Diffusion of Liquids. 429 



intensity of the attractions acting upon them at any particular 

 instant of their course. Moreover the velocities with which 

 these two atoms move may be quite different, and what may be 

 spoken of as the velocity of the molecule NaCl in solution is 

 compounded of the velocities of its two atoms Na and CI. 



The action of the galvanic current on such a system would 

 then be as follows : — The atoms would still continue to fly 

 about, but in directions more nearly parallel to that of a line 

 joining the two electrodes ; and the tendency of these latter 

 to draw to the one side the negative atoms, and to the other 

 the positive, would be facilitated by their continually recur- 

 ring states of liberty. If a partition be imagined in the vessel 

 in which electrolysis is taking place, say halfway between 

 the two electrodes and perpendicular to the line joining them, 

 it is clear that more positive than negative atoms pass this 

 partition in a positive direction, and more negative than posi- 

 tive atoms pass it in a negative direction in a given time. 

 But positive atoms moving in a negative direction, and nega- 

 tive atoms moving in a positive direction, compensate for each 

 other, and there remains only the excess of motion in each 

 direction ; which amounts to saying that a certain number of 

 positive atoms cross the partition in a positive direction, and 

 a certain number of negative atoms cross it in a negative direc- 

 tion, in a given time. " These two numbers need not be the 

 same, as they depend not only upon the impulsive force which 

 for both is the same, but also on the degree of mobility, which 

 for several reasons can be different for different substances"*. 



Now the ratio of the number of negative atoms which cross 

 the partition to the number of molecules which may be con- 

 sidered as decomposed in a given time is designated by Hit- 

 torf, in his well-known papers on the "Wanderung der 

 Ionen"f, with the letter n, and called by him the u Ueberfuhr- 

 ungszaM" He shows further that this n is equal to the ratio 

 of the velocity of the negative atoms to the sum of the velo- 

 cities of both negative and positive atoms %. Kohlrausch, in 

 his papers referred to above, expresses this relation thus, 



where v is the velocity of the negative and u that of the posi- 

 tive atoms; and from the numerous values of n given by Hit- 

 torf for each substance he has deduced those which correspond 



* Clausius, loc. cit. 



t Pogg. Ann. lxxxix. p. 176 ; xcvii. p. 1 ; ciii. p. 1 ; cvi. p. 337. 



X See Pogg. Ann. ciii. p. 20, and cvi. p. 353 » 



