278 EXCITED RADIO-ACTIVITY [CH. 
emanation is almost completely lost by heating the wire to a white 
heat. Miss F. Gates! found that the activity was not destroyed 
by the intense heat, but manifested itself on neighbouring bodies. 
When the active wire was heated electrically in a closed cylinder, 
the activity was transferred from the wire to the interior surface 
of the cylinder in unaltered amount. The rate of decay of the 
activity was not altered by the process. By blowing a current of 
air through the cylinder during the heating, a part of the active 
matter was removed from the cylinder. Similar results were found 
for the excited activity due to radium. 
F. von Lerch (loc. cit.) determined the amount of activity 
removed at different temperatures. The results are shown in the 
following table for a platinum wire excited by the thorium ema- 
nation. 





T ae Percentage of | 
emperature | activity removed 
Heated 2 minutes ... A 800° C. 0 
then » minute more  .». 1020° C. 16 
” ” 3 ” ” 1260° C. 52 
a eye i . 1460° C. 99 


It is not possible to settle definitely from these experiments 
whether the active matter is actually volatilized at a high tempe- 
rature or is removed by disintegration of the surface of the wire. 
All the metals so far tried apparently lose their activity at about» 
the same temperature. 
181. Emission of heat. It has been shown in sections 105, 
106, and 163, that the radium emanation, together with the 
secondary products which arise from it, is responsible for about 
75 per cent. of the total heat emission observed for radium. 
The gradual decay to a minimum of the heat emission of the 
radium for the first few hours after the emanation is removed 
is due to the gradual decay of the excited activity produced by 
the occluded emanation in the radium itself. In a similar way, the 
gradual increase of the heating effect of the separated emanation 
1 Phys. Review, p. 300, 1903. 
