Radioactivity of Radium and Thorium. 447 



§ 2. Rate of Decay of the Activity of the Radium Emanation. 



It has long been known that the activity of the radium 

 emanation decays very slowly, and special methods were 

 found necessary for its determination. The emanation mixed 

 with air was obtained from a solution of radium chloride kept 

 in a closed bottle, and was stored over mercury in an ordinary 

 gas-holder. From time to time equal quantities were mea- 

 sured off by a gas pipette and delivered into the testing- 

 vessel. The latter consisted of an air-tight brass cylinder 

 carrying a central electrode insulated by an ebonite stopper 

 provided with a guard-ring connected to the earth. A 

 sufficient voltage to obtain the saturation-current was applied 

 to the outside of the cylinder, and the inner electrode was 

 connected with the electrometer, with a suitable capacity in 

 parallel. The contents of the gas-holder were thoroughly 

 mixed by shaking, and a definite volume measured off by the 

 pipette and blown into the cylinder which w r as then closed 

 air-tight. The ionization-current immediately after the intro- 

 duction of the emanation furnished the measure of the activity 

 of the latter. The measurements were repeated at suitable 

 intervals over a period of 33 days before the effect became 

 too small to be accurately determined. 



The following table expresses the results : — 



Time in Hours. Relative Activity. 



100 



20-8 85-7 



187-6 24-0 



354-9 6-9 



521-9 1-5 



786-9 0*19 



It will be seen that the activity falls in a geometrical pro- 

 gression with the time and decays to half value in 3" 71 days. 

 If I is the original activity and 1^ the activity after time t, 



Taking as the value of X the mean value deduced from the 

 last four observations of the above table, we find 



X = 2-16.10-«=j^, 



when t is expressed in seconds. 



In these measurements the effect of excited activity was 

 eliminated by taking the current immediately after the intro- 

 duction of the emanation into the cylinder. In a closed space 

 the ionization-current steadily increases after the introduction 



