PHYSICS: K.-L. YEN 
97 
molecules, drags the latter more towards it and thus has a smaller effective 
mean free path. The negative ion, with the smaller attractive force, has 
a greater effective mean free path. As the mobility varies directly with the 
mean free path, it can be easily seen why the negative ions have a greater 
mobility than the positive ions. 
But the above explanation would not be applicable to the case where the 
ratio of the negative to the positive mobility is less than unity, for that would 
mean that the attractive force between the negative and the uncharged mol- 
ecule is greater than that between the positive ion and the uncharged mol- 
ecule, which would be impossible according to the theory upon which the 
explanation is based. Fortunately, in such cases the differences between the 
positive and the negative mobilities are so small that they are well within the 
limits of experimental fluctuations; consequently, until it is conclusively es- 
tablished that there are cases where the positive mobilities are greater than 
the negative by a quantity much too great to be accounted for by experimental 
conditions, the above explanation seems to be the most reasonable one so far 
advanced. 
The detailed paper has been communicated to the Physical Review. 
1 Latty, R. F., London, Proc. R. Soc, (A), 84, 1910. 
2 Kovarick, A. F., Physic. Rev., Ithaca, N. Y., 30, 1910, (415). 
3 Todd, Phil. Mag., London, (Ser. 6), 22, 1911, (791); June, 1913. 
4 Townsend, J. S., London, Proc. R. Soc, (A), 85, 1911. 
5 Franck, J., Ann. Physik, Leipzig, 22, 1906, (972). 
6 Moore, Physic. Rev., Ithaca, N. Y ., 1912. 
7 Haines, Phil. Mag., London, 30, 1915; 31, 1916. 
8 Ratner, Ibid., 32, 1916. 
9 Chattock, Ibid., 48, 1899, (401). 
10 Wellisch, London, Phil. Trans. R. Soc, (A), 209, 1909. 
11 Wellisch, Amer. J. Sci., New Haven, May, 1915; Phil. Mag., London, March, 1916. 
12 Loeb, Physic Rev., Ithaca, N. Y., 8, 1916, (633); these Proceedings, 2, 1916, (633). 
13 Rutherford, Cambridge, Proc Phil. Soc, 9, 1898, (410). 
14 Franck, Ann. Physik, Leipzig, 21, 1906, (985). 
15 Townsend, Electricity in Gases, Oxford, 1915. 
16 Rutherford, Phil. Mag., London, 21, 1911, (669). 
17 Millikan, The Electron, Chicago, 1917. 
18 Wellisch, Phil. Mag., London, 32, 1917, (199). 
