the fundamental nature of matter are expressible in terms employed,
more than two thousand years ago, by the old 'master of those that
know.' Such a criticism, however, would involve forgetfulness of the
fact, that the connotation of these terms, in the mind of the modern,
is almost infinitely different from that which they possessed in the
mind of the ancient, philosopher. In antiquity, they meant little more
than vague speculation; at the present day, they indicate definite
physical conceptions, susceptible of mathematical treatment, and
giving rise to innumerable deductions, the value of which can be
experimentally tested. The old notions produced little more than
floods of dialectics; the new are powerful aids towards the increase
of solid knowledge.

[Sidenote: (2) Conservation of energy.]

Everyday observation shows that, of the bodies which compose the
material world, some are in motion and some are, or appear to be, at
rest. Of the bodies in motion, some, like the sun and stars, exhibit a
constant movement, regular in amount and direction, for which no
external cause appears. Others, as stones and smoke, seem also to move
of themselves when external impediments are taken away. But these
appear to tend to move in opposite directions: the bodies we call
heavy, such as stones, downwards, and the bodies we call light, at
least such as smoke and steam, upwards. And, as we further notice
that the earth, below our feet, is made up of heavy matter, while the
air, above our heads, is extremely light matter, it is easy to regard
this fact as evidence that the lower region is the place to which
heavy things tend--their proper place, in short--while the upper
region is the proper place of light things; and to generalise the
facts observed by saying that bodies, which are free to move, tend