uld continue to give out heat and light forever. But
those advance thinkers who had grasped the import of the doctrine of
conservation could at once appreciate the force of Thomson's doctrine
of dissipation, and realize the complementary character of the two
conceptions.

Here and there a thinker like Rankine did, indeed, attempt to fancy
conditions under which the energy lost through dissipation might be
restored to availability, but no such effort has met with success, and
in time Professor Thomson's generalization and his conclusions as to the
consequences of the law involved came to be universally accepted.

The introduction of the new views regarding the nature of energy
followed, as I have said, the course of every other growth of new ideas.
Young and imaginative men could accept the new point of view; older
philosophers, their minds channelled by preconceptions, could not get
into the new groove. So strikingly true is this in the particular case
now before us that it is worth while to note the ages at the time of the
revolutionary experiments of the men whose work has been mentioned as
entering into the scheme of evolution of the idea that energy is merely
a manifestation of matter in motion. Such a list will tell the story
better than a volume of commentary.

Observe, then, that Davy made his epochal experiment of melting ice by
friction when he was a youth of twenty. Young was no older when he
made his first communication to the Royal Society, and was in his
twenty-seventh year when he first actively espoused the undulatory
theory. Fresnel was twenty-six when he made his first important
discoveries in the same field; and Arago, who at once became his
champion, was then but two years his senior, though for a decade he had
been so famous that one involuntarily thinks of him as belonging to an
elder generation.

Forbes was under thirty when he discovered the polarization of heat,
which pointed the way to Mohr, then thirty-one, to the mechanical
equivalent. Joule was twenty-two in 1840, when his great work was
begun; and Mayer, whose discoveries date from the same year, was then
twenty-six, which was also the age of Helmholtz when he published his
independent discovery of the same law. William Thomson was a youth just
past his majority when he came to the aid of Joule before the British
Society, and but seven years older when he formulated his own doctrine
of the dissipation of energy. And Clausius and Rankine,