418 History of Luminescence 



and it also owes something to chance. Since cathode rays and X-rays 

 excite luminescence, why not look for penetrating radiation which 

 might be emitted by fluorescent and phosphorescent substances in 

 general. In addition to the visible light of phosphorescence, it is 

 quite possible that some other peculiar radiation ^® might also be 

 produced. Such was the reasoning of Antoine Henri Becquerel 

 (1852-1908) , who tested the hypothesis and communicated the re- 

 sults to the French Academy of Sciences on February 24, 1896. 

 Becquerel exposed a number of phosphorescent substances to light 

 and placed them on a photographic plate wrapped in black paper, 

 which was later developed. Many samples gave negative results, but 

 among them, fortunately, was a uranium potassium sulphate, a salt 

 whose phosphorescence he had previously studied. When the plate 

 exposed to this salt was developed it showed a definite blackening 

 that was not due to vapors, which he ruled out by placing a thin 

 sheet of glass between the salt and the plate. It was quite certain 

 that the uranium salt emitted a penetrating ray. 



Somewhat later,^' on an overcast day, Becquerel placed his 

 uranium specimen and paper-covered plate in a drawer, awaiting 

 a sunny day for the exposure, and then decided to develop the 

 plate to see whether any darkening had occurred without the sun- 

 light. To his astonishment the uranium salt affected the plate even 

 without excitation of phosphorescence. Evidently a penetrating 

 radiation which had nothing to do with phosphorescence came 

 from the uranium salt and affected the plate. 



Later experiments indicated that other uranium compounds also 

 emitted penetrating radiation and that the property was charac- 

 teristic of uranium itself. In fact, at that time no other known 

 phosphor could have replaced the uranium salt, and any uranium 



trical Engineers and the American Electrochemical Society in New York City, where 

 he exhibited post-cards of the blue grotto of Capri printed with phosphorescent paint. 

 He also demonstrated Edison's calcium tungstate lamp or " X-ray lamp," essentially a 

 Crookes' tube whose interior walls were coated with calcium timgstate. The tube 

 glowed when connected to a large induction coil, owing to cathode rays striking the 

 tungstate crystals. Hammer quoted Langley on the firefly as the most efficient source 

 of light and Sir Oliver Lodge as saying that if the secret of the firefly were known, 

 a boy turning a crank could furnish sufficient energy to light an entire electric circuit. 



Another popular writer with a special interest in " Intra-atomic Energy " (see 

 Annual Report Smithsonian Institution, 263-293, 1903) was Gustave LeBon. His books 

 were entitled: Evolution de la matiere (Paris, 1905; English 1907) and Evolution of 

 forces (New York, 1908) , both of which involve discussion of luminescence and in- 

 visible light. 



^« H. Muraoka (1896) claimed that penetrating radiation accompanied firefly light, 

 but the effect was shown to be due to chemical action on the photographic plate 

 from the paper with which it was wrapped. 



^^ See the series of papers by H. Becquerel in the Comptes Rendus of the French 

 Academy (1896-1897). 



