6 DISCOVERY AND NATURE OF RADIOACTIVITY 



rays. Some thirteen minerals were found to possess this property, 

 and among them pitchblende was the most active.* One of the first 

 results of their work was the discovery of foloniitm^ the first 

 substance emitting Becquerel rays to be isolated from pitchblende. 

 It was named by Madame Curie from Poland, her native country. 

 In this paper Madame Curie ^^ also proposed the term radioactive for 

 all substances giving rise to rays of this nature. Later in the same 

 year M. and Mme. Curie and Bemont''' announced the discovery of 

 radium, f and the fact that it was a new element was confirmed spec- 

 troscopically by Demarcay.*^ The atomic weight of radium, as 

 determined by Mme. Curie, is 225, while Wilde's "* determination 

 gives 232. The atomic weight of uranium is 240. The results of 

 Mme. Curie's researches up to the year 1904 are embodied in her 

 TheseJ'^ 



It remains now to trace very briefly the researches that have led 

 to a clear understanding of the nature of radioactivity, and its general 

 distribution in nature. 



The Complexity of the Rays : In 1899 Rutherford ^"^ made the 

 discovery that the rays of uranium are complex, consisting of at 

 least two kinds, to which he gave the names « rays and /5 rays. 



Cathode Rays from Radium ; Beta Rays : In the same year 

 Giesel,'^^ Meyer and von Schweidler,"'' and Becquerel^ all discovered 

 independently that rays from radium were deflected by a magnet, 

 and in the following year (1900) BecquereP showed that their 

 behavior in the magnetic field was quite similar to that of cathode 

 rays. Evidently, then, here was a type of cathode ray given off by 

 certain bodies spontaneously, and at atmospheric pressure. Follow- 

 ing up these experiments, M. Curie ^ demonstrated, as Rutherford 

 had the year previously for uranium, that the rays given off by 

 various radioactive bodies are complex, consisting of at least two 

 kinds of rays, one deviable by the magnetic field, and the other not. 

 The deviable rays, said M. and Mme. Curie, •^'' are charges of nega- 

 tive electricity. In 1902 Rutherford ^"^ announced that the negatively 

 charged particles emitted by both uranium and radium are similar 

 in all respects to cathode rays. The /9 particles vary considerably in 

 velocity between certain limits, thus introducing a complexity into 

 the nature of the /9 ray. 



*Afanasjew (1900) has since examined 51 minerals, and found that all containing 

 uranium and thorium can blacken the photographic plate. 

 t See also citation No. 40. 



