346 EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



whence 



We cannot assign a precise value to ATF P , since the quantity of 

 chlorine which was evolved in the form of gas is not stated. But 

 the value of -ATF P must lie between 290 cal - and 580 caL , probably 

 nearer to the former. 



The great difference in the results of the two series of experiments 

 relating to electrodes of zinc and platinum in hydrochloric acid is 

 most naturally explained by supposing some difference in the con- 

 ditions of the experiment, as in the concentration of the acid, or in 

 the extent to which the substitution of zinc for hydrogen took place.* 

 That which it is important for us to observe in all these cases is that 

 there are conditions under which heat is absorbed in a galvanic or 

 electrolytic cell, so that the galvanic cell has a greater electromotive 

 force than can be accounted for by the diminution of its energy, and 

 the operation of electrolysis requires a less electromotive force than 

 would be calculated from the increase of energy in the cell, especially 

 when the work done against the pressure of the atmosphere is taken 

 into account. 



It should be noticed that in all these experiments the quantity 

 represented by AQ (which is the critical quantity with respect to 

 the point at issue) was determined by direct measurement of the heat 

 absorbed or evolved by the cell when placed alone in a calorimeter. 

 The resistance of the circuit was made so great by a rheostat placed 

 outside of the calorimeter that the resistance of the cell was regarded 

 as insignificant in comparison, and no correction appears to have been 

 made in any case for this resistance. With exception of the' error 

 due to this circumstance, which would in all cases diminish the heat 

 absorbed in the cell (or increase the heat evolved), the probable error 

 of AQ must be very small in comparison with that of (V'V")Ae, 

 or with that of Ae, which were in general determined by the com- 

 parison of different calorimetrical measurements, involving very much 

 greater quantities of heat. 



In considering the numbers which have been cited, we should 

 remember that when hydrogen is evolved as gas the process is in 

 general very far from reversible. In a perfect electrochemical 



*It should perhaps be stated that in his extended memoir published in 1877 in the 

 At&moires dee Savants Strangers, in which he has presumably collected those results 

 of his experiments which he regards as most important and most accurate, M. Favre 

 does not mention the absorption of heat in a cell of this kind, or in the similar cell in 

 which cadmium takes the place of zinc. This may be taken to indicate a decided 

 preference for the later experiments which showed an evolution of heat. Whatever 

 the ground of this preference may have been, it can hardly destroy the significance 

 of the absorption of heat, which was a matter of direct observation in repeated experi- 

 ments. See Comptes Rendus, t. Ixviii, p. 1305. 



