85 



plored the whole space traversed by the secondary rays, which 

 chamber the secondary rays would, as a rule, completely 

 cross if they entered it, it may be taken that he really com- 

 pared the number of f3 particles in the secondary beam with 

 the number of those in the primary. The numbers which 

 he obtained varied from 15% to 50%, according to the sub- 

 stance, which is the order of things we should expect if the 

 secondary were simply scattered primary radiation. Again, 

 the loss of velocity of the cathode particles, which is found 

 to occur on scattering at a plate, presuming the secondary 

 radiation to be scattered primary, is just what we should ex- 

 pect. In the case of the a rays no secondary radiation other 

 than S rays has been found; but a small reflection of canal 

 rays has been observed, e.g., by Fuchtbauer (Phys. Zeit., 

 March 1, 1906). Barkla has shown that the secondary radi- 

 ation produced by X-rays consists in part of scattered pri- 

 mary radiation especially when the surface struck is of mate- 

 rial whose atomic weight is low. The only cases in which a 

 secondary radiation appears that is neither 6 radiation nor 

 reflected primary are those in which /3 rays are produced at 

 the impact of X- or y rays, and in which X-rays are produced 

 by cathode rays. It is remarkable that in the former of these 

 cases there is very great difficulty in accounting for the high 

 speed which is possessed by the secondary radiation, caused by 

 X-rays and y-.rays (Wien, Ann. d. Phys., December 28. 1905). 

 It may well be that further research will bring these cases 

 into better agreement with the rest. 



The next question which it is interesting to consider in 

 relation to the various types of radiatior. is that of the law 

 of absorption in passing through matter. 



Absorption in the case of the material radiations appears 

 to be due to two main causes : loss of energy, which causes a 

 gradual loss of speed; and scattering, which means a diminu- 

 tion in the number of particles in the primary beam. There 

 is a possibility of a third, viz., absorption of the flying particle 

 by an atom which it is traversing. 



In the case of the a particle, I have shown that the first 

 of these causes operates alone, so that the particle pursues a 

 rectilinear course throughout its career (Australasian Asso- 

 ciation for the Advancement of Science, January, 1904; Phil. 

 Mag., December, 1904). It is the absence of any effective 

 amount of scattering that makes the study of the motion of 

 an individual a particle comparatively simple. The loss of 

 energy in traversing an atom, or more exactly the probable 

 loss in crossing a given space occupied by an atom, is nearly 



