) and Ruth Bakken (right), in the value and signifi- an exchange of Chemistry Laboratory, Oslo, ca 1930 (T. Kronnen and A. C. Pappas) cance (27)" were letters between both obtained from Kasimir Fajans and Elizabeth Róna (23). In the last of Ellen Gleditsch, who had returned, by then, to her na- the correspondence, Fajans commented (24): tive country of Norway. These values were so close to You probably have not received any information from that predicted by Soddy for pure lead produced at the Vienna about the fate of Dr. Stephanie Horovitz. I end of the uranium decay series that the group displace- learned about it from a mutual relative at Warzawa. ment law could no longer be in doubt. Stephanie moved there after World War I and after her parents had died in Vienna to join her married Gleditsch was born on December 29, 1879 in sister. She was not active in chemistry and both were Mandal, in southern Norway (28).