568 Prof. Rutherford and ~Mv. Soddy on 



given for radium, for in this case the point measured is the 



temperature at which some of the thorium emanation first 

 escaped condensation, and not the point at which volatilization 

 begins of that already condensed. In place of the T-tube and 

 gas-holder of fig. 2 (PL XIV.) a highly emanating thorium 

 compound was placed in series with the gas-stream, and the 

 temperature T observed at which the emanation first began 

 to make its appearance in the testing-vessel. The following 

 are some of the result- obtained : — 



Gas. 



c. c. per Sec 



onci. 



T. 



Hydrogen . 



. . '71 





— 155= C. 





1-38 





-159 



Oxygen . 



. . -dS 





— 155°-5 





•58 





-15fr 





■58 





— 155°*5 



If these result- are compared with those obtained for the 

 radium emanation it will be seen that the temperatures with 

 an equal gas-stream in the two cases are very nearly the 

 same. Thus, in the determinations in an oxygen stream of 

 *58 c. c. per second, the radium emanation commenced to 

 volatilize at — 15 5 C , and some of the thorium emanation 

 escapes condensation at — 155°*5 G. It was at first thought 

 that this result indicated that the condensation-points of the 

 two emanations were identical. It will be recalled that no 

 difference could be detected in the chemical properties of the 

 two emanations, both being quite unaffected by the most 

 powerful chemical reagents. Yet it seemed improbable that 

 the two emanations could be materially identical on account 

 of the completely distinct character of their radioactive 

 properties. Not only are their rates of decay widely different, 

 being 5000 times faster in the one case than the other, but 

 the excited activities they give rise to are also completely 

 different, not only in the rate of decay, but even in the 

 number of changes through which they apparently pass 

 before their activity disappears. In the course of further 

 work a very distinct difference of behaviour in the con- 

 densation phenomena in the two cases was brought to light. 

 It was observed that some of the thorium emanation was 

 condensed at temperatures as much as 30° above the point 

 of complete condensation. The curve fig. 5 (PL XIY.) is an 

 example of the results obtained. The maximum ordinate 

 taken as 100 represents the amount of emanation entering 

 the testing-vessel at temperatures far above the point at 

 which condensation commence-. This amount begins to 

 decrease at about — 120 c , and becomes le>s than 1 per cent. 



