Papers on Contact Electricity. 45 



the potential — E; so that the potential-difference is equal to 

 2E, which must be measured by the heat of combustion of the 

 zinc. If we now unite the zinc with any indifferent metal, 

 for example platinum, then a part of the electricity will flow 

 over to the platinum until both the metals have a common 

 potential + P. The free tension on the zinc becomes now 

 — E-f-P, that of the platinum + P; therefore the potential- 

 difference between zinc and platinum is now — E, which is 

 measured by half the heat of combustion of the zinc. If, 

 therefore, the heat-value of the Daniell's cell be A (the heat- 

 values, as is well known, must have reference to the chemical 

 equivalents of the substances), and the heat of combustion of 

 the zinc be B, then the potential-difference between zinc and 



platinum would be equal to x-r-. If the metal connected with 



the zinc is oxidized in the air, the manner of considering 

 the subject still remains the same, or the potential-difference 

 between the metals is measured by half the difference of the 

 heats of combustion." 



It will be observed that Prof. Exner speaks of the differ- 

 ence of potentials between a metal and its oxide as deter- 

 minable from the heat of combustion ; we presume, then, this 

 difference must be a constant, what in fact he calls 2E. Now 

 let us consider the second part of his statement given above; 

 and we find that by joining platinum metallically to zinc the 

 platinum and zinc have a common potential which he calls 

 -j-P; yet the difference of potential between the platinum 

 and the oxide of zinc is now only +E, or half the previous 

 difference between zinc and its oxide. Although, then, he 

 commences by saying that the difference of potential between 

 zinc and its oxide is measured by the heat of combustion, 

 which we know, per chemical equivalent, is a constant, he ends 

 by concluding that the difference of potential is not a constant. 



But it may be suggested that as his language is a little 

 vague, and not in accordance with that employed in the 

 modern mathematical theory of electricity, perhaps he means 

 that there is a certain charge of electricity in the oxide which 

 is a constant. This, however, does not appear to be his mean- 

 ing, since a very simple consideration will show that if the 

 charge of electricity in the oxide were a constant, the differ- 

 ence of potentials between zinc and platinum in metallic con- 

 tact, as measured by the Volta experiments, would not be, as 

 experiment shows, fairly constant for clean dry zinc and plati- 

 num (viz. 0*981 volt), but would be a variable depending on 

 the condensing-arrangement employed. 



[Addition } Dec. 21st. — This latter argument, Mr. Brown 



