1892.] Voltaic Cells with Fused Electrolytes. 79 



As regards the quality of the substance used, the zinc, lead, iron, 

 tin, and copper, and the chlorides of magnesium and tin, were pur- 

 chased as pure. The other metals and chlorides were the substances 

 as supplied by the chemical manufacturers under their respective 

 labels. In the case of metals forming more than one chloride, that 

 one was chosen in which a chlorine molecule C1 2 gave the highest 

 heat of combination. The corresponding formulae are stated in the 

 tables of results. The heats of combination employed are Thomsen's. 



Table I gives the results obtained with the chlorides of the metals 

 as stated, together with the theoretical values ; the unit employed 

 being the electromotive force of a Daniell cell, which for the Daniell 

 I used may be taken as about 1*13. 



In calculating the theoretical value, it is assumed that the reaction 

 is simply the substitution, in the double fluid electrolyte, of the metal 

 with higher combining heat for that with lower ; and, therefore, that 

 the theoretical electromotive force is, according to Thomson's law, 

 simply proportional to the difference of their combining heats with 

 chlorine, which is the anion of the electrolyte. This difference, 

 divided by 50,130, the heat equivalent of a Daniell cell, gives the 

 theoretical electromotive force for each pair, in terms of the Daniell. 

 In practice, it is more convenient to divide, once for all, the heat of 

 combination of each metal by the heat equivalent of the Daniell. The 

 quotients thus found are placed under their respective metals in the 

 table. It seems possible that these numbers may represent the true 

 differences of potential at contact of metal and liquid. 



The theoretical electromotive force for any pair of metals is then 

 given by the difference of their respective numbers. In the table, 

 this will be found in heavy type, at the intersection of the vertical 

 and horizontal lines passing through any given pair of metals ; the 

 metal to the right forming the negative pole of the arrangement. 

 Immediately below each theoretical electromotive force, is placed 

 that found by experiment, being the mean of the number of sets of 

 experiments indicated by the figure in brackets below it. The highest 

 and lowest observed values are stated to the right and left of it. 



In this table it will be observed that, while the mean experimental 

 result for any combination of the metals zinc, lead, and tin agrees 

 fairly well with Thomson's law ; those combinations in which silver, 

 copper, or cadmium occur do not exhibit such good agreement ; and 

 closer examination shows that much more consistent results could be 

 obtained by applying a constant correction for each metal. I have 

 determined, by a tentative process, the best values for these correc- 

 tions, and the results are shown in Table II. It is only a question of 

 differences; and if the correction for cadmium be taken as zero the 

 corrections for all the other metals will be positive. A justification 

 for this selection of cadmium will be found in a later paragraph. 



