814 Dr. Norman Campbell on 



collisions o£ the rays originally liberated with other atoms, 

 the independence of this state o£ the field through which the 

 rays pass after liberation might possibly be explained. And 

 again, the occurrence of collisions between the liberation of 

 the rays and their emergence would explain the remarkable 

 absence of rays emerging with speeds much greater than 

 that required for ionization. On the other hand, there is no 

 evidence that the 8 rays which emerge come from a layer 

 of finite thickness ; the layer from which they come is 

 certainly thinner than the thinnest material films obtain- 

 able, but then these layers are not less than 100 molecules 

 thick. 



A second explanation of the discrepancy might be obtained 

 by supposing that the 11 volts, which is the minimum speed 

 of the primary rays necessary for ionization, is not the least 

 •energy which must be communicated by those rays to an 

 electron in order to cause it to appear as a 8 ray. According 

 to Lenard, 11 volts is the energy necessary to enable the 

 primary rays to penetrate within the atom at all and to act 

 upon the electrons within it ; the energy which they would 

 have to give to an electron in order to liberate it when they 

 act on it may be quite different. 



If W were much smaller, (3) might be made to agree 

 much more nearly with experiment, and the predictions of 

 the theory as to the ionizing power of fast ft rays might 

 still be in accordance with observation if the estimate of the 

 number of electrons per atom were proportionally reduced ; 

 it is to be noted that the estimate given by Thomson as a 

 result of this theory (2 or 3 times the atomic weight) is con- 

 siderably greater than that based on considerations of the 

 scattering of a and /3 rays. On the other hand, if the 

 primary rays have to penetrate through an opposing field in 

 order to enter the atom, the 8 rays would have to emerge 

 through the same field, and the velocity with which they 

 emerged would not be that which they received directly 

 from the primary rays. 



11. When the primary rays are not very fast and T is 

 comparable with W, (3) indicates that the proportion of 

 £ rays having very small velocities ought to increase rela- 

 tively to those having higher velocities. On the other hand, 

 the figures in Table I. seem to show that the contrary pro- 

 position is true, and that the slower primary rays excite the 

 ■smaller proportion of very slow /3 rays. But when T is 

 small, there is no doubt that the forces holding the electrons 

 to the atoms have to be taken into account. Kamsauer (13) 

 bas pointed out that Thomson's theory does not agree with 



