ae sa 
198 RADIO-ACTIVE EMANATIONS [CH. 
reached a constant value. By drawing a steady stream of air 
through the vessel the value of the current was much reduced. It 
was also observed that the radiations could apparently pass through 
large thicknesses of paper, which completely absorbed the ordinary 
a radiation. 
In an investigation of these peculiar properties of thorium 
compounds, the writer’ found that the effects were due to an 
emission of radio-active particles of some kind from the thorium 
compounds. This “emanation,” as it was termed for convenience, 
possesses the properties of ionizing the gas and acting on a photo- 
graphic plate, and is able to diffuse rapidly through porous 
substances like paper and thin metal foil. 
The emanation, like a gas, is completely prevented from escap- 
ing by covering the active matter with a thin plate of mica. The 
emanation can be carried away by a current of air; it passes 
through a plug of cotton-wool and can be bubbled through solutions 
without any loss of activity. In these respects, it behaves very 
differently from the ions produced in the gas by the rays from 
active substances, for these give up their charge completely under 
the same conditions. 
Since the emanation passes readily through large thicknesses 
of cardboard, and through filters of tightly packed cotton-wool, it 
does not seem likely that the emanation consists of particles of 
dust given off by the active matter. This point was tested still 
further by the method used by Aitken and Wilson, for detecting 
the presence of dust particles in the air. The oxide, enclosed in 
a paper cylinder, was placed in a glass vessel, and the dust was 
removed by repeated small expansions of the air over a water 
surface. The dust particles act as nuclei for the formation of 
small drops and are then removed from the air by the action of 
gravity. After repeated expansions, no cloud was formed, and the 
dust was considered to be removed. After waiting for some time 
to allow the thorium emanation to collect, further expansions were 
made but no cloud resulted, showing that for the small expansions 
used, the particles were too small to become centres of condensa- 
tion. The emanation then could not be regarded as dust emitted 
from thorium. 
1 Phil. Mag. p. 1, Jan. 1900. 
