vill] RADIO-ACTIVE EMANATIONS 249 
charge and have a mass about twice that of the hydrogen atom. 
The emanations do not possess the property of permanently radiat- 
ing, but the intensity of the radiations diminishes according to an 
exponential law with the time, falling to half value, in the case of 
thorium in one minute, and in case of radium in about four days. 
The law of decay of activity does not seem to be influenced by 
any physical or chemical agency. 
The emanation particles gradually break up, each particle as it 
breaks up expelling a charged body. The emanation after it has 
radiated ceases to exist as such, but is transformed into a new 
kind of matter, which is deposited on the surface of bodies and 
gives rise to the phenomena of excited activity. This last property, 
and the connection of the emanation with it, is discussed in detail 
in the succeeding chapter. 
