poken of as entities. It is much more philosophical
to speak of matter and motion, for in the absence of motion there is no
energy, and the energy varies with the amount of motion; and
furthermore, to understand any manifestation of energy one must inquire
what kind of motion is involved. This we do when we speak of mechanical
energy as the energy involved in a body having a translatory motion;
also, when we speak of heat as a vibratory, and of light as a wave
motion. To speak of energy without stating or implying these
distinctions, is to speak loosely and to keep far within the bounds of
actual knowledge. To speak thus of a body possessing energy, or
expending energy, is to imply that the body possesses some kind of
motion, and produces pressure upon another body because it has motion.
Tait and others have pointed out the fact, that what is called potential
energy must, in its nature, be kinetic. Tait says--"Now it is impossible
to conceive of a truly dormant form of energy, whose magnitude should
depend, in any way, upon the unit of time; and we are forced to conclude
that potential energy, like kinetic energy, depends (even if unexplained
or unimagined) upon motion." All this means that it is now too late to
stop with energy as a final factor in any phenomenon, that the _form of
motion_ which embodies the energy is the factor that determines _what_
happens, as distinguished from how _much_ happens. Here, then, are to be
found the distinctions which have heretofore been called forces; here
is embodied the proof that direct pressure of one body upon another is
what causes the latter to move, and that the direction of movement
depends on the point of application, with reference to the centre
of mass.

It is needful now to look at the other term in the product we call
energy, namely, the substance moving, sometimes called matter or mass.
It has been mentioned that the idea of a medium filling space was
present to Newton, but his gravitation problem did not require that he
should consider other factors than masses and distances. The law of
gravitation as considered by him was--Every particle of matter attracts
every other particle of matter with a stress which is proportional to
the product of their masses, and inversely to the squares of the
distance between them. Here we are concerned only with the statement
that every particle of matter attracts every other particle of matter.
Everything then that possesses gravitativ