members now that Robert Hooke contested with Newton
the discovery of the doctrine of universal gravitation? The judgment of
posterity is unjust, but it is inexorable. And so we can little doubt
that a century from now one name will be mentioned as that of the
originator of the great doctrine of the conservation of energy. The man
whose name is thus remembered will perhaps be spoken of as the Galileo,
the Newton, of the nineteenth century; but whether the name thus
dignified by the final verdict of history will be that of Colding, Mohr,
Mayer, Helmholtz, or Joule, is not as, yet decided.

LORD KELVIN AND THE DISSIPATION OF ENERGY

The gradual permeation of the field by the great doctrine of
conservation simply repeated the history of the introduction of every
novel and revolutionary thought. Necessarily the elder generation, to
whom all forms of energy were imponderable fluids, must pass away before
the new conception could claim the field. Even the word energy, though
Young had introduced it in 1807, did not come into general use till some
time after the middle of the century. To the generality of philosophers
(the word physicist was even less in favor at this time) the various
forms of energy were still subtile fluids, and never was idea
relinquished with greater unwillingness than this. The experiments of
Young and Fresnel had convinced a large number of philosophers that
light is a vibration and not a substance; but so great an authority as
Biot clung to the old emission idea to the end of his life, in 1862, and
held a following.

Meantime, however, the company of brilliant young men who had just
served their apprenticeship when the doctrine of conservation came upon
the scene had grown into authoritative positions, and were battling
actively for the new ideas. Confirmatory evidence that energy is a
molecular motion and not an "imponderable" form of matter accumulated
day by day. The experiments of two Frenchmen, Hippolyte L. Fizeau and
Leon Foucault, served finally to convince the last lingering sceptics
that light is an undulation; and by implication brought heat into the
same category, since James David Forbes, the Scotch physicist, had shown
in 1837 that radiant heat conforms to the same laws of polarization
and double refraction that govern light. But, for that matter, the
experiments that had established the mechanical equivalent of
heat hardly left room for doubt as to the immateriality of this
"impondera