180 Mr. D. C. H. FJorance : A Study of the Ionization 



plate o£ lead than from a plate of aluminium, the curve 

 (A — C) is greater than (B — D). From the curve (A — C) 

 (fig. 8) the apparent coefficient of absorption of the ft rays 

 is observed to be approximately 0'046 cm. -1 ; and it has 

 already been shown that the absorption coefficient of the 

 ft rays from Ur X determined under similar conditions was 

 004 cm." 1 . 



There are more " incident " ft rays ejected from lead than 

 from aluminium ; hence the difference between the curves 

 A and B, or C and J), will give a general indication of the 

 behaviour of a certain proportion of the incident ft rays. 

 The coefficient of absorption of these incident ft rays = 

 0*056 cm. -1 approximately. The emergent ft rays appear 

 to be more penetrating than the incident ft rays ; and this is 

 to be anticipated just as in the case of scattered ft rays, which 

 are more absorbable the greater the angle through which 

 they have been deflected. 



Beatty (Phil. Mag. August 1910) has found that the 

 incident corpuscular radiation produced by a homogeneous 

 beam of X rays was similar in penetrating power to the 

 emergent corpuscular radiation. 



When the upper plate was aluminium foil and the lower 

 plate changed from lead to aluminium, the curves were 

 similar to those in which aluminium was the upper plate. 

 The absorption coefficient for the emergent ft rays in this 

 case was 0'04 cm. -1 . 



In all these experiments the/3 radiation appears to be inde- 

 pendent of the plates except for multiple reflexion, and the 

 emergent radiation rather more penetrating than the incident. 



The curves of fig. 7 are all concave to the pressure axis. 

 Bragg predicted that with a light substance for the upper 

 plate the curve should be convex to the pressure axis. An 

 attempt was made to obtain this condition by making the 

 upper plate of Al foil and having a column of air 24 cm. 

 high above the plate. In order to reduce still further the 

 scattered ft rays, the top of the vessel was lined with carbon 

 and the side of the vessel lined with ebonite coated with 

 Al foil. Of course this has not reduced the problem to the 

 simple one that Bragg assumes — namely, no radiation from 

 the sides of the vessel and none from the upper plate. The 

 result thus obtained is similar to that in which an aluminium 

 plate was used. At lower pressures the ionization current is 

 slightly greater for aluminium foil, and at the high pressures 

 less than the current given when the upper plate is aluminium. 

 Before discussing the curves further let us briefly consider 

 the theory of the ionization by 7 rays. 



