430 Dr. J. H. Long on the 



to dilute solutions. He has also shown that v, the velocity {Be- 

 weglichkeit, mobility) of a negative atom, as CI, is the same in 

 all its compounds from which it may be electrolyzed, in HC1, 

 KC1, BaCl 2 , and so on. The same is also true for any positive 

 atom, asK in KC1, KOH, KNO s , &c. 



If the values of n for any series of salts as found by Hittorf 

 and reduced by Kohlrausch be compared with the rates of dif- 

 fusion of the same, it will be found that in the cases where n 

 is large the rate of diffusion is small ; and, indeed, in each 

 series the product of n and the latter, which I shall call d, is 

 nearly a constant quantity. Considering, for instance, the 

 dyad chlorides, we have 



% d. Id. 



2 2 



BaCl 2 . . . -618 450 277 



SrCl 3 . . . '655 432 283 



CaCl 2 . . . -673 429 288 



MgCl 2 . . . -682 392 267 



In the case of the salts KC1, KBr, and KI we have 



n. d. nd. 



KC1 ... -515 803 413 



KBr . . . -514 811 416 



KI . . . -505 823 415 



and in the case of the sulphates, 



d. 



n 



2' 2 



MgS0 4 . . . -630 348 219 



ZnS0 4 . . . -640 332 212 



CuS0 4 . . . -645 316 204 



From these tables it appears, comparing any two substances 

 in each series, that 



n : n' : : dl : d; 

 that is, 



— — - : -J——, : : a! : d. 



But, as remarked above, v has been shown to be equal to 

 v r ; hence 



d : d' : : v + u : v + u r ; 



that is, the rate of diffusion of a salt is proportional to the sum 

 of the velocities with which its component atoms move during 

 electrolysis. 



We shall now be able to understand the analogy between 



