Electromotive Forces in the Voltaic Cell. 345 
in each cell. This energy is the same in all the cells, but not 
in all does it take the same form. In the zinc-platinum cell 
it mainly results in driving the current forward. In the zinc- 
zinc cell it wholly results in a Peltier (or Bouty) generation 
of heat. In going out of the cell by a cathode zinc plate, it 
has to move hydrogen towards it, and (ipso facto) oxygen 
away from it, in opposition to the strong chemical attraction ; 
thus it will do work and liberate energy, which, since there is 
nothing better to do, must exhibit itself as heat. At an iron 
surface less heat is generated, and at a copper less still ; but, 
at any cathode which attracts oxygen, some heat must be 
generated by a current made to do work in opposition to this 
attraction. 
In the zinc-zine cell there is no propulsion of electricity at 
all by the cell; on one side, where the current enters, zinc is 
dissolved and the current helped forward with the full energy 
(or nearly the full energy) of the combination, so that no (or 
nearly no) waste energy or heat is there produced ; but on 
the other side, where the current leaves, the same combination 
is (not exactly undone but) opposed and the current hindered 
with (probably something less than) the full energy of the 
combination, and there the heat of combination is generated. 
Thus, regarding the passage of hydrogen to the cathode as 
a virtual separation of O (or SQ,) from it, we may say in 
general that in any one of the above cells used as a volta- 
meter, the energy available for helping the current on is that 
represented by the difference between the combination-energies 
of the substances respectively attacked and liberated ; 7. e. 
Zn, SO,—H,, SO,; but that besides this, the combination 
M, SO, is virtually undone ; and, since the energy of this com- 
bination appears asa generation of heat at the cathode, it is so 
much to be subtracted from the propelling force available for 
the current, only the balance being left for this purpose, viz., 
Zn, S0,—H,, SO,—M, SO,. 
Whether one ought to write SQ, or O in these expressions I 
am not sure, but it is not essential to decide this at present. 
Another way of regarding the matter is to say that the 
force propelling the current is that due to the difference of 
energies Zn,O—M,O, but that as soon as a current actually 
passes and hydrogen is liberated it coats the cathode more or 
less thickly, and an extra term must be subtracted from the 
above to represent the opposition force of this hydrogen. 
The efficacy of this hydrogen as a current-opposer must 
