Lawrence Badash
Decortment of History of Science and Med~cine Yale University New Haven, Connecticut
The Discovery of Thorium's Radioactivity
W h i l e l\Iaric Curie's rntry into the field of r:rdioartivity single-handedly raised the interest in this srience from t h r low level to wl~ichit h:~dfallen (1) sinre Henri Bequcrcl's discovery of the phenomenon two yea,rs earlier, shr wts anticipated in her first major discovery hy the physicist, Gcrhard Carl Schmidt,. Early i n 1896, Brcquerel had found invisihle, penetri~t,ingra~liationrmitted from cryst,als of the doublc sulfat,e of potaisii~n~and uranium. Within a few months he had tralwl this activity to nletallic uranium, and investigated the physical properties of these rays a t some Icngth. One tilay infer that Becquerd considered the phcnomenot~to he of an atomic nature, sit~reit, was wsso~,iatedwith ele~i~ental uranium, but during the folloving t,wo years, while he dolninated the small rcscarch efforts in this field, he made no such explicit ,statement. X7hether hc deliberately tested other elemcnts for tlic cnlission of invisihle, pcnrt,rating rays me [lo not k n o \ ~ ; no investigation of this sort was reportnl. Inevitably, however, the linomn elements in the pcriodic table would be examined for this property, and only one other than uraniunl would h r found t,o exhihit it: thorium. (This excludes the radioactive Gotopes of elrmrt~tsthen known, e.g., lead and bismuth.)
The Author is at Camhridge University during the 19G5-66 sear, continuing his study of the history of radiaartivity, while s u l w r t e d by a NATO fellowship and NSF research grant, GS GR4. He wishes lo expre* his thanks and appreciation to Derek J. de Solla Prire, of Yale University, with whom he has had many vnluahle discussions an this subject. The word "radic+aetive2' was coined by Mme. Curie a few months latw in a paper describing the discovery of polonium and written with her husband: "Sur une suhatmce nouvelle radio-active, contenne ~ R I I Sla pechblende," Contpt. Rend., 127, li.+'iX (July 18, I X W ) .
Schn~idl'sdiscovery of thorium's radioactivity' was rcported to the Pliysiral Soricty of Berlin on February 4, 1898 ( 2 ) , some t,lVomonths brfore Marie Curie aonounred the samc result in Paris ( 3 ) . Presumably, he had been led to the study of "Becquerel rays" by a familiarity with the physical properties of uranium compounds. Yet lie puhlishcd no prior research on radioaetivit,y and :~rroml~lishedvirtually nothing in this are:&after his disrovery.
Gerhard C. Schmidt 11 865-1 9 4 9 )
Fmm Deufrch Senioren der Phydk lLeipzig. 19361, with the permirrion of Prof. Dr.-lng. E. Bruche. Photograph courtesy of Prof. Dr. H. Solie and Dr. I. A. Schneyer.
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S ~ h n i d was t born in L O I I ( ~i l ~ l I 1Ri.i. I 111,r(wiv(,(l his schooling in several Gertn.ur citich, atrtl liis 1'111) i l l 1891, from the University of Basel. He mi~sa 1rofessor of physics at the University of Erlangcn from 1901-04, Konigsberg from 1904-08, and Munster from 1908 until his ret,irelnent in 1935. Hc remained in Minster, in good health through the second World War, and died of a stroke in 1949 a t the age of 84. At the time of our interest, however, he was a Pvivatdocent in Erlangen, and assistant t,o Eilhard Wiedemann in the Physical Institute of the University (4). Wiedemann was one of the leaders in the study of
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phosphorescence and fluoresceuce during the nineteenth century, and with him Schmidt published many papers on luminescence, spectroscopy, electrical discharges, and cathode rays. Many of Wiedemann's students naturally pursued their own researches in these fields, and Schmidt also acted as adviser to some of them (5). Becquerel himself had begun his work with phosphorescent uranium crystals in the belief that the new rays were somehow associated with luminescence. It is therefore probable that Schmidt, because of his supervision of students working with uranium and his own research, had a similar background for the study of uranium rays. Thus he had merely to jump into this field pioneered by Becquerel, hut now dormant. This Schmidt did in late 1897 or early 1898. Using uranium materials, of which the laboratory no doubt had a good supply, he probably repeated some of the experiments that Becquerel, or Elster and Geitel, or Kelvin, Beattie, and Smoluchowski had described in the scientific literature. Then came his inspiratiouare there elements other than uranium which emit these rays? I n his search among the contents of the periodic table (he never indicated how systematic or extensive this search was), Schmidt tested many elements and a variety of compounds, including fluorspar, retene, oil of turpentine, and resin. Some of the compounds, such as those named, were studied because other investigators had reported rays issuing from them which were capable of blackening a photographic plate. Schmidt, however, distinguished them from uranium rays by their inability to impart a conductivity to air. Among the elements, only thorium was found to have an activity resembling that of uranium. Schmidt's report was sent from Erlangen to the Physical Society of Berlin on January 25, 1898, and read there the following week. It was later reprinted in expanded form in the Annalen der Physilc und Chemie, commonly known as Wiedemann's Annulen, since it was edited by Gustave and Eilhard Wiedemann, father and son (6). Finally, for the benefit of those French scientists who had seen only Marie Curie's report of April 12, the Academy of Sciences in Paris published a summary of Schmidt's findings the following month (7). By hoth electrical and photographic tests thorium and its compounds were found to have an activity comparable to that of uranium. Schmidt did not indicate by what method his discovery was made, thoughfollowing it he employed hoth techniques to determine the properties. I n this task he logically pursued the pattern set by Becquerel, who had, in fact, closely copied Roentgen's procedure for the investigation of X-rays. Using an Exner electroscope, Schmidt found that, in equal times, the gold leaves fell 38 divisions without thorium nearby, and 42 divisions when the thorium was brought near. But since the time in question was six minutes, this was a rather small effect to detect. I t is likely, therefore, that the discovery was made by the photographic method, and Schmidt then tried to confirmit with the electroscope. Better results were obtained with a Hallwachs electrometer, using an arrangement suggested by Elster and Geitel. Looking at this data there is no question that the thorium imparted a conductivity to the air. Further, the deflection of the electrometer indicator varied with the amount of mate220 / Journal of Chemical Education
rial present, affording conclusive proof of the source. Schmidt also found that a compound of thorium (e.g., thorium oxide, sulfate, or nitrate), left on a photographic plate which was wrapped in light-tight paper. would hlacken (expose) the plate in a day or two. Thin metal cutouts placed under the thorium layer left silhouettes on the photographic plate, and different substances were seen to vary in their opacity to the rays. Interestingly, wheu testing to determine if the absorption of the rays was proportional to the thickness of the intervening material, he found not much more absorption in many layers of tinfoil than in one. He roneluded that "the emitted rays from thorium are as little homogeneous as the uranium-or Roentgen rays" (8). I n general, the physical properties of thorium rays were found to be similar to those of uranium rays, with one major exception. Schmidt could find no evidence of polarization when the rays were passed through crossed tourmalines. Repetition of other Becquerel experiments, however, convinced him that the rays were refracted and probably reflected. Schmidt erred in these conclusions, in part led on by Becquerel, i.e., because of the latter's influence, Schmidt somewhat uncritically interpreted his experimental findings to show a positive effect. Within a year, Ernest Rutherford (9) presented evidence to the contrary, and hoth he and Marie Curie (10) suggested that uranium and thorium radiation were not electromagnetic in nature. But in early 1898, Schmidt concluded that thorium rays resemble both uranium rays and X-rays, but differ from each by failure of polarization and existence of refraction, respectively. Like Becquerel, he made no explicit statement that radioactivity is apparently an atomic property, although he did suggest that the phenomenon is related to the high atomic weights of uranium (240) and thorium (232). Yet in this period, suffering from an overabundance of different types of rays from numerous ~ubstances,~ the mere variation on a theme offered by thorium rays was not enough to resurrect interest in radioactivity. Schmidt announced his discovery of thorium's activity on February 4,1898; Marie Curie on April 12. Neither paper, however, caused any discernible activity among other scientists. It took the subsequent discoveries of polonium and radium by the Curie team to awaken sustained interest in this subject. New additions to the periodic table could be comprehended and justly valued, hut the extension of a phenomenon as basic as electricity, magnetism, or gravity could not then be appreciated. Literature Cited
(1) L..Arne?. J. Phvs.. 33. 128 (1965). , . BADASH. (2) . , 14 (1898). , ~ SCHMIDT. , - G,y C.. ~,vwhand; ~ h v s i k~. e i~.e r l i ,n17. -. (3) C r m r ~ M., , Compt. Rend., 12;, 1101 (1898). A,, Physifcalische Bldtter, 6 , 30 (1950); POGGEN(4) KRATZER, DORFF, J. C., Biographiseh-Lite7a7isehe8 Handubrterbueh der Ezakten Naturwissmsehejta, series 6 and 7. (5) See for example ARNOLD W., Ann. Physik, 61, 313 (1897), and DEUSGEN, E.,Ann. Physik, 66, 1128 (1898). (6) SCHMIDT, G. C., Ann. Physik, 65, 141 (1898). (7) SCHMIDT. G. C.., Comwt. Rend.. 126. 1264 118981. (8j SCHMIDT; G. C., Ann. ~hysik,'65,i41 (1898). E., Phil. Mag., 47, 109 (1899). (9) RUTHERFORD, M.,Revue Ghdrale des Sciences, 10, 41 (1899). (10) CURIE,
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=E.g., cathode rays, X-rays, canal rays, discharge rays, and rays from glow worms, fire flies, freshly-cleaned metallic surfaces, luminescent materials, etc. See Badash ( I ) .
