436 Prof. W. H. Bragg on the Properties and 



that is neither S radiation nor reflected primary rays, are 

 those in which [3 rays are produced at the impact of X or 

 7 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 7 rays (Wien, Ann. d. P/iys., 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 radiation, 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 

 proportional to the square root of the atomic weight, and the 

 effects appear to be exactly additive. 



On the other hand, if we consider a stream of ft particles 

 projected into matter, and attempt to find the history of their 

 motion, we are faced with a problem of great complexity. 

 If we look for an answer expressed statistically, we must find 

 the number of particles in each unit volume of the absorbing- 

 matter as a function of the time, the velocity, and the direc- 

 tion of motion, ff, on the other hand, we try to follow the 

 motion of any one particle, we must find the chance that the 

 particle considered has any particular position, velocity, and 

 direction of motion at any given time ; which is really 

 equivalent to finding the function just mentioned. Moreover, 

 the data are very uncertain. We know so little of the interior 

 of the atom that we are unable to say with what forces the 

 electrons will be influenced when it penetrates within ; 

 whether, for example, we may neglect the action of the 

 positive electricity of the atom, and consider only the electrons 



