632 Mr. R. D. Kleeman on the Secondary Cathode 



primary electrons is probably in any particular gas approxi- 

 mately the same for the ft and y rays, since the velocities of 

 the secondary rays from a substance are approximately the 

 same; and the amount of secondary ionization produced by 

 the same number of primary electrons is therefore the same 

 for the ft and y rays. From this it follows that the ratio of the 

 secondary ionization per c.c. with the ft rays to the secondary 

 ionization in the same gas with the 7 rays, is the same for 

 all gases at the same pressure; and since the total ionization 

 per c.c. is the same for the ft and y rays, this must also be 

 true for the ratio of the primary ionizations. If the primary 

 and secondary ionizations in a gas are each expressed in terms 

 of these quantities in air, these ratios are both equal to unity. 

 Thus the ionization by ft or 7 rays in a gas a in terms of 

 that in air, may in each case be denoted by n a m a , and in the 

 case of a gas b by n b m b , and so on, where n a , n b , &c. denote 

 the primary ionizations, and m a , m h , &c. the secondary 

 ionizations. 



Now, if the primary ionizations are given by the values 

 of M l5 or M 2 it follows that m a > m b > &c, since the atomic 

 ionizations increase less rapidly with the atomic weight than 

 the values of M x and M 2 . This means that the ionizing power 

 of an electron from an atom in a gas composed of the same 

 kind of atoms, decreases with increase of atomic weight of the 

 atoms. This result is very improbable : and it follows therefore 

 that the values found for M : and M 2 probably do not represent 

 the primary ionizations. This point will now be considered. 



If the secondary cathode rays from the various substances 

 have not the same ionizing power in air, the values found 

 for Mj and M 2 will not represent exactly the quantities M x 

 and M 2 according to the definition. If the ionizing power 

 of the secondary cathode rays increases with increase of 

 atomic weight, this would have the tendency to give values 

 for M t and M 2 which increase too rapidly with the atomic 

 weight. Now, although the ionizing power of the secondary 

 cathode rays may vary to some extent with the nature of the 

 substance by which they are emitted, it is very improbable 

 that it varies to such an extent as to account for the above 

 discrepancy. It is more likely that, besides the penetrating- 

 rays generated in a substance, more easily absorbable rays 

 are generated which do not get free of the layer, and are 

 therefore not taken into account in the measurements of 

 secondary radiation. If this is the correct explanation it 

 would follow, since the ionization in a gas does not increase 

 so rapidly with increase of atomic weight as the values of 

 Mj and M 2 , that the amount of more easily absorbable radiation 

 produced decreases with increase of atomic weight. 



