DISCOVERY AND NATURE OF RADIOACTIVITY 5 



Rays of Niewenglowski : The phenomenon of X rays was 

 always associated with phosphorescence, and Henri Poincare'^'' had 

 already suggested that the two phenomena might bear a causal rela- 

 tion to each other. Experiments with phosphorescent sulfide of zinc, 

 by Henry, *^" led to the discovery that a coating of that substance on a 

 body, otherwise opaque to the X rays, rendered the body transparent 

 to them. Rays that could penetrate matter opaque to ordinary light 

 were obtained by Niewenglowski^^ in 1896 with several phosphores- 

 cent bodies after exposing them to sunlight. He obtained the image 

 of a piece of money on sensitive paper that was protected from light 

 rays by rays from phosphorescent sulfide of calcium that had been 

 exposed to sunlight. 



Becquerel Rays and the Discovery of Radioactivity ; In 

 the same eventful year of 1896 Becquerel ^ confirmed Niewen- 

 glowski's results, and experimented with, among other substances, 

 various salts of uranium. Finally he ^ demonstrated that exposure 

 to sunlight was not necessary, but that salts of uranium that had 

 never been exposed to light gave out invisible rays that could pass 

 through opaque objects and darken a photographic plate. Further- 

 more, while uranic salts are phosphorescent, the uranous salts are 

 not, though both possess the property of radioactivity. Thus it was 

 shown that the phenomenon is not necessarily connected with phos- 

 phorescence. 



It was recognized^ that Becquerel rays were very similar to X rays, 

 and since all the salts of uranium, whether they had ever been ex- 

 posed to light or not, and whether crystallized or dissolved, gave rise 

 to the rays, the latter were thought to be due to uranium. Experi- 

 ment showed that metallic uranium was strongly active. "Uranium," 

 said Becquerel,'' " is the first example of a metal manifesting a phe- 

 nomenon of the nature of an invisible phosphorescence." These in- 

 visible rays from uranium that can pass through matter opaque to 

 ordinary light and darken a photographic plate, are known as 

 Becquerel rays. 



The Discovery of Radium : After Becquerel's discovery. Mon- 

 sieur and Madame Curie, of Paris, began to examine different min- 

 erals containing uranium in order to see if they gave off Becquerel 



Rontgen's first hint of the rays was their effect on barium platino-cyanide paper (Nov. 

 8, 1895). In the following year Troost "* announced that artificial hexagonal blend 

 gave off X rays, and could be substituted for the Crookes tube in many experiments. 



