﻿Thorium and Radium Emanations. 361 



wondered at, as their concentration may be many times as 

 large as the concentration of the emanation. 



In none of the experiments with thorium emanation nor 

 with mixed emanations has any trace of the maxima pre- 

 viously observed been found, and their presence in the earlier 

 experiments, therefore, seems to be due to special circum- 

 stances. The fact that they were not found when P 2 5 was 

 enclosed in the tube, suggests that the presence or absence 

 of water vapour may have a deciding influence. They might, 

 of course, be due to the formation of a molecular complex, 

 but at present there is no evidence on which a decision as to 

 their cause can be based. 



It is worth noting that these experiments involve, for the 

 first time, the consideration of the time which an atom 

 spends in the gaseous phase in equilibribrium with a liquid. 

 In the case of thorium emanation, a determination of this 

 magnitude might be based on methods similar to those here 

 employed, if the true condensation curve could be deter- 

 mined. 



i Summary. 



(1) When thorium and radium emanations are mixed with 

 air at atmospheric pressure, thorium emanation appears to be 

 condensed about 5° C. above the radium emanation. 



(2) This apparent difference is probably due to the radium 

 emanation in the gaseous phase over the condensed phase 

 being swept away by the air current. 



(3) As the concentration of the emanation in a highly 

 exhausted tube diminishes the emanation becomes more easily 

 condensed. 



(4) In certain circumstances the condensation curve of 

 radium emanation exhibits two maxima, one about —75° C. 

 and the other about — 161° C. It is suggested that the 

 existence of this property may be dependent on the presence 

 of water vapour. 



(5) The existence of at least one maximum in the con- 

 densation curve is confirmed by studying a glass tube, 

 containing a large quantity of radium emanation, floating 

 vertically in liquid air. A ring of bright luminosity occurs 

 just above the surface of the liquid air, and this ring is not 

 due to a condition of the glass nor to a concentration of the 

 active deposit away from the emanation. 



(6) Internal changes are liable to take place inside the 

 tube which will completely alter the condensation curve 

 obtained. 



(7) At liquid-air temperature in a highly exhausted tube 



