G. F. Barker — Radioactivity of Thorium Minerals. 161 



Akt. XYII. — Radioactivity of Thorium Minerals •^'^ by 

 George F. Barker. 



The discovery of the spontaneous radioactivity of nraninm 

 and its compounds, by Becquerel in 1896f has affected most 

 profoundly our views not only of the constitution of matter, 

 but also of the relations existing between matter and energy. 

 The extraordinarily delicate means which he proposed for the 

 detection of this radioactivity, first the photographic method, 

 and second the electric, made possible the remarkable dis- 

 covery by the Curies and Bemont in 189 8:J: of the new element 

 radium, whose quantity in uraninite is so infinitesimal as to be 

 far beyond our power of detection by the balance, or even by 

 the spectroscope. Indeed, only the enormous ratio of the 

 energy of radium to the radioactive matter concerned in emit-, 

 ting it, enabled it to be recognized even by the electrometer. 



In February, 1898, G. C. Schmidt presented to the Physical 

 Society of Berlin§ a preliminary paper on the rays emitted by 

 the element thorium and its compounds. This paper was 

 printed in extenso in Wiedemann's Annalen in the following 

 April. II On the 12th of the same month, a communication 

 from Mme. SModowska Curie was presented by Lippmann to 

 the Academie des Sciences of Paris^f giving an account of her 

 examination of the radioactivity of a number of substances, 

 including the compounds of thorium. Like Schmidt, she used 

 both the photographic and the electric methods in her investi- 

 gations, making the latter quantitative. Two condenser j)iates 

 were employed, 8 centimeters in diameter, separated 3 centi- 

 meters from each other, and having a potential difference of 

 100 volts maintained between them. A uniform layer of the 

 substance to be examined, finely pulverized, was placed on the 

 lower plate ; and since the emitted rays ionized the air between 

 the plates and rendered it conducting, a current traversed the 

 condenser and charged the electrometer to which the upper 

 plate was connected. Knowing the capacity of the electro- 

 meter, inasmuch as the speed of deviation of the needle is 

 proportional to the current-strength, it is possible to determine 

 the current in absolute measure. It was found preferable, 

 however, to ?neasure the counter potential difference required 

 to maintain the electrometer at zero. As its charge was feeble, 

 this compensation was accomplished by means of a piezoelec- 



* Read before the National Academy of Sciences, April 22, 1903. 



t Comptes Rendus, cxxii, 420, 501, 689, 1896. 



X Comptes Rendus, cxxvii, 1215, December, 1898. 



§ Verb. Phys. Ges. Berlin, xvii, 14, February, 1898. 



II Ann. Pbys. Cbem., II, Ixv, 141-151, April 15, 1898. 



■ff Comptes Rendus, cxxvi, 1101, April 12, 



