6 Professor William H. Bragg [Jan. 27, 



plates. We might go more into detail, and find the distribution of 

 those that are returned ; we should then have data from which we 

 might determine in some measure the distribution of the centres of 

 force inside the atom. We cannot follow this up now, but I would 

 like to draw your attention to a curious indication which we obtain 

 when we compare the results for gold with those which Madsen found 

 for aluminium. They show that the hghter metal turns back fewer 

 yS particles, and that its power of absorbing a stream of rays is rather 

 an absolute abstraction of energy. There is clearly an actual absorp- 

 tion effect, which is to be distinguished from the scattering effect. 

 Indeed, the two effects are obviously of different importance in the two 

 cases. When a fi ray strikes a gold atom it must be much more liable 

 to deflection than when it strikes the lighter atom of aluminium. 

 On the other hand, I think it can be shown clearly that in ploughing 

 through aluminium atoms there is a relatively quicker absorption 

 of energy. We may illustrate this by a rough model. Let us stand 

 an electromagnet upright on the table, and let us suspend an- 

 other magnet so that it can swing over the fixed one and just 

 clear it. If we draw back the swinging magnet and let it go to- 

 wards the fixed one, the currents running so that the two repel, then 

 as the moving magnet tries to go by there will be a deflection 

 depending on the relative speed, the closeness of approach, and the 

 strength of the poles. This may represent the turning aside of an 

 electron by a centre of force inside an atom. Now let the magnet at 

 the table be supported by a spiral spring so as to be still upright, but 

 have some freedom of motion ; then, when the experiment is repeated, 

 the swinging magnet pushes the other more or less to one side ; it is 

 less deflected, but it has to give up some of its energy. This is exactly 

 what happens in the case of the j3 particle. The centre of force in 

 the gold atom behaves like the stiffer electromagnet on the table ; it 

 deflects the electron more, but robs it of less energy in doing so. It 

 will not do to suppose the gold atom to differ from the aluminium 

 atom simply in the number of centres of force, such as electrons, 

 which it contains, if it is supposed that they all act independently. 

 There is some other fundamental difference, equivalent to a difference 

 in the stiffness with which the electrons are set in their places. There 

 are two things to be expressed in the behaviour of the atom towards 

 the /3 particle, as has been pointed out several times. H. W. Schmidt 

 has actually calculated them from experiments which gave them 

 indirectly and somewhat approximately. The method I have just 

 outlined gives one of them directly, viz. that which is called the 

 scattering coefficient ; and I think the other can also be found directly 

 by a method which will serve as an illustration of the behaviour of 

 y rays. 



We must first, however, consider the part which y and x rays 

 play generally in this theory. Workers are by no means agreed as 

 to the proper way in which to regard them, but there is no need to 



