178 



§2. 



The following tables contain the results of the determina- 

 tion of the constant of specific ionisation due to a rays. These 

 have all been made recently,, except when the contrary is 

 stated. I have rejected a large number of earlier measure- 

 ments. In the case of each experiment with a gas, the value 

 of lU for air was found immediately before and im- 

 mediately afterwards. It varies somewhat from day 

 to day, and generally increases during any long 

 series of experiments, since the warmth and dry- 

 ness are conducive to the better retention of the emanation. 

 The radium plate is not quite in so good a condition as it used 

 to be, being covered with a very thin film of dirt and grease 

 (mainly from the taps). This could, no doubt, be removed by 

 a red heat, but I am unwilling to handle the radium film so 

 roughly just now. The presence of the film slightly lowers the 

 ranges, about '5 to 1 mm. in air ; and rather blurs the cor- 

 ners of the ionisation curves : — 



Mean, I'So 



Carbon Bisulphide. 



Mean, I'Jl 



