elf to be a much better
working hypothesis. Heat, up to that time, and indeed much later, was
regarded as an imponderable substance, _caloric_; as a thing which was
absorbed by bodies when they were wanned, and was given out as they
cooled; and which, moreover, was capable of entering into a sort of
chemical combination with them, and so becoming latent. Rumford and
Davy had given a great blow to this view of heat by proving that the
quantity of heat which two portions of the same body could be made to
give out, by rubbing them together, was practically illimitable. This
result brought philosophers face to face with the contradiction of
supposing that a finite body could contain an infinite quantity of
another body; but it was not until 1843, that clear and unquestionable
experimental proof was given of the fact that there is a definite
relation between mechanical work and heat; that so much work always
gives rise, under the same conditions, to so much heat, and so much
heat to so much mechanical work. Thus originated the mechanical theory
of heat, which became the starting-point of the modern doctrine of the
conservation of energy. Molar motion had appeared to be destroyed by
friction. It was proved that no destruction took place, but that an
exact equivalent of the energy of the lost molar motion appears as
that of the _molecular_ motion, or motion of the smallest particles of
a body, which constitutes heat. The loss of the masses is the gain of
their particles.

[Sidenote: Earlier approaches towards doctrine of conservation.]

Before 1843, however, the doctrine of conservation of energy had been
approached Bacon's chief contribution to positive science is the happy
guess (for the context shows that it was little more) that heat may be
a mode of motion; Descartes affirmed the quantity of motion in the
world to be constant; Newton nearly gave expression to the complete
theorem; while Rumford's and Davy's experiments suggested, though they
did not prove, the equivalency of mechanical and thermal energy.
Again, the discovery of voltaic electricity, and the marvellous
development of knowledge, in that field, effected by such men as Davy,
Faraday, Oersted, Ampere, and Melloni, had brought to light a number
of facts which tended to show that the so-called 'forces' at work in
light, heat, electricity, and magnetism, in chemical and in mechanical
operations, were intimately, and, in various cases, quantitatively
related. It