1] RADIO-ACTIVE SUBSTANCES 9 
The ionization of the gas by phosphorus does not, however, 
seem to be due to a penetrating radiation like that found in the 
case of uranium, but rather to a chemical action taking place at 
its surface. The compounds of phosphorus do not show any 
activity, and in this respect differ from uranium and the other 
active bodies. 
Le Bon! has also observed that quinine sulphate, if heated and 
then allowed to cool, possesses for a short time the property of 
discharging both positively and negatively electrified bodies. It 
is necessary, however, to draw a sharp line of distinction between 
phenomena of this kind and those exhibited by the naturally radio- 
active bodies. While both, under special conditions, possess the 
property of ionizing the gas, the laws controlling the phenomena 
are quite distinct in the two cases. For example, only one com- 
pound of quinine shows the property, and that compound only 
when it has been subjected to a preliminary heating. The action 
of phosphorus depends on the nature of the gas, and varies with 
temperature. On the other hand, the activity of the naturally 
radio-active bodies is spontaneous and permanent. It is exhibited 
by all compounds, and is not, as far as is yet known, altered by 
change in the chemical or physical conditions. 
9. The discharging and photographic action alone cannot be 
taken as a criterion as to whether a substance is radio-active or 
not. It is necessary in addition to examine the radiations, and to 
test whether the actions take place through appreciable thicknesses 
of all kinds of matter opaque to ordinary light. For example, a 
body giving out short waves of ultra-violet light can be made to 
behave in many respects like a radio-active body. As Lenard? has 
shown, short waves of ultra-violet light will ionize the gas in their 
path, and will be rapidly absorbed in the gas. They will produce 
strong photographic action, and may pass through some substances 
opaque to ordinary light. The similarity to a radio-active body is 
thus fairly complete as regards these properties. On the other 
hand, the emission of these light waves, unlike that of the radiations 
from an active body, will depend largely on the molecular state 
1 C. R. 130, p. 891, 1900. 
2 Drude’s Annal. 1, p. 498; 3, p. 298, 1900. 
