example, contains a store of oxygen, which may unite with, and
consume, a metal immersed in it; it is from this kind of combustion
that we are to derive the heat and light employed in our present
course.

The generation of this light and of this heat merits a moment's
attention. Before you is an instrument--a small voltaic battery--in
which zinc is immersed in a suitable liquid. An attractive force is at
this moment exerted between the metal and the oxygen of the liquid;
actual combination, however, being in the first instance avoided.
Uniting the two ends of the battery by a thick wire, the attraction is
satisfied, the oxygen unites with the metal, zinc is consumed, and
heat, as usual, is the result of the combustion. A power which, for
want of a better name, we call an electric current, passes at the same
time through the wire.

Cutting the thick wire in two, let the severed ends be united by a
thin one. It glows with a white heat. Whence comes that heat? The
question is well worthy of an answer. Suppose in the first instance,
when the thick wire is employed, that we permit the action to continue
until 100 grains of zinc are consumed, the amount of heat generated in
the battery would be capable of accurate numerical expression. Let
the action then continue, with the thin wire glowing, until 100 grains
of zinc are consumed. Will the amount of heat generated in the battery
be the same as before? No; it will be less by the precise amount
generated in the thin wire outside the battery. In fact, by adding the
internal heat to the external, we obtain for the combustion of 100
grains of zinc a total which never varies. We have here a beautiful
example of that law of constancy as regards natural energies, the
establishment of which is the greatest achievement of modern science.