y determine the amount of heat
generated by its combustion. But zinc can be burnt not only in air
but in liquids. It is thus burnt when acidulated water is poured over
it; it is also thus burnt in the voltaic battery. Here, however, to
obtain the oxygen necessary for its combustion, the zinc has to
dislodge the hydrogen with which the oxygen is combined. The
consequence is that the heat due to the combustion of the metal in the
liquid falls short of that developed by its combustion in air, by the
exact quantity necessary to separate the oxygen from the hydrogen.
Fully four-fifths of the total heat are used up in this molecular
work, only one-fifth remaining to warm the battery. It is upon this
residue that we must now fix our attention, for it is solely out of it
that we manufacture our electric light.

Before you are two small voltaic batteries of ten cells each. The two
ends of one of them are united by a thick copper wire, while into the
circuit of the other a thin platinum wire is introduced. The platinum
glows with a white heat, while the copper wire is not sensibly warmed.
Now an ounce of zinc, like an ounce of coal, produces by its complete
combustion in air a constant quantity of heat. The total heat
developed by an ounce of zinc through its union with oxygen in the
battery is also absolutely invariable. Let our two batteries, then,
continue in action until an ounce of zinc in each of them is consumed.
In the one case the heat generated is purely domestic, being liberated
on the hearth where the fuel is burnt, that is to say in the cells of
the battery itself. In the other case, the heat is in part domestic
and in part foreign--in part within the battery and in part outside.
One of the fundamental truths to be borne in mind is that the sum of
the foreign and domestic--of the external and internal--heats is fixed
and invariable. Hence, to have heat outside, you must draw upon the
heat within. These remarks apply to the electric light. By the
inter-mediation of the electric current the moderate warmth of the
battery is not only carried away, but concentrated, so as to produce,
at any distance from its origin, a heat next in order to that of the
sun. The current might therefore be defined as the swift carrier of
heat. Loading itself here with invisible power, by a process of
transmutation which outstrips the dreams of the alchemist, it can
discharge its load, in the fraction of a second, as light and