vir] RADIO-ACTIVE EMANATIONS 201 
the gas, which is a measure of the activity of the emanation, 
diminishes according to an exponential law with the time like the 
activity of the products Ur X and Th X. The rate of decay is, 
however, much more rapid, the activity of the emanation decreas- 
ing to half value in about one minute. According to the view 
developed in section 124, this expresses the result that half of the 
Current 

4 
Time in Minutes 
Fig. 38. 
emanation particles have undergone change in one minute. After 
an interval of 10 minutes, the current due to the emanation is 
very small, showing that practically all the emanation particles 
present have undergone change. 
The decrease of the current with time is an actual measure of 
the decrease of the activity of the emanation, and is not m any 
way influenced by the time taken for the ions produced to reach 
the electrodes. If the ions had been produced from a uranium 
compound, the duration of the conductivity for a saturation voltage 
would only have been a fraction of a second. 
The rate of decay of the activity of the emanation is independ- 
ent of the electromotive force acting on the gas. This shows that 
