20 JBumstead — Heating Effects produced by Rimtgen Rays. 



Now if the conclusion, toward which the present experiments 

 point, be accepted, it is not surprising that the " fraction " a 

 should be greater than unity ; for if atomic energy is set free 

 it is not unlikely that it is through the mechanism of these 

 easily absorbed secondary rays. Of course the data from 

 which the above calculation has been made are not sufficiently 

 accurate to give any weight to the actual number obtained for 

 a ; but it is sufficiently clear that a considerable number of 

 these easily absorbed secondary rays must be generated, of 

 which only a very small fraction can escape from the metal. 



This conclusion in itself makes it somewhat improbable that 

 a large part of the energy could be transformed into the more 

 penetrating secondary rays and so carried away ; but it is desir- 

 able to consider the question of this type of rays independ- 

 ently. Sagnac, who has made an extensive series of experi- 

 ments upon secondary rays* finds that heavier metals in gen- 

 eral give out more -intense, and more easily absorbed, rays (of 

 the type now under consideration) than lighter metals ; and, in 

 particular, that this is true of lead as compared with zinc. On 

 the other hand, Townsend (loc. cit.) finds that lead is an excep- 

 tion among the heavier metals and, as a matter of fact, gives 

 out less intense and more penetrating rays (of this type) than 

 zinc, and this result has recently been confirmed by Evef. If 

 Sagnae's result were true in general, it would of course dispose 

 of all possibility of explaining the present experiments by 

 means of the energy of the secondary rays ; and even from 

 Townsend's observations (when allowance is made for the dif- 

 ference of absorption by air) it is impossible to conclude that 

 the total ionizing power of the secondary rays from zinc is 

 much greater than (or even as great as) that of the rays from 

 lead. But leaving these results out of account, I think it is 

 possible to show that, with the rays which I used and under 

 the experimental conditions, it is not possible to attribute the 

 results obtained to this cause. 



The secondary rays from metals are always more easily 

 absorbed than the primary rays which generate them. Barkla^; 

 has shown that the secondary radiation from gases and light 

 solids is practically identical in character with the primary 

 radiation ; and this had previously been found to be the case 

 for such substances as paraffin by Sagnac (loc. cit.). But all 

 experimenters, so far as I am aware, are agreed that the secon- 

 dary radiation from lead, zinc and other similar metals is less 

 penetrating than the primary ; and no evidence has been 

 obtained of any secondary rays more penetrating than the rays 



*Ann. Chim. Phys., xxii, p. 493, 1901. 

 fPMl. Mag., Yiii, p. 674, 1904. 

 tProc. Koy. Soc, Ixxiv, p. 474, 1905. 



