278 ME. E. M. WELLISCH ON THE MOBILITIES OF THE 



Mention has already been made (vide section 8) of the increase in value of the 

 product pk at low pressures in the case of air, nitrous oxide and carbon dioxide, and 

 of the diminution in the case of vapours, e.g. ethyl chloride, when the pressures 

 approach the vapour pressure at the temperature under consideration. Such 

 deviations from the law pk = constant could be ascribed to variations in the size 

 of the cluster constituting the ion ; however, they follow readily from the expression 

 (a) of the mobility if we take into account the deviation from the law of MAXWELL 

 which states that the coefficient of viscosity of a gas or vapour is independent of its 

 density. In the case of gases it is known that 77 diminishes as the pressure is reduced 

 beyond a certain value;* such a diminution would, according to the theory here 

 given, produce an increase in the value of pk. In the case of vapours it has been 

 established that r/ increases rapidly as the saturated state is approached ; in fact, as a 

 result of WARBURG and VON BABO'S experiments on the viscosity of CO 2 at high 

 pressures. MEYER! came to the conclusion that the experimental values above certain 

 pressures could be explained only by supposing carbon dioxide to behave as a liquid, 

 the density of which is practically independent of pressure ; an increase in the value 

 of 77 would, according to the expression (a), diminish the product pk, a result in 

 accordance with experimental observations. 



10. Summary. 



1 . The velocities of the positive and negative ions produced by Rontgen rays in 

 4 gases and 15 vapours have been measured at normal temperatures over a wide 

 range of pressures and under different electric intensities. LANGEVIN'S null method 

 was employed throughout. 



2. For a constant pressure the velocity of the ion was found to vary as the electric 

 intensity. 



3. It was found that, in general, the mobility (k) of the ion varied inversely as the 

 pressure (p). In the case of nitrous oxide and carbon dioxide there was a slight 

 increase in value of the product pk both for the positive and negative ions as the 

 pressure was reduced below about 7 cm. of mercury. In the case of ethyl chloride 

 there was a marked decrease in the value of pk as the vapour approached the 

 saturated state ; there is reason to believe that such a diminution would appear in 

 the case of all the vapours in the neighbourhood of the saturated state. 



4. In the case of vapours there was, in general, little difference in the values of 

 the positive and negative mobilities. The mobility of the positive ion was found 

 greater than that of the negative for aldehyde, ethyl alcohol, -aceton, sulphur dioxide, 

 ethyl chloride, pentane, ethyl acetate, methyl bromide, and ethyl iodide. 



5. There appeared to be no direct relation between mobilities and molecular 



* Vide JEANS, loc. tit., p. 253. 

 t Loc. tit., Art. 90. 



