Barss — Ionization hij Collision in Gases and Vapors. 239 



series of observations, that could be detected by the mano- 

 meter or the direct deflections of the electrometer. 



In the above table, L is the distance between the plates and 

 8 the corresponding electrometer deflection in centimeters per 

 second. 



Discussion of Carves. 



The curves at the higher pressures in all the vapors employed 

 give evidence of some extraordinary but characteristic behavior. 

 The curve for a smaller distance between the plates rises more 

 abruptly than that for a greater distance ; so that the curves 

 for two different distances cross each other. As the pressure 

 is diminished, this process is somehow reversed, the curves 

 becoming approximately parallel and finally diverging as the 

 pressure becomes still further reduced. This is contrary to the 

 behavior of gases in so far as we have experimental evidence, 

 and is contrary to the facts that would be expected from 

 Townsend's theory. According to this theory, for any pres- 

 sure, provided it is kept constant, the curves corresponding to 

 different distances between the plates should diverge, as they 

 do at the lower pressure. 



We have, therefore, experimental evidence which points to 

 a distinct difference between gases and vapors, in so far as 

 ionization by collision is concerned. 



Values of a in sulphur dioxide at a pressure of 0"5 cm and in 

 ether at a pressure of - 6 cm (pressures at which the curves 

 diverge) were calculated from the formula stated on page 230 

 and are given in Table V. The values of a for different dis- 

 tances between the plates agree sufficiently well to lead to the 

 conclusion that Townsend's theory, as developed for gases, 

 holds in vapors at low pressures. 





Table V. 







S0 2 . pressure 0"5 cm . 





X 



o(L=l cm ) a(L = 0-8 cm ) 



a (L = 0-o cm ) 



280 



0-17 





37U 



0-45 0-48 





500 



1-75 1-75 



1-90 



580 



3-41 3-25 

 Ether, pressure 0'6 cm . 



3-44 



X 



a (L = l-5 cm ) a (L — l cm ) 



o(L = 0-5 cm ) 



360 



0-33 0-31 





530 



1-75 2-02 



1-8 



610 



4-13 



4-3 



