the Cause and Mature of Radioactivity. 577 



IV. Effect of Conditions upon Emanating Power. 



The experiments just described brought out two new points. 

 Thorium hydroxide possessed an emanating power which 

 increased with time since preparation, and when it attained its 

 maximum it was much greater than that of the oxide. Before 

 any further work was undertaken, it was necessary to make a 

 close study of the influence of conditions upon the emanating 

 power of thorium compounds. 



Effect of Temperature. — The effect of increase of tem- 

 perature on the emanating power of thoria has already been 

 fully investigated by one of us (Phi/s. Zeit. ii. p. 429, 1901). 

 The results, stated briefly, show that an increase in tem- 

 perature up to a certain limit, in the neighbourhood of a red 

 heat, correspondingly increases the emanating power. At 

 the maximum this is between three and four times that at the 

 ordinary temperature, and is maintained at this increased 

 value for several hours without any sign of diminution with 

 time. When the thoria is allowed to cool, the emanating 

 power then returns to the neighbourhood of the normal value. 

 If. however, the limit of temperature given is exceeded, 

 de-emanation sets in, and even while the high temperature is 

 maintained, the emanating power falls rapidly to a fraction 

 of its former value. On cooling, the substance is found to he 

 more or less de-emanated. It is of interest that no increase of 

 emanating power is observed when de-emanation commences. 



These experiments were extended to include the effects of 

 cooling. The platinum tube which contained the thoria was 

 surrounded with a felt jacket containing a mixture of solid 

 carbon dioxide and ether. The emanating power immediately 

 fell to 10 per cent, of its former value. On removing the 

 cooling agent it again rose quickly to nearly the normal. 



In another experiment some thoria was surrounded in a 

 platinum crucible with a mixture of solid carbon dioxide and 

 ether, and kept in a vacuum for several hours. On removing 

 it, and allowing its temperature to rise, it possessed much 

 the same value as an ordinary sample, and after standing 

 some time in the air it was again tested, and no difference 

 could be detected between the two. 



Thus changes in temperature produce very marked simul- 

 taneous changes in emanating power, but between the 

 limits of —110° and an incipient red heat no permanent 

 alteration in the value occurs. 



Effect of Moisture. — Dorn (loc. cit.) had noticed that 

 moisture produced a moderate increase in the power of thoria 



