g and
guiding influence of those reasons and causes which were hypostatised
in his ideal 'Forms.' In modern science, the conception of the
inertia, or resistance to change, of matter is complex. In part, it
contains a corollary from the law of causation: A body cannot change
its state in respect of rest or motion without a sufficient cause.
But, in part, it contains generalisations from experience. One of
these is that there is no such sufficient cause resident in any body,
and that therefore it will rest, or continue in motion, so long as no
external cause of change acts upon it. The other is that the effect
which the impact of a body in motion produces upon the body on which
it impinges depends, other things being alike, on the relation of a
certain quality of each which is called 'mass.' Given a cause of
motion of a certain value, the amount of motion, measured by distance
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 its