28 Wellisch — Motion of Ions and Electrons through Gases. 



values of X/j?. It should be remembered, however, that at 

 the lowest pressures employed we are nearing the conditions 

 for which the mobility law would be no longer valid even for 

 an unchanging ion. A simple calculation gives that in air for 

 jp = -05™™ and with 1 volt fall of potential, the positive ion 

 makes about 330 collisions in traversing the distance of 2*^°" 

 between the electrodes. It is surprising that the mobility law 

 should be so nearly valid at this stage ; the explanation is 

 probably that the velocity (3"3 X 10* cm./sec.) acquired by the 

 ion after describing freely a distance equal to the mean free 

 path is still smaller than the mean velocity of thermal agitation 

 of the molecules (4-6 X lO"*). If we suppose that the mobility 

 law is valid as long as the mean velocity of agitation pre- 

 dominates, we find by calculation that in air the law should 

 hold for values of X/p up to 20. Loeb^ has recently made a 

 series of determinations of the mobilities of the ions in air 

 under high electric fields and has shown that the mobilities 

 remain normal at atmospheric pressure for field strengths up 

 to 12,450 volts per cm.; the law pJc = const, was verified for 

 values of X/p up to about 20. 



The preceding considerations indicate that the notion of an 

 ion as a cluster is unnecessary ; the cluster theory must depend 

 for its continued existence on arguments essentially different 

 from those which have hitherto been advanced. Moreover, 

 it should be stated that evidence of a more direct nature in 

 favor of the single molecule theory has of late years been forth- 

 coming. Chattock and Tyndallf in their experiments on the 

 point discharge showed that the absorption of positive ions of 

 hydrogen by a metal corresponded to a withdrawal of two 

 atoms of hydrogen from the gas. Erikson;}: in experimenting 

 with regard to the variation of ionic mobility with changes in 

 temperature concluded that his results were not explicable by 

 the notion of clusters. In the present experiments it has been 

 shown that an electron passes unencumbered through ordinary 

 gases at considerable pressures notwithstanding the strong elec- 

 tric field which is associated with it ; it is hard to reconcile this 

 fact with the basic idea of the cluster theory, viz., that the 

 cluster of molecules is held together by the electric held asso- 

 ciated with the ion. 



Leaving the question as to the nature of the gas ion, we may 

 now with advantage consider another outstanding problem of 



*Loeb, Proc. Nat. Ac. Sc, vol ii, No. 7, p. 345, 1916; also Phys, Rev., 

 viii, p. 633, 1916. Loeb has misunderstood me when he states that I verified 

 the law pk = const, for the ions in air up to values of X/p as high as 34*5 ; 

 as a matter of fact I maintained merely that the negative ions were still in 

 evidence for this value, whereas Townsend's theory would necessitate their 

 complete disappearance for a value of X/p equal to 0"2. (E. M. W.) 



f Chattock and Tyndall, Phil. Mag., xvi, p. 24, 1908 ; also loc. cit., p. 601. 



j:Erikson, Phys. Rev., vol. vi, p. 345, 1915. 



