228 REPORT — 1897. 



between the rates at which salt is taken from the neighbourhoods of the 

 anode and kathode gives also the ratio between the velocities of the 

 kation and anion. 



Thus, by measuring the contents of vessels containing the electrodes 

 before and after the passage of the current, we can determine the ratio 

 between the velocities of the two ions in any given case of electrolysis. 

 Many such investigations have been made by Hittorf,' Lenz,^ Loeb and 

 Nemst,^ and Kistiakowsky.* An account of their methods and results 

 will be found in Professor Ostwald's ' Lehrbuch der AUgemeinen Chemie,' 

 2nd edition, vol. ii., p. 598, and most of the numerical results obtained 

 are included in a table compiled by T. C. Fitzpatrick and published in the 

 previous portion of this report, which appeared in 1893. 



A further step was taken by Professor F. Kohlrausch in the year 1879.^ 

 Kohlrausch introduced a satisfactory method of measuring the conduc- 

 tivity of electrolytes by means of alternating currents, and showed that, 

 from a knowledge of the conductivity, the sum of the opposite ionic 

 velocities (i.e. the velocity with which the opposite streams of ions travel 

 past each other) could be calculated. 



Faraday's work showed that the passage of a definite quantity of elec- 

 tricity through the solution involves the decomposition of a definite mass 

 of electrolyte, which varies as its chemically equivalent weight and as 

 the quantity of electricity. Thus the quantity of electricity which must 

 pass in order to decompose the equivalent weight of an electrolyte in 

 grams is independent of the nature of the electrolyte. 



We may therefore represent the facts by considering the process of 

 electrolysis to be a kind of convection, the ions moving through the 

 solution and carrying their charges with them. Each univalent ion may 

 be supposed to carry a certain definite charge, which we may take to be 

 the true natural unit of electricity ; each divalent ion carries twice as 

 much, and so on. 



Let us take, as an example, the case of an aqueous solution of hydro- 

 chloric acid whose concentration is w gram-equivalents per cubic centi- 

 metre. 



There will then be m gram-equivalents of hydrogen ions and the same 

 number of chlorine ions in this volume. Let us suppose that on each 

 gram-equivalent of hydrogen there reside -f q units of electricity, and on 

 each gram-equivalent of chlorine ions —q units. If u denote the average 

 velocity of the hydrogen ions, the positive charge carried per second across 

 unit area normal to the flow is m q u. Similarly, if v be the average 

 velocity of the chlorine ions, the negative charge carried in the opposite 

 direction is m q v. But positive electricity moving in one direction is 

 equivalent to negative electricity moving in the other, so that the total 

 •*eurrent, C, is to 5- (u + v). 



Now let us consider the amounts of hydrogen and chlorine liberated at 

 the electrodes by this current.^ At the kathode, if the chlorine ions were 

 at rest, the excess of hydrogen ions would be simply those arriving in one 



' Pogq. Ann. 1853-9, vol. Ixxxix. pp. 177 ; xcviii. p. 1 ; ciii. p. 1; cvi. pp. 337,513. 

 = Mem. Petersh. Ak. 1882, vol. ix. p. 30. 

 ' Zeits. physikal. Chem. 1883, vol. ii. p. 948. 

 * Zeits. physikal. Chem. 1890, vol. vi. p. 97. 

 ' Wied. Ann. vol. vi. p. 160. 



° This modification of Professor Lodge's method of developing Kohlrausch's equa- 

 tion was suggested to the writer by Professor G. F. FitzGerald. 



