392 Prof. W. H. Bragg on the Consequences of 



scattering of ft rajs by supposing a real secondary radiation 

 to be added to a reflected primary. The experiments are 

 simple. When a stream o£ ft rays falls at an angle of, say, 

 45° upon an aluminium plate, it is found that the ft rays 

 which leave the plate on the incidence side are not distri- 

 buted symmetrically about a normal to the plate, but show a 

 maximum in a direction which is separated by the normal 

 from that of the incident stream. When the plate is of lead 

 or any other substance having a high atomic weight, the 

 effect is much less marked. In fact it looks as if there was 

 a confused specular reflexion at the surface of the plate 

 coupled with a radiation scattered in all directions. McClel- 

 land therefore divides the scattered rays into two groups, the 

 first of which consists of ft rays from the primary stream 

 reflected by the surface of the plate like light by a mirror, 

 the second of a set of true secondary rays. 



Let us first consider the question of specular reflexion. 

 All the evidence w r e have regarding the actions and reactions 

 between atoms and radiant entities shows that each atom 

 when in collision with an entity has to bear the shock alone : 

 it receives no support from its neighbours, even when they 

 form parts of the same molecule, a fortiori when they are 

 only neighbouring atoms in the surface of a plate such as 

 McClelland used. It is this which makes radioactive 

 measurements independent of physical and chemical con- 

 ditions. The point seems to be firmly established now, for 

 though at times evidence has been brought forward which 

 has at first appeared to contradict the principle, more careful 

 examination has always shown the evidence to have been 

 mistaken. The principle may be expressed by the statement 

 that the action of a molecule on one of the radiant entities is 

 the sum of the effects of the actions of its component atoms, 

 no allowance for constitutive influences being necessary. 



*One or two examples will be sufficient. 

 The stopping power of a molecule for a rays is the sum of 

 the stopping powers of the individual atoms of the molecule. 

 During 1908 I measured as carefully as possible the stopping- 

 powers of a number of gases which were prepared in a 

 very pure state by Dr.Rennie and Dr. Cooke of the Adelaide 

 University. The range of the a particle can be measured to 

 much less than one per cent. The additive principle was 

 found to be true within the errors of experiment ; both for 

 stopping powers measured with respect to Ra C, and those 

 measured with respect to the cc particles of Iva A. The two 

 sets are not quite the same*. 



* Bragg, Phil. Mag. April and Sept. 1907. 



