CHAPTER VIII. 
RADIO-ACTIVE EMANATIONS. 
129. Introduction. A most important and striking property 
possessed by radium, thorium, and actinium, but not by uranium or 
polonium, is the power of continuously emitting into the surround- 
ing space a material emanation, which has all the properties of a 
radio-active gas. This emanation is able to diffuse rapidly through 
gases and through porous substances, and may be separated from 
the gas with which it is mixed by condensation by the action of 
extreme cold. This emanation forms a connecting link between 
the activity of the radio-elements themselves and their power of 
exciting activity on surrounding objects, and has been studied more 
closely than the other active products on account of its existence in 
the gaseous state. The emanations from the three active bodies all 
possess similar radio-active properties, but the effects are more 
marked in the case of the emanation from radium, on account of 
the very great activity of that element. 
Thorium Emanation. 
130. Discovery of the emanation. In the course of 
examination of the radiations of thorium, several observers had 
noted that some of the thorium compounds, and especially the 
oxide, were very inconstant sources of radiation, when examined in 
open vessels by the electrical method. Owens! found that this 
Inconstancy was due to the presence of air currents. When a 
closed vessel was used, the current, immediately after the intro- 
duction of the active matter, increased with the time, and finally 
} Phil. Mag. p. 360, Oct. 1899. 
