tr» 
<= 
. 
206 RADIO-ACTIVE EMANATIONS [CH. 
substances but are unable to pass through a thin plate of mica; 
both behave like a temporarily radio-active gas, mixed in minute 
quantity with the air or other gas in which they are conveyed. 
136. Decay of activity of the emanation. Very little 
emanation escapes from radium chloride in the solid state, but 
the amount is largely increased by heating, or by dissolving the 
compound in water. By bubbling air through a radium chloride 
solution, or passing air over a heated radium compound, a large 
amount of emanation may be obtained which can be collected, 
mixed with air, in a suitable vessel. 
Experiments to determine accurately the rate of decay of 
activity of the emanation have been made by P. Curie, and 
Rutherford and Soddy*. In the experiments of the latter, the 
emanation mixed with air was stored over mercury in an ordinary 
gas holder. From time to time, equal quantities of air mixed with 
the emanation were measured off by a gas pipette and delivered 
into a testing vessel. The latter consisted of an air-tight brass 
cylinder carrying a central insulated electrode. A saturation voltage 
was applied to the cylinder, and the inner electrode was connected 
to the electrometer with a suitable capacity in parallel. The 
saturation current was observed immediately after the mtroduction 
of the active gas into the testing vessel, and was taken as a measure 
of the activity of the emanation present. The current increased 
rapidly with the time owing to the production of excited activity 
on the walls of the containing vessel. This effect is described in — 
detail in chapter Ix. 
The measurements were made at suitable intervals over a period 
of 33 days. The following table expresses the results, the initial 
activity being taken as 100. 
Time in hours Relative Activity 
0 100 
20°8 85°7 
187°6 24:0 
3549 69 
521-9 15 
786°9 0°19 
1 C. R. 135, p. 857, 1902. 2 Phil. Mag. April, 1903. 
