C)'2G Electrical Discliarge from a Point to a Plane, 



produced by the positive than by the negative discharge. 

 It is more probable that the clouds produced by ultra-violet 

 light and by these discharges are due to a common cause — 

 the Meissner effect. 



We thus arrive at the following conclusions : — 



(1) A discharge from a point in a gas in which it induces 



no chemical reaction, produces no nuclei larger thtin 

 negative ions. The current is carried entirely by 

 ions and by no larger particles of the electrodes 

 or gas. 



(2) No discharge from a point can be obtained in which 



positive ions exist unaccompanied by negative ions. 



The importance of this second conclusion shoidd be noted : 

 it is certainly worthy of remark that there should always be 

 negative ions present in the field of the positive discharge. 



Helium. — A few experiments were undertaken with this 

 gas to ascertain whether the phenomena of the discharge 

 were in any way different in character for a monatomic gas. 

 In estimating the expansion required to give condensation 

 on negative ions in such a gas, account must be taken of the 

 dependence of the cooling effect due to expansion^ which 

 causes condensation, on the racio of the specific heats of 

 the gas. 



If Ti, T2 are the temperatures before and after expansion 

 we have 



T.- 



I 



ir- 



Hence to get condensation on negative ions in a gas with 



T 

 different value of y, we must keep ^ constant and have 



J-i 



On standardizing the apparatus with radium rays the 



V9 



value 1"15 was obtained for — to give condensation on 

 negative ions in helium. * 



Then (1*15)'^'"^ = (1*25)^"^, when 7 refers to air or 

 hydrogen, and 7^ to helium. If 7=1-41, this gives y= 1*66, 

 aValue deduced for heliam on other grounds. If an accuracy 

 greater than 1 per cent, is not desired, there is probably no 

 simpler or more efficient method than this for determining 7 

 for any given gas. 



On making experiments on the discharge of the same 

 nature as those on the other gases tried, the results obtained 



