J. W. Gihhs — Eqtdlihrmm of Heterogeneous Substances. 517 



( V' — T^") Ae = 34825 '■»'• AQ-2\\^ ™'-, 



whence 



Z?«-^Trp=: 36938. 



We cannot assign a precise value to z/ TT>, since the quantity of chlo- 

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

 value of —z/ TFp must lie between 290'''''- and 580™'-, probably nearer 

 to the former. 



The great diiFerence 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 condi- 

 tions 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 ai'e 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 accovmted 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, — espe- 

 cially 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 rep- 

 resented by Z/ ^ (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 circimistance, which would in all cases diminish the heat 

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

 oi A Q must be very small in comparison with that of ( V -^ V") Ae, 

 or with that of zJf, which were in general determined by the compar- 



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

 Memoires des Savants Etrangers, 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 Com.ptes Bendus, t. Ixviii, p. 1305. 



