the Shape of the Atom. 237 



or the distribution of the radiation, must depend to a certain 

 extent on the nature of the atom. The distribution of the 

 secondary radiation outside an atom produced by a {3 ray 

 must obviously depend largely on tbe amount of matter of 

 the atom traversed by the /3 ray. Therefore, if the atom is 

 spherical in shape a given increase in the atomic weight 

 would produce less change in the magnitude of the mean 

 angle of distribution when tbe atom is large than when it is 

 small, since the change in the diameter of the atom will be 

 smaller in the former case than in the latter. We should 

 therefore expect the limiting distribution of the secondary 

 radiation to depend less on the nature of the atom the greater 

 its atomic weight. 



This could be tested by means of the following arrange- 

 ment. Suppose we have a number of slabs of different 

 kinds of matter of infinite thickness, and some of the atoms 

 in each slab eject electrons at right angles to one of the faces 

 of the slab with the same velocity, which is also independent 

 of the nature of the matter. The radiation from both of the 

 faces of a slab would consist of real secondary radiation and 

 deflected primary rays. Now when we are dealing with 

 slabs of infinite thickness the ratio of the two radiations from 

 the opposite sides of a slab is independent of the density of 

 the slab ; it is also independent of the proportion of the 

 atoms which eject /3 rays. These ratios for slabs of different 

 substances would therefore afford some information on the 

 relative limiting distribution of the secondary radiation round 

 the atom. 



Now the experiments of Prof. Bragg and Dr. Madsen* on 

 the secondary cathode rays ejected by 7 rays satisfy the 

 above conditions. They showed that the electrons ejected 

 by 7 rays move in the direction of propagation of the rays, 

 and if the rays are hardened by being first passed through a 

 thick screen of lead, the velocity of these electrons is practi- 

 cally independent of the nature of the matter in which they 

 are produced. They next carried out a set of measurements 

 of the amounts of secondary cathode radiation from the two 

 sides of plates of different materials sufficiently thick to give 

 the maximum amount of radiation — which is equivalent to 

 dealing with plates of infinite thickness, the 7 rays passing- 

 through each plate at right angles to one of the surfaces. The 

 ratios of the radiations from the two opposite sides of each 

 plate for hard 7 rays have been calculated from a table given 



* Phil. Mag. xvi. pp. 918-9£9 ; Dec. 1908. 



