THERMODYNAMICS OF THE VOLTAIC CELL. TF 
in different cells ; they find that for all the cases which they have 
examined the equation 
dE 
5 T 
holds good within the limits of experimental error. 
Hence we obtain 
= > (i) 
Bees Ue 
oq 
an equation which Jahn* has applied to the determination of heats 
of combustion. These investigations of Jahn and Gill cover 
between them such a wide range of chemical actions as to warrant 
us in regarding equation (D)—provisionally at least—as generally 
applicable. + 
PR yt ax, uD) 
6. Llectromotive Force and Dissociation. 
In a communication addressed to the Electrolysis Committee of 
the British Association,{ Willard Gibbs expresses the relation 
between e. m. f. and eke of formation in a different form. 
Denoting by T, the “temperature of transformation” —z.e., the 
temperature at which the chemical action which gives rise to the 
current would go on indiscriminately in either directions—Gibbs 
goes on to assume that the cell may be treated as a case of Carnot’s 
cycle,|| and from this assumption he easily deduces— 
E=H. =e’ (E) 
where H has the same signification as in (C) and (D). 
This equation is of great practical importance, inasmuch as if 
any two of the quantities E, H, and T, be known the third 
can be determined by applying it. Cohen‘) has verified it through- 
out in several cases, and has made it the basis of an electrical 
method of determining transformation temperatures, which gives 
results in very close agreement with those obtained by other 
methods, where such are applicable. WVan’t Hoff, Cohen, and 
Bredig** in a joint memoir combine the equation of Gibbs with 
that of Helmholtz and compare the results with experiment, thus 
verifying both at once. 
* Wied. Ann. xxxvii, p. 408. 
7 This equation was indeed assumed by Lodge even before Jahn’s work had rendered the 
assumpticn justifiable. See B. A. Report, 1887, p. 340. 
t B. A. Rep. 1886, p. 888. See also a criticism by Prof. Lodge, B. A. Rep. 1887, p. 340 ; 
and Prof. Gibbs's reply, B. A. Rep. 1888, p. 343. 
§ The lowest temperature of complete dissociation of a chemical compound is a special 
case of a transformation temperature. 
|| Whether or no this assumption is in general justifiable seems to be a little doubtful ; 
and, so far, Gibbs’ proof may be regarded as open to question. His result however appears, 
as far as experiments have conducted us up to the present, to be quite correct. 
J] Zeitsch. fiir Phys. Chem. xiv, pps. 53, 535; Cohen attributes the relation to Van’t 
Hoff, but Gibbs’ investigation preceded Van ’t Hoff’s. 
** Zeitsch. fiir phys. Chem. xvi, p. 453. 
