10 RADIO-ACTIVE SUBSTANCES [cH. 
of the compound, or on temperature and other physical conditions. 
But the great point of distinction lies in the nature of the radia- 
tions from the bodies in question. In one case the radiations behave 
as transverse waves, obeying the usual laws of light waves, while m 
the case of a naturally active body, they consist for the most part 
of a continuous flight of material particles projected from the 
substance with great velocity. Before any substance can be called 
“yadio-active ” in the sense in which the term is used to describe 
the properties of the natural radio-active elements, it is thus 
necessary to make a close examination of its radiations; for it is 
unadvisable to extend the use of the term “radio-active” to 
substances which do not possess the characteristic radiating 
properties of the radio-active elements which we have described, 
and the active products which can be obtained from them. Some 
of the pseudo-active bodies will however be considered later in 
chapter IX. 
10. Thorium. In the course of an examination of a large 
number of substances, Schmidt* found that thorium, its compounds, 
and the minerals containing thorium, possessed properties similar 
to those of uranium. The same discovery was made independently 
by Mme Curie®. The rays from thorium compounds, like those 
from uranium, possess the properties of discharging electrified 
bodies and acting on a photographic plate. Under the same 
conditions the discharging action of the rays is about equal in 
amount to that of uranium, but the photographic effect is 
distinctly weaker. 
The radiations from thorium are more complicated than those 
from uranium. It was early observed by several experimenters 
that the radiation from thorium compounds, especially the oxide, 
when tested by the electrified method, was very variable and 
uncertain. A detailed investigation of the radiations from thorium 
under various conditions was made by Owens*®. He showed that 
thorium oxide, especially in thick layers, was able to produce 
conductivity in the gas when covered with a large thickness of 
paper, and that the amount of this conductivity could be greatly 
1 Wied. Annal. 65, p. 141, 1898. 2 CO. R, 126, p. 1101, 1898. 
3 Phil, Mag. Oct. 1899. 
