'produced h>j ft and y Rays at High Pressures. 179 



the ft rays emerging from the plate next to the ionization- 

 chamber. The plate o£ lead 9*2 mm. thick was placed near 

 the vessel to reduce to a minimum the effect of the scattering 

 in the substances of smaller atomic weights. The curve 

 (fig. 6, PL I.) is similar to that obtained by Bragg (Phil. Mag. 

 Dec. 1908). 



The Ionization due to the y Rays of Radium. 



The experimental arrangement is shown in fig. 1. The 

 source of y rays was 30 mgrs. of radium bromide. It is 

 impossible to secure a perfect cone of y rays on account of 

 the scattering they undergo whenever they traverse matter. 

 Precautions were, however, taken to obtain an approximate 

 cone of rays. A conical opening was cut in a large block of 

 lead, and the radium was placed in a definite position at the 

 apex. Four lead slides, each 3 mm. thick, were placed in 

 the positions marked in order to cut out the softer 7 rays. 

 Practically all the 7 rays entering the ionization-vessel 

 proceed from this conical opening. The space between the 

 plates is traversed by the main cone of 7 rays together with 

 those 7 rays scattered from the original coue by the plates. 

 Besides the ft rays produced by the 7 rays from the two 

 plates, there are also ft rays set up in the walls of the vessel 

 due to the scattered 7 rays. 



Experiments were carried out — 



(1) Keeping the bottom plate the same and varying 



the upper plate — Pb, Al, or Al-foil ; 



(2) Keeping the upper plate the same and reversing 



the bottom plate Al or Pb ; 



(3) Varying the distance between the plates in each of 



the two previous arrangements. 



Fig. 7 (PL I.) shows the results obtained when the plates 

 were kept at a constant distance of 1 cm. apart. In curve A 

 the lower plate was aluminium and the upper plate lead, and 

 in curve C the lower plate was lead. When the 7 rays pass 

 through 1'5 cm. of lead there are more emergent ft rays 

 from aluminium than from lead. Hence, if the curve C is 

 subtracted from the curve A, or D from B, then the differ- 

 ence should give the approximate effect of the additional 

 ft rays from the aluminium plate. The difference between 

 the curves gives an indication of what happens to a certain 

 proportion of the corpuscular emergent radiation when the 

 pressure is varied. Since more ft rays are scattered from a 



N2 



