836 Prof. Barkla and Mr. Philpot : Ionization in Gases and 



corresponding final curves are shown in fig. 2. In trie ex- 

 periments with etlryl bromide and methyl iodide, the full 

 ionization-pressure curve was obtained experimentally when 

 the ends of the ionization-chamber were of carbon as well as 

 when of gold. Fig. 3, for instance, shows the corresponding 



Fig. 3. 





























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b(C 



































































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P 3 J . 



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G 

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5 10 § 15 



PXESSUftE OF y/IPOUR OFCH s l( Ct,fty£ ^ ^ <DS ) 



A and B curves for methyl iodide. They are specially inter- 

 esting because of their particular form. The curve A is 

 concave upwards because there is a net loss of radiation by 

 the methyl iodide at the ends when these are of carbon. 

 Curve B is only slightly convex upwards because there is only 

 a slight gain of radiation at the ends from the gold. 



It is evident from these curves then that at atmospheric 

 pressure, air and all heavier gases completely absorbed the 

 corpuscular radiation from the two end faces ; and that ethyl 

 bromide and methyl iodide produced complete absorption of 

 the end corpuscular radiation at much lower pressures. The 

 critical pressures for air, 2 H 5 Br, and CH 3 I — that is, the 

 pressures at which no corpuscular radiation was able to get 

 across a chamber 1 centimetre in thickness — were 66, 16, and 

 12 centimetres of mercury respectively. If we multiply 



