270 MR. E. M. WELLISCH ON THE MOBILITIES OF THE 



8. Discussion of Results. 



A first deduction from the experimental results is that for a given electric intensity 

 the velocity of an ion varies inversely as the pressure ; while, for a constant pressure, 

 the velocity is directly proportional to the electric intensity. 



LANGEVIN found that in air over a range of pressures varying from 7 '5 cm. to 

 143 cm. of mercury the product of the pi-essure and the mobility of the positive ion 

 was sensibly constant ; but, in the case of the negative ion, this product showed a 

 marked increase in value when the pressure was reduced below 20 cm. ; this result 

 was interpreted as denoting a simplification in the structure of the negative ion at 

 relatively low pressures. The values of the mobilities given for CO 2 and N 2 O in the 

 tables show an increase in the product pk at low pressures both for the positive and 

 negative ions. As pointed out before, the experimental ei'ror tends to increase when 

 the mobilities are measured at low pressures ; on this account no attempt was made 

 to obtain measurements at pressures below 1 cm. of mercury. 



In the case of vapours the pressures were in general chosen so that the vapour 

 under consideration was well removed from its condensation point ; under these 

 circumstances no tendency of the product pk to increase with diminution of pressure 

 was observed. However, attempts were made to observe whether there was any 

 deviation from the law pk = constant as the vapour approached the saturated state ; 

 great difficulty was incurred on account of the tendency of the vapour to condense 

 on the insulation, nevertheless, in the case of ethyl chloride, the product pk 

 showed a marked decrease in value both for the positive and negative ions as the 

 pressures were increased beyond 200 mm.* It is known from experiments on 

 saturated vapours f that the density of a vapour increases more rapidly than 

 would be expected from BOYLE'S law as the vapour approaches saturation, in other 

 words, there is a tendency for the vapour molecules to form aggregations ; this 

 would result in a diminution of the value of the product pk, either in consequence 

 of an increased complexity of the ionic structure, or of an increased collision 

 frequency between the ion and the molecules of the vapour. Another explanation 

 of the alteration in value of pk is given below, under section 9 ; on this view the 

 alteration depends not so much on the increase of density as on the marked increase 

 in viscosity which has been shown! to occur when a vapour approaches the saturated 

 state. 



PRZIBRAM has recently measured the ionic mobilities in several vapours at a 

 pressure of 1 atmosphere, the temperatures being chosen so as to just suffice to 

 prevent condensation ; the vapours were ionised by the a rays from polonium, and a 



* The vapour pressure of ethyl chloride at 10 C. is 691 mm. 



t Cf. MEYER, ' Kinetic Theory of Gases,' 2nd edition, chapter 4. 



{ Cf. MEYER, loc. til., chapter 7. 



' Wien. Berichte,' Ila, Bd. 117, p. 665, 1908. 



