56 Mr Chittock, The Migration Constants of 



Now let a definite volume V c.c. of the solution round the 

 cathode be separated from the rest and stirred, this volume being 

 large enough to contain the whole region in which any change 

 of concentration has taken place. The concentration of this 

 portion will then be less than its original value by an amount 



S?i = 1000^, 

 q V 



hn being measured in gram-equivalents per litre. By measuring 

 the change of concentration we can therefore find the value of p, 

 and hence that of ujv. A second determination of the same 

 quantity can be obtained in a similar way from the change of 

 concentration at the anode. 



In determining the migration constants of very dilute solu- 

 tions, it would be quite impossible to measure the changes of 

 concentration by the ordinary methods of chemical analysis. The 

 measurement of the electrical conductivity of such a solution 

 affords however a sufficiently delicate method of obtaining its 

 concentration, and this method has been employed in the pre- 

 sent work. 



The experiments here described show that the apparent value 

 of the migration constant of a solution of hydrochloric acid increases 

 considerably as the concentration is diminished. It was thought 

 that this result indicated a decrease in the velocity of the hydrogen 

 ion as the cause of the low conductivity of the solution. A re- 

 cently published paper by Whetham and Paine* shows however that 

 another explanation is more probable. These authors have carried 

 out a series of measurements on solutions of sulphuric acid, by 

 a method similar in principle to that which has been employed 

 by the present writer, and have found a change in the migration 

 constant, which is similar, though smaller in amount, to that 

 which has been obtained in the case of hydrochloric acid. They 

 find that the change can be explained on the supposition that the 

 conductivity of the solvent water is partly due to the presence 

 of a salt formed from a weak acid and a weak base, such as 

 ammonium carbonate ; this substance may be present in sufiicient 

 quantity, owing to the absorption of atmospheric ammonia and 

 carbon dioxide. A small quantity of ammonium carbonate would 

 account for the observed change in the migration constant, and 

 also for the decrease in the apparent equivalent conductivity of 

 a dilute solution of acid or alkali. They therefore conclude 

 that their experiments give no evidence in favour of the hypo- 

 thesis that the velocity of the hydrogen ion becomes smaller at 

 great dilution. 



The writer's experiments on hydrochloric acid had not been 

 * Proc. Boy. Soc. lxxxi. A, p. 58. 



