36 Mr Crowther, On the Relation between Ionization and 



the gas before reaching the walls of the ionization chamber, the 

 ionization-pressure curve as before should have the form 



I = Ap + Bp\ 



When, however, the pressure is high enough to cause the 

 complete absorption in the gas of the soft secondary rays from 

 the gas, the ionization produced by these secondary rays will be 

 simply proportional to their energy, that is, simply proportional to 

 the pressure of the gas ; and the whole ionization in the gas will 

 again follow a simple pressure law. 



10 



Fig. 2. 



15 

 ~F*ressure 



zo 



M.m 



The apparatus employed consisted of a gas-tight brass box 

 with an aluminium window to admit the rays ; the ionization was 

 measured between a pair of parallel plates placed inside the box 

 but outside the direct line of action of the primary beam, which 

 was limited and defined by perforated lead screens. 



A second aluminium window at the far side of the box allowed 

 the primary rays to pass out of the box, so that the amount of 



