44 Messrs. Ayrton and Perry on Prof. Exner's 



Daniell's cell, thus calculated, and the value obtained by a 

 direct measurement with an absolute electrometer would not 

 have been arrived at by Sir William had he had at his disposal 

 the more accurate determinations that have since been made 

 of the heats of combustion. This, however, as we think may 

 be gathered from Dr. Wright's paper, does not in the least 

 disprove the accuracy or detract from the utility of Sir Win. 

 Thomson's, or rather Joule's, theory, but merely shows that 

 the products of combustion in the Daniell's cell are not exactly 

 what have usually been supposed ; for it has to be remem- 

 bered that if energy is developed or absorbed by occlusion of 

 gases, or by any other physical action that has not hitherto 

 been included in chemical actions, this must all be taken into 

 account in the calculation of electromotive force. 



II. Now Prof. Exner expresses the opinion that the so- 

 called contact electricity is produced by the oxidation of the 

 metals in contact with the oxygen of the air, in the same way 

 as electricity is evolved by the oxidation of the zinc in gal- 

 vanic cells. He says he has proved that this is true, and 

 states, in consequence, that the electromotive force between 

 two metals in contact with the air must be measured and 

 expressed by the heats of combustion. The exact relation- 

 ship which, in his opinion, ought to exist between the contact 

 electromotive force of, say, zinc and copper and the difference 

 of the heats of combustion of equivalent quantities of these 

 metals is the fundamental part of his paper. 



He says, at the bottom of page 598, Heft 4, Band ix. Wiede- 

 mann's Annalen, 1880, as we have translated it : — " We know 

 that in cells each chemical process produces a potential-dif- 

 ference which is proportional to its heat-value ; and in the 

 case of the oxidation of a metal in air, we should expect 

 that the potential-difference between a metal and the oxide 

 produced would be proportional to the heat of combustion. 

 Therefore any metal which, when insulated in the air, oxidizes, 

 must contain a certain quantity of separated positive and 

 negative electricity ; and it is obvious that these charges are 

 inactive towards outside bodies. These separated electricities 

 cannot surpass a certain tension-difference ; for the observed 

 tension is always a constant, no matter if the oxidation con- 

 tinues or not. From this it would appear that the quantities of 

 electricity produced in excess through continual oxidation are 

 neutralized again with the production of the corresponding 

 quantity of heat. 



" If now, for example, a piece of zinc through oxidizing in 

 the air has a potential +E, the oxidized layer, or rather the 

 layer of air which is in contact with it, has, on the contrary, 



