n to
another we can never secure more than an equivalent quantity; that, in
short, "to create or annihilate energy is as impossible as to create or
annihilate matter; and that all the phenomena of the material universe
consist in transformations of energy alone." Some philosophers think
this the greatest generalization ever conceived by the mind of man. Be
that as it may, it is surely one of the great intellectual landmarks
of the nineteenth century. It stands apart, so stupendous and so
far-reaching in its implications that the generation which first saw the
law developed could little appreciate it; only now, through the vista of
half a century, do we begin to see it in its true proportions.

A vast generalization such as this is never a mushroom growth, nor does
it usually spring full grown from the mind of any single man. Always a
number of minds are very near a truth before any one mind fully grasps
it. Pre-eminently true is this of the doctrine of the conservation of
energy. Not Faraday alone, but half a dozen different men had an inkling
of it before it gained full expression; indeed, every man who advocated
the undulatory theory of light and heat was verging towards the goal.
The doctrine of Young and Fresnel was as a highway leading surely on
to the wide plain of conservation. The phenomena of electro-magnetism
furnished another such highway. But there was yet another road which led
just as surely and even more readily to the same goal. This was the road
furnished by the phenomena of heat, and the men who travelled it were
destined to outstrip their fellow-workers; though, as we have seen,
wayfarers on other roads were within hailing distance when the leaders
passed the mark.

In order to do even approximate justice to the men who entered into
the great achievement, we must recall that just at the close of the
eighteenth century Count Rumford and Humphry Davy independently showed
that labor may be transformed into heat; and correctly interpreted this
fact as meaning the transformation of molar into molecular motion. We
can hardly doubt that each of these men of genius realized--vaguely, at
any rate--that there must be a close correspondence between the amount
of the molar and the molecular motions; hence that each of them was in
sight of the law of the mechanical equivalent of heat. But neither of
them quite grasped or explicitly stated what each must vaguely have
seen; and for just a quarter of a century no on