﻿Thorium and Radium Emanations. 359 



However, it was found in this case that the quantities o£ 

 thorium and radium emanations condensed on different dates 

 remained appreciably constant, and that therefore the pro- 

 portional diminution effect was not an invariable property 

 of the mixed emanations. 



Thorium Emanation alone. 



An opportunity was taken after all the radium emanation 

 had decayed to determine the condensation curve for thorium 

 emanation alone in this tube. 



The experiments were carried out as explained on page 353, 

 and the values for E and the fraction of emanation con- 

 densed as stated on page 354. 



The curve obtained is shown in fig. 8, curve B, and it is 

 seen that the greater part of the emanation is condensed from 

 -162° C. to -180° C. 



Theory of the Experiment. 

 It is difficult to compare the preceding experiments, where 

 the fraction of the emanation condensed at a given tempe- 

 rature has been studied, with the condensation of an ordinary 

 gas where the vapour-pressure of the gas rather than the 

 fraction condensing is a function of the temperature. It may, 

 however, be pointed out that even were the radium and thorium 

 emanations physically identical as regards volatility, a differ- 

 ence in the direction found, namely, that the shorter lived 

 emanation will appear more volatile, is to be expected under 

 certain circumstances. In the case of thorium emanation 

 there is a steady supply of, say, x atoms per second to the 

 gaseous phase and a steady disappearance of the same number 

 per second, Ax from the condensed phase, and (1— A)x from 

 the gaseous phase, where A is the fraction condensed. Equi- 

 librium is established when Ax more atoms enter the con- 

 densed phase per second than leave it. In the case of radium 

 emanation, the numbers entering and leaving the condensed 

 phase are equal. In the case of mixed emanations, indis- 

 tinguishable in volatility, the ratio of the concentration of 

 thorium to radium emanation must be higher in the gaseous 

 than in the condensed phase, since the ratio of thorium to 

 radium emanation condensing is higher than the ratio of 

 thorium to radium emanation volatilizing. Hence the thorium 

 emanation must appear more volatile. Whether or not this 

 effect is large will involve the absolute time which, on the 

 average, a molecule of emanation spends in the gaseous 

 phase before entering the condensed phase. If this is com- 

 parable with the period o£ average life of the atom, tho effect 

 will be marked. 



