448 Prof. Rutherford and Mr. Soddy on the 



of the emanation, very rapidly at first and then more gradually 

 until it attains a maximum after 5 or 6 hours nearly twice 

 as great as at first, and then decreases according to the above 

 law of the decay of the emanation . If when the maximum is 

 attained the emanation is blown out of the testing-vessel the 

 excited activity remains, representing about one-half of the 

 total activity, concentrated on the negative electrode. In 

 the actual measurements it was usual to take a series of 

 readings at regular short intervals after the introduction of 

 the emanation. The proportion of the current due to the 

 emanation could be easily deduced from these readings, for 

 the proportionate increase due to excited activity is for any 

 given interval independent of the quantity and " age " of the 

 emanation, and is the same throughout the whole series of 

 measurements. 



During the progress of the work a very similar result to 

 the one above given for the rate of decay of the radium 

 emanation has been published by P. Curie (Comptes Rendus, 

 1902, cxxxv. p. 857) under the title " On the Time-Constant 

 characteristic of the Disappearance of Radioactivity induced 

 by Radium in a Closed Space.' 3 It is an interesting example 

 of how a measurement of the rate of decay of the excited (or 

 induced) activity in a closed space containing the radium 

 emanation gives in reality the rate of decay of the latter, and 

 not of the excited activity at all. The latter decays at the 

 same rate in the free air as when sealed up in glass. 



M. Curie measured the penetrating radiation emitted from 

 a sealed glass tube containing the radium emanation ; and 

 since the latter gives no penetrating rays (compare § 5), the 

 effect measured was solely due to the excited activity on the 

 walls produced by the emanation. As we have seen, after an 

 initial period of some hours this effect attains a maximum, 

 and then represents the equilibrium-point when the decay of 

 excited activity is balanced by the continuous production of 

 fresh active matter from the emanation. As the activity of the 

 emanation decays, so also does its power of exciting activity 

 decrease in like ratio. Hence the excited activity in a closed 

 tube containing the radium emanation furnishes a measure of 

 the activity of the emanation itself. After the initial period 

 of a few hours has elapsed, its constant of decay is the same 

 as that of the emanation. If at any time the emanation is 

 blown out of the tube, the rate of decay changes at once to 

 the time-rate of decay of the excited activity, as M. Curie's 

 experiments themselves show. 



M. Curie performed his experiments under the most varied 

 conditions and the rate of decay of the activity was unaffected 



