is the nutriment of the current. Thus the heat generated by the
thermo-current in a distant wire is simply that originally imparted to
the pile, which has been first transmuted into electricity, and then
retransmuted into its first form at a distance from its origin. As
water in a state of vapour passes from a boiler to a distant
condenser, and there assumes its primitive form without gain or loss,
so the heat communicated to the thermo-pile distils into the subtler
electric current, which is, as it were, recondensed into heat in the
distant platinum wire.

In my youth I thought an electro-magnetic engine which was shown to me
a veritable perpetual motion--a machine, that is to say, which
performed work without the expenditure of power. Let us consider the
action of such a machine. Suppose it to be employed to pump water
from a lower to a higher level. On examining the battery which works
the engine we find that the zinc consumed does not yield its full
amount of heat. The quantity of heat thus missing within is the exact
thermal equivalent of the mechanical work performed without. Let the
water fall again to the lower level; it is warmed by the fall. Add
the heat thus produced to that generated by the friction, mechanical
and magnetical, of the engine; we thus obtain the precise amount of
heat missing in the battery. All the effects obtained from the
machine are thus strictly paid for; this 'payment for results' being,
I would repeat, the inexorable method of nature.

No engine, however subtly devised, can evade this law of equivalence,
or perform on its own account the smallest modicum of work. The
machine distributes, but it cannot create. Is the animal body, then,
to be classed among machines? When I lift a weight, or throw a stone,
or climb a mountain, or wrestle with my comrade, am I not conscious of
actually creating and expending force? Let us look at the antecedents
of this force. We derive the muscle and fat of our bodies from what
we eat. Animal heat you know to be due to the slow combustion of this
fuel. My arm is now inactive, and the ordinary slow combustion of my
blood and tissue is going on. For every grain of fuel thus burnt a
perfectly definite amount of heat has been produced. I now contract
my biceps muscle without causing it to perform external work. The
combustion is quickened, and the heat is increased; this additional
heat being liberated in the muscle itself. I lay hold of