4 RADIO-ACTIVE SUBSTANCES [CH. 
The characteristic property of these radiations, besides their 
penetrating power, is their action on a photographic plate and 
their power of discharging electrified bodies. In addition, a 
strongly radio-active body like radium is able to cause marked 
phosphorescence and fluorescence on some substances placed near 
it. In the above respects the radiations possess properties 
analogous to Réntgen rays, but it will be shown that, for the 
major part of the radiations emitted, the resemblance is only 
superficial. 
The most remarkable property of the radio-active bodies is 
their power of spontaneously and continuously radiating energy at 
a constant rate, without, as far as is known, the action upon them 
of any external exciting cause. The phenomena at first sight 
appear to be in direct contradiction to the law of conservation of 
energy, since no obvious change with time occurs in the radiating 
material. The phenomena appear still more remarkable when it 
is considered that the radio-active bodies must have been steadily 
radiating energy since the time of their formation in the earth’s 
crust. 
Immediately after Rontgen’s discovery of the production of 
X rays, several scientists were led to examine if any natural 
bodies possessed the property of giving out radiations which could 
penetrate metals and other substances opaque to light. As the 
production of X rays seemed to be in some way connected with 
cathode rays, which cause strong fluorescent and phosphorescent 
effects on various bodies, the substances first examined were those | 
that were phosphorescent when exposed to light. The first obser- 
vation in this direction was made by Niewenglowski1, who found 
that sulphide of calcium exposed to the sun’s rays gave out some 
rays which were able to pass through black paper. A little later 
a similar result was recorded by H. Becquerel? for a special 
calcium sulphide preparation, and by Troost* for a specimen of 
hexagonal blend. These results were confirmed and extended in 
a later paper by Arnold*. No satisfactory explanations of these 
somewhat doubtful results have yet been given, except on the 
view that the black paper was transparent to some of the light 
1 C. R. 122, p. 385, 1896. 2 CO, R. 122, p. 559, 1896. 
3 C. R. 122, p. 564, 1896. 4 Wied. Annal. 61, p. 316, 1897. 
