A HISTORY OF SCIENCE 



Academy of Sciences that the well-known chemical 

 compound calcium sulphide, when exposed to sunlight, 

 gave off rays that penetrated black paper. He had 

 made his examinations of this substance, since, like 

 several others, it was known to exhibit strong fluores- 

 cent or phosphorescent effects when exposed to the 

 cathode rays, which are known to be closely connected 

 with the X-rays. This discovery was followed very 

 shortly by confirmatory experiments made by Bec- 

 querel, Troost, and Arnold, and these were followed in 

 turn by the discovery of Le Bon, made almost simulta- 

 neously, that certain bodies when acted upon by sun- 

 light give out radiations which act upon a photographic 

 plate. These manifestations, however, are not the 

 effect of radio-activity, but are probably the effects 

 of short ultra-violet light waves, and are not produced 

 spontaneously by the substances. The radiations, or 

 emanations, of the radio-active substances, on the 

 other hand, are given out spontaneously, pass through 

 substances opaque to ordinary light, such as metal 

 plates, act upon photographic plates, and discharge 

 electrified bodies. The substances uranium, thorium, 

 polonium, radium, and their compounds are radio- 

 active, radium being by far the most active. 



The first definite discovery of such a radio-active 

 substance was made by M. Henri Becquerel, in 1896, 

 while making some experiments upon the peculiar ore 

 pitch-blende. Pitch-blende is a heavy, black, pitchy- 

 looking mineral, found principally at present in some 

 parts of Saxony and Bohemia on the Continent, in 

 Cornwall in Great Britain, and in Colorado in America. 

 It is by no means a recently discovered mineral, having 



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