898 Messrs. J. C. Chapman and S. H. Piper on 



radiation the experimental evidence has been insufficient to 

 lead to an exact theory of the processes resulting in its pro- 

 duction. Several possioilities suggest themselves, which we 

 wish to examine and upon which a few simple experimental 

 results throw some light. 



It has been shown by Townsend, Perry, Dora, and others, 

 that another type of radiation is emitted in addition to the 

 secondary X radiations. This consists of negatively charged 

 particles identical with those found in a vacuum-tube. 

 Sadler * has recently shown that in most cases they only 

 appear with the homogeneous X radiation, and it is possible 

 there are no exceptions to this rale. This connexion between 

 the two types of radiation suggests the possibility that the 

 homogeneous X radiation may be produced as the result of 

 the sudden alteration in velocity of an ejected electron, or 

 again, it may be due to the subsequent bombardment of 

 other atoms by the ejected electron, the radiation taking- 

 place either from the colliding electron or from the atom 

 struck. 



Several objections can, however, be urged against the 

 hypothesis that the radiation arises from the ejected electron. 

 Firstly, the velocities with which the electrons leave the 

 substance, that is, the velocities of ejection, have been found 

 to vary over wide limits by using various primary beams. 

 Yet, whatever these velocities, the secondary X radiation 

 given off is of constant penetrating power and perfectly 

 homogeneous. Secondly, the same velocity of ejection from 

 the atoms of different substances is accompanied by homo- 

 geneous beams varying greatly in penetrating power. These 

 facts seem to show that the radiation cannot proceed from 

 the electron at its ejection. 



Another explanation is that the characteristic secondary 

 radiation results from the bombardment of atoms by these 

 ejected corpuscles. In this case the radiation may proceed 

 from the colliding electron or the atom so struck. The same 

 arguments that have been used above go to disprove that it 

 is the expelled electron which is the source of the radiation, 

 for although the velocities of these electrons differ greatly 

 before collision when a heterogeneous primary beam is 

 used, yet the secondary radiation is of constant penetrating 

 power. 



The following experiment in addition points to the con- 

 clusion that it is not the atom which is bombarded that emits 

 the homogeneous radiation. 



* Phil. Mag. March, 1910. 



