238 Barss— Ionization by Collision in Gases a?u/ Vapors. 



vapor pressure of the particular substance being used. Tin's 

 pressure was reduced by means of a water pump and tbe 



3» I was refilled with vapor. This process was repeated 

 several times SO that when readings were made a very small 

 percentage of air was present with the vapor. Three sets of 

 curves were obtained in each vapor corresponding to three 

 different pressures, each set containing three curves corre- 

 sponding to three different distances between the plates. 

 These curves are shown in figs. 4 to 8 inclusive. In the vapor 

 curves as in the air curves, ordinates represent ionization cur- 

 rents and abscissae represent the field strength in volts per 

 centimeter. In tigs. 4. 6 and 8, the values of field strength for 

 the middle set of curves are given at the top of the figures. A 

 characteristic series of readings is given in Table IV. It was 

 found that, for high pressures (9'9 cm , 5-3 cm and 2'9 C,U in SO,), 

 the ionization corresponding to saturating field strength was 

 proportional to the pressure. This fact was observed for all 

 vapors: and the result enabled smaller pressures, which could 

 not be measured accurately by the open manometer, to be cal- 

 culated. 



The vapors were not admitted into the chamber at pressures 

 high enough to produce any appreciable permanent condensa- 

 tion on the walls of the apparatus ; for after each vapor was 

 used, the ionization in air was tested, the results varying only 

 as the sensitiveness of the electrometer varied. In every case 

 there was no change in pressure during the time taken for a 



Table IV. 

 S0 2 (pressure r6 cm ). See fig. 4. 



X 



6 (L = l cm ) 



c (L = 0-8* m ) 



<? (L =0 -5"") 



50 





6-30 





80 



7-9S 





2-85 



100 





6-30 





160 



7*98 





2-9 



200 





6-31 





280 







2-9 



420 



8-03 







525 









780 





6-30 



3-35 



795 





6-41 





845 



8-40 







1045 



9-50 





6-4 



1056 





7-15 





1200 



11-10 





13 



1300 





10-50 





1500 





18-1 





