CHAPTER VII.
RADIO-ACTIVE EMANATIONS.
138. 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 surrounding 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.
139. 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[1] found that this
inconstancy was due to the presence of air currents. When a
closed vessel was used, the current, immediately after the introduction
of the active matter, increased with the time, and finally
- ↑ Owens, Phil. Mag. p. 360, Oct. 1899.
