724 Prof. Rutherford and Mr. Richardson : Analysis 



radiations of certain elements. His results showed, as had 

 long been supposed, that the 7 rays were identical in general 

 properties with X rays and possessed the fundamental pro- 

 perty of exciting characteristic 7 rays. In a paper entitled 

 " The origin of /3 and 7 rays from radioactive substances," 

 Rutherford * put forward the view that the 7 rays from 

 radioactive substances were to be regarded as the charac- 

 teristic radiations of the respective elements set up by the 

 escape of a or /3 rays from them. On this basis an expla- 

 nation was given of the numerous groups of homogeneous 

 /3 rays emitted by radium B and C, and their connexion with 

 the 7 rays was outlined. If this were the case, each type of 

 characteristic radiation emitted should be absorbed according 

 to an exponential law by an absorbing substance of low 

 atomic weight like aluminium. 



The present experiments were undertaken with a view of 

 testing this hypothesis. It will be seen that this analysis 

 brings out that the 7 radiation from radium B consists of at 

 least two and possibly of three distinct types, and from 

 radium of a single type, probably corresponding in pene- 

 trating power to the characteristic radiations to be expected 

 from elements of their atomic weight. 



Experimental Arrangement. 



In the preliminary investigations the source of 7 rays 

 consisted of about 50 millicuries of radium emanation enclosed 

 in an a ray tube with thin walls. The thickness of glass was 

 equal in stopping power for a, rays to about 2 cm. of air. 

 In order to get rid of the effect of the primary /3 rays, the 

 source was placed between the pole-pieces of a powerful 

 electromagnet. The 7 rays passed horizontally into a thin- 

 walled ionization vessel (3x5x7 cm.) CD, fig. 1, placed at 

 the side of the electromagnet, in which the ionization was 

 measured in the usual way by means of an exhausted electro- 

 scope E of dimensions 5x5x5 cm. The ionization produced 

 by the 7 rays in the electroscope E was negligibly small 

 compared with that produced in the ionization vessel CD. 

 The sides of the vessel CD, through which the 7 rays passed, 

 consisted of thin sheets of mica equivalent in stopping-power 

 for the u particles to about 2 cm. of air. This was done in 

 order to increase relatively the ionization due to the softer 

 7 rays that might be present. The inside of the vessel was 

 lined throughout with aluminium. In most experiments the 

 pole-pieces of the electromagnet were about 2 cm. apart. 



* Kutherford, Phil. Mag. Oct. 1912. 



