travelled in a certain time, which it will produce in a given quantity
of matter, say a cubic inch, is not always the same, but depends on
what that matter is--a cubic inch of iron will go faster than a cubic
inch of gold. Hence, it appears, that since equal amounts of motion
have, _ex hypothesi_, been produced, the amount of motion in a body
does not depend on its speed alone, but on some property of the body.
To this the name of 'mass' has been given. And since it seems
reasonable to suppose that a large quantity of matter, moving slowly,
possesses as much motion as a small quantity moving faster, 'mass' has
been held to express 'quantity of matter.' It is further demonstrable
that, at any given time and place, the relative mass of any two bodies
is expressed by the ratio of their weights.

[Sidenote: Mechanical theory of heat.]

When all these great truths respecting molar motion, or the movements
of visible and tangible masses, had been shown to hold good not only
of terrestrial bodies, but of all those which constitute the visible
universe, and the movements of the macrocosm had thus been expressed
by a general mechanical theory, there remained a vast number of
phenomena, such as those of light, heat, electricity, magnetism, and
those of the physical and chemical changes, which do not involve molar
motion. Newton's corpuscular theory of light was an attempt to deal
with one great series of these phenomena on mechanical principles, and
it maintained its ground until, at the beginning of the nineteenth
century, the undulatory theory proved itself 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