IONS PRODUCED BY RONTGEN RAYS IN GASES AND VAPOURS. 



271 



blast method was employed in the measurement of the mobilities. The following 

 results are given in PRZIBRAM'S paper : 



111 the cases of ethyl alcohol, aceton, ethyl ether, and methyl acetate, it is noticeable 

 that the mobilities are much smaller than when measured at pressures well removed 

 from the saturated state and reduced to a pressure of 760 mm. of mercury in 

 accordance with the law pk = constant. It is probable that in the case of every 

 vapour, just as has been shown in the particular case of ethyl chloride, the values of 

 pk would decrease if pressures were chosen so as to make the vapour approach 

 saturation. As an analogous illustration of this point, it is worthy of mention that 

 PHILLIPS* has shown that the ionic mobilities when measured in air at atmospheric 

 pressure vary as the absolute temperature over a wide range of temperatures ; but, 

 in the neighbourhood of the temperature of boiling liquid air, the mobilities are 

 distinctly smaller than would be the case if this law of variation were applicable. 



A preliminary examination of the results which are given in Tables I. and II. shows 

 that there is no direct dependence of the 1 ionic mobility on the molecular weight ; it 

 might have been expected that the mobilities would decrease as the molecular weight 

 increased, but that this is not always true is seen by comparing the cases of ammonia 

 (molecular weight 17 and mean ionic mobility - 77 cm./sec.) and oxygen (molecular 

 weight 32 and mean mobility 1'58 cm./sec.). A better way of considering the results 

 would be to divide the list into two groups, placing in one group those gases which 

 have a relatively small critical temperature, and in the other those (the so-called 

 vapours) which have a relatively high critical temperature ; it will then be seen that 

 the high mobilities belong without exception to the gases of the first group ; the 

 apparently small values of the ionic mobilities in certain cases, e.g. ammonia and 

 aldehyde, can then be conceived as being due to large cohesive forces between their 

 molecules evinced by their relatively high critical temperatures. An attempt to 

 deduce the mobility values from purely theoretical considerations is given later under 

 section 9. 



It is noticeable also from an inspection of the tables that, especially in the case of 



* 'Roy. Soc. Proc.,' A, vol. 78, p. 167, 1906. 



