Secondary 7 Radiation. 243 



we know that 7 rays in passing through matter give rise to 

 ft rays and that ft rays excite 7 rays, the amount of the 

 transformed 7 radiation is far greater than could be explained 

 on this point of view. 



It is necessary to adopt the first view, that the primary 

 7 rays during the process of scattering lose energy and are 

 in consequence modified in type. With a lead radiator, 

 after the secondary 7 radiation has passed through 0'084cm. 

 of lead, the radiation appears to be similar to that produced 

 from an aluminium or zinc radiator. 



In the case of radiators such as platinum, mercury, and 

 lead, there is besides the ordinary type of secondary 7 radia- 

 tion an " extra radiation " of a more absorbable type. The 

 amount of this radiation is small, but its effect can be con- 

 siderably increased when methyl iodide is used in the electro- 

 scope. This radiation is mainly detectable as a returned 

 radiation from the incident surface of the radiator. Although 

 a large number of experiments have been made it is impossible 

 at present to say with certainty that this extra-radiation is a 

 characteristic radiation of the radiator concerned. When 

 platinum was used as a radiator this extra-radiation was 

 more marked, and it appeared to be softer than that from 

 mercury or lead. Another probable explanation is that the 

 extra-radiation is due to j3 rays excited in the material of 

 the plate ; since the & rays rapidly lose energy in passing 

 through matter, this might explain the presence of the small 

 percentage of soft 7 rays. 



Summary. 



1. An investigation has been made on the emergent 



and returned secondary 7 radiations, using radium 

 emanation as a source of primary 7 rays. The 

 experiments confirm the author's earlier results that 

 the secondary 7 radiation becomes gradually less 

 penetrating the greater the angle it makes with the 

 original direction of the primary beam. The pene- 

 trating power depends to a certain extent on the 

 thickness of the radiator and on the screening of the 

 emanation. The secondary 7 radiation is completely 

 heterogeneous. 



2. The secondary 7 radiation for all radiators is different 



in type from the primary radiation, and it appears 

 that in the process of scattering some modification 

 has taken place, 



R 2 



