Mr. J. Brown. 



[June 16, 



in the double salt does not decompose the potassium chloride, but 

 only reacts with its own chloride. Hence we have grounds for 

 assuming that potassium chloride remains practically inert, so far 

 as concerns any direct chemical action between it and the metal 

 immersed. The potassium chloride is, however, active in another 

 way, viz., in combining with the various metallic chlorides to form 

 their double salts ; and the heat of this combination should no doubt 

 be taken into account. Unfortunately, the heats of formation of 

 these double salts appear to be unknown. If they are nearly equal, 

 so that their differences are small in comparison with the whole heat 

 equivalent of the cell, their effect will be negligible. A comparison 

 of Tables I and II shows that this is probably so, but naturally the 

 results of these experiments with double salts cannot be compared 

 with theory quite so satisfactorily as those with simple chlorides. 



In any given experiment the two chlorides employed should be either 

 both double or both single, to avoid the complication which would 

 otherwise be introduced by the formation of a double salt on one side 

 and not on the other. In the preliminary stage of this work, some 

 experiments were made without observance of this rule, and the dis- 

 cordant results obtained showed that it cannot be disregarded. It 

 was therefore necessary to form double chlorides for all the metals 

 employed ; and the combinations whose formulae appear in Table III 

 were found, either by reference to the books or by experiment, to be 

 suitable. 



The experiments were carried out in the same way as the first set, 

 and the results are given in Table III, where for magnesium, iron, 

 and aluminium, they are stated in the same form as in Table I. For 

 the other metals, only single observations, as a rule, were made, for 

 comparison with Table I. The corresponding numbers in both tables 

 aie at least of the same order of magnitude, the difference only in 

 one case appearing in the first decimal place. 



