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 mechanical theory of
heat. This study, moreover, will not only have the effect of making
known to us interesting facts in electricity, but will throw some
light on the phenomena of electricity themselves."

The eminent professor was thus expressing the general opinion of his
contemporaries, but he certainly seemed to have felt in advance that
the new theory was about to penetrate more deeply into the inmost