de in such a manner as
to introduce no new force. It will thus be seen that this quantity
well expresses the capacity possessed by a system for modifying the
state of a neighbouring system to which we may suppose it connected.

Now this theorem of pure mechanics was found wanting every time
friction took place--that is to say, in all really observable cases.
The more perceptible the friction, the more considerable the
difference; but, in addition, a new phenomenon always appeared and
heat was produced. By experiments which are now classic, it became
established that the quantity of heat thus created independently of
the nature of the bodies is always (provided no other phenomena
intervene) proportional to the energy which has disappeared.
Reciprocally, also, heat may disappear, and we always find a constant
relation between the quantities of heat and work which mutually
replace each other.

It is quite clear that such experiments do not prove that heat is
work. We might just as well say that work is heat. It is making a
gratuitous hypothesis to admit this reduction of heat to mechanism;
but this hypothesis was so seductive, and so much in conformity with
the desire of nearly all physicists to arrive at some sort of unity in
nature, that they made it with eagerness and became unreservedly
convinced that heat was an active internal force.

Their error was not in admitting this hypothesis; it was a legitimate
one since it has proved very fruitful. But some of them committed the
fault of forgetting that it was an hypothesis, and considered it a
demonstrated truth. Moreover, they were thus brought to see in
phenomena nothing but these two particular forms of energy which in
their minds were easily identified with each other.

From the outset, however, it became manifest that the principle is
applicable to cases where heat plays only a parasitical part. There
were thus discovered, by translating the principle of equivalence,
numerical relations between the magnitudes of electricity, for
instance, and the magnitudes of mechanics. Heat was a sort of variable
intermediary convenient for calculation, but introduced in a
roundabout way and destined to disappear in the final result.

Verdet, who, in lectures which have rightly remained celebrated,
defined with remarkable clearness the new theories, said, in 1862:
"Electrical phenomena are always accompanied by calorific
manifestations, of which the study belongs to the mechan