﻿180 Dr. M. Levin on the Origin of the ft Rays 



from commercial thorium always contains impurities, which 

 are difficult to remove, and the complete volatilization of the 

 active deposit was not easy under such conditions. Thorium 

 A was almost completely driven off by heating the platinum 

 dish for 10 minutes with a blowpipe, but some thorium B 

 still remained behind after heating to the melting-point of 

 iron by means of a blowpipe-furnace, kindly provided by 

 Dr. Stansfield. Consequently, both the a and /3 ray activities 

 were initially higher than those shown in fig. 1. As thorium 

 B decayed, the a and (3 ray activities at first decreased, passed 

 through a minimum and gradually increased again in exactly 

 the same way as in the curves shown in fig. 1. 



In a recent investigation, Dr. Halm has shown that the 

 active deposit of thorium contains two distinct a ray products, 

 called thorium B and thorium C*. If each of these four 

 a ray products present with thorium X supplies an equal 

 proportion of the total equilibrium activity, the activity due 

 to thorium X and the emanation together should be 50 per 

 cent, of the total. Xow when the active deposit is com- 

 pletely removed, the initial a activity observed is due to 

 thorium X and the emanation together; for, in consequence 

 of its rapid rate of change, the latter reaches its equilibrium 

 value in a few minutes after the platinum dish is removed 

 from the furnace. The maximum activity is reached about 

 one day later, when the active deposit is in equilibrium with 

 the thorium X. During this time the activity of the thorium 

 X has been diminishing, and consequently the maximum 

 activity observed is less than the theoretical value, which 

 would have been obtained if the active deposit had initially 

 been in equilibrium with the thorium X. This theoretical 

 value can be determined in a simple way by extrapolation 

 from the curve of decay of thorium X. In this way it was 

 deduced that the theoretical initial value was 126, showing 

 that the activity of thorium X and the emanation together is 

 48 per cent, of this maximum, thus agreeing with the assump- 

 tion made above. 



In a similar manner, the initial percentage of the /3 activity 

 is reduced to 2'61 percent. In other experiments the values 

 found were 3*06, 2*75, and 2*92 per cent. If thorium X 

 gave out (3 rays, one would expect about 50 per cent, for the 

 initial value of the /3 activity— an amount of another order of 

 magnitude from that observed. Therefore we conclude that 

 the transformation of thorium X into the emanation is 

 accompanied only by the emission of a rays. 



It will be seen from fig. 1 that the a ray activity reaches 

 * Loc. cit. 



