780 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1908. 



discharge for potentials above 300 volts; Rutherford later called this 

 the " saturation current." 



Finall}^, by studying the modification of the velocity of discharge 

 produced by the interposition of lamina of different substances, Bec- 

 querel showed the complexity of the emanation emitted by the ura- 

 nium. From now on these radiations were called " Becquerel rays." 



All these results Avere verified by Rutherford, who extended them, 

 characterizing by their absorption two classes of rays: The a 

 rays, very active and greatly absorbed by the air; the /S rays, less 

 active and much less absorbed. He applied to gases, rendered con- 

 ductive by these rays, the theory of ionization, which J. J. Thomson 

 was then developing, and showed the identity of the phenomena 

 produced in the air by the Becquerel rays and the Rontgen rays. 



While Becquerel's results were being verified in England, M. and 

 Mme. Curie in France and Schmidt in Germany were searching for 

 this emanation from other bodies. Mme. Curie and Schmidt dis- 

 covered it simultaneously in thorium. Mme. Curie found that all 

 active bodies contained either uranium or thorium. She determined 

 by the quartz-piezo-electrical method of Curie that each compound of 

 uranium, whatever its history, possessed the same power of discharg- 

 ing — the same radio-activity — using the name adopted by Curie. 

 The two Curies then tried to isolate the body endowed with the 

 property of radio-activity ; and by an immense amount of work, using 

 Becquerel's rays and Curie's piezo-electrical method in their analyses, 

 they finally discovered polonium and then radium. Using pure 

 uranium as the unit of radio-activity, radium chloride has an activity 

 of 1,800,000. 



Becquerel could now continue his researches with the extremely 

 active products placed at his disposal by the Curies. His earlier 

 experiments he repeated with polonium and radium, and showed, by 

 his absorption tests, that polonium emits an emanation different from 

 that of radium. Utilizing the admirable collection of phosphorescent 

 compounds left by his father, Becquerel did not delay in establishing 

 several new properties of the emanation from the new products, 

 showing that each of them emitted a complex bundle of rays excit- 

 ing in a special manner the diverse phosphorescent substances. By 

 his absorption method he showed that the very penetrating rays excite 

 the double sulphate of uranium and polonium and that the most 

 penetrating rays excite the diamond. Finally, he noted that the 

 radium emanation can give back to bodies the property which they 

 may have lost of becoming phosphorescent by being heated. This 

 may also be accomplished by means of the electric discharge. 



Becquerel noted that the radium emanation gives to chlorine a 

 phosphorescence much more persistent than that produced by ordi- 

 nary light, and compared this with the similar result produced by 



