J. W. Gihhs — Eqidlihrmm of Heterogeneous Snhatances. 511 



woiild have an electromotive force acting in the direction of a current 

 wliich would carry the hydrogen from the denser to the rarer mass. 

 Certainly the gas could not be carried in the opposite direction by 

 an external electromotive force without the expenditure of as much 

 (electromotive) work as is equal to the mechanical work necessary to 

 pump the gas from the one arm of the tube to the other. And if by any 

 modification of the metallic electrodes (which remain unchanged by 

 the passage of electricity) we could reduce the passive resistances to 

 zero, so that the hydrogen could be carried reversibly from one mass 

 to the other without finite variation of the electromotive force, the only 

 possible value of the electromotive force would be represented by the 



expression t-y-^ as a very close approximation. It will be observed 



that, although gravity plays an essential part in a cell of this kind 

 by maintaining the difference of pressure in the masses of hydrogen, 

 the electromotive force cannot possibly be ascribed to gravity, since 

 the work done by gravity, when hydrogen passes from the denser to 

 the rarer mass, is negative. 



Again, it is entirely improbable that the electrical currents caused 

 by differences in the concentration of solutions of salts, (as in a cell 

 containing sulphate of zinc between zinc electrodes, or sulphate of 

 copper between copper electrodes, the solution of the salt being of 

 unequal strength at the two electrodes,) which have recently been 

 invesugated theoretically and experimentally by MM. Helmholtz and 

 Moser,* are confined to cases in which the mixture of solutions of 

 diflerent degrees of concentration will produce heat. Yet in cases in 

 which the mixture of more and less concentrated solutions is not 

 attended with evolution or absorption of heat, the electromotive force 

 must vanish in a cell of the kind considered, if it is determined 

 simply by the diminution of energy in the cell. And when the mix- 

 ture produces cold, the same rule would make any electromotive force 

 impossible except in the direction which would tend to increase the 

 difference of concentration. Such conclusions as would be quite 

 irreconcilable with the theory of the phenomena given by Professor 

 Helmholtz. 



A more striking example of the necessity of taking account of the 

 variations of entropy in the cell in a priori determinations of electro- 

 motive force is afforded by electrodes of zinc and mercury in a solu- 

 tion of sulphate of zinc. Since heat is absorbed when zinc is dissolved 



* Annalen der Physik und Chemie. Neue Folge, Band iii, February, 18"? 8. 

 Trans. Conn. Acad.. Vol. III. 65 Junk, 1878, 



