Rays emitted by Substances exposed to 7 Rays. 621 



produced by the secondary cathode rays in these experiments r 

 it may be stated that in one experiment, when the radiating 

 substance was aluminium, a leak o£ 2319 divisions in 40 

 seconds was obtained; and when the radiating substance was 

 lead, a leak of 2295 divisions in 30 seconds was obtained. 

 The value o£ (mc—uc) corresponded, therefore, to a leak of 

 741 divisions in 40 seconds. The capacity of the whole 

 system was about 200 E.s.u., and the electrometer gave a 

 deflexion of 4010 divisions for one volt*. 



The secondary 7 radiation, which was approximately elimi- 

 nated in these experiments, was, with the thickness of the 

 layers of substances used, in most cases so small as to be 

 comparable with the experimental error, that is, equal to. two 

 or three per cent, of the total leak obtained. 



The amount of secondary cathode radiation from a 

 plate of a substance increases with the thickness of the 

 plate up to a certain thickness, beyond which there is no 

 increase of secondary radiation, the plate producing then the 

 maximum amount of secondary radiation. It is therefore 

 necessary to consider whether the thickness of substances 

 used in these experiments gave the maximum amount of 

 secondary radiation. 



The secondary cathode rays from a substance exposed to 

 y rays are nearly, though not quite, so penetrating in cha- 

 racter as those obtained with the ft rays, according to S. J. 

 Allen (Phys. Review, vol. xxiii. p. 82, August 1906). 

 Therefore the thickness of a radiating plate of a given sub- 

 stance producing the maximum amount of secondary cathode 

 radiation must be about the same as is necessary with ft rays, 

 that is a thickness of about 2 mm. in the case of lead. The 

 layers of substances in these experiments were *4 cm. thick, 

 and therefore gave approximately the maximum amount of 

 secondary radiation. 



An experiment by Mackenzie f ought to be mentioned in 

 this connexion. Mackenzie found that the maximum amount 

 of secondary cathode and 7 radiation from a lead plate 

 exposed to the ft and 7 rays of a quantity of radium, was 

 obtained with a thickness of about 2 mm. of lead, while the 

 thickness of the plate which produced the maximum amount 

 of secondary radiation, when it was exposed to the 7 rays 

 alone, was about 7 mm. The explanation of this seems to 

 be that the secondary 7 radiation, when the plate is exposed 

 to the 7 rays alone, is larger in comparison with the total 

 radiation (which consists of secondary 7 and secondary 

 * 10 divisions correspond to 1 mm, on scale, 

 t Phil. Mag. July 1907, p. 184. 



Phil. Mag. S. 6. Vol. 14. No. 83. Nov. 1907. 2 T 



