very far afield from the dreamings of the eighteenth-century
philosophers; the electron of J. J. Thompson shows many points of
resemblance to the formless centre of Boscovich.

Whatever the exact form of the molecule, its outline is subject to
incessant variation; for nothing in molecular science is regarded as
more firmly established than that the molecule, under all ordinary
circumstances, is in a state of intense but variable vibration. The
entire energy of a molecule of gas, for example, is not measured by its
momentum, but by this plus its energy of vibration and rotation, due
to the collisions already referred to. Clausius has even estimated
the relative importance of these two quantities, showing that the
translational motion of a molecule of gas accounts for only three-fifths
of its kinetic energy. The total energy of the molecule (which we call
"heat") includes also another factor--namely, potential energy, or
energy of position, due to the work that has been done on expanding,
in overcoming external pressure, and internal attraction between the
molecules themselves. This potential energy (which will be recovered
when the gas contracts) is the "latent heat" of Black, which so long
puzzled the philosophers. It is latent in the same sense that the energy
of a ball thrown into the air is latent at the moment when the ball
poises at its greatest height before beginning to fall.

It thus appears that a variety of motions, real and potential, enter
into the production of the condition we term heat. It is, however,
chiefly the translational motion which is measurable as temperature;
and this, too, which most obviously determines the physical state of the
substance that the molecules collectively compose--whether, that is to
say, it shall appear to our blunt perceptions as a gas, a liquid, or a
solid. In the gaseous state, as we have seen, the translational motion
of the molecules is relatively enormous, the molecules being widely
separated. It does not follow, as we formerly supposed, that this
is evidence of a repulsive power acting between the molecules. The
physicists of to-day, headed by Lord Kelvin, decline to recognize any
such power. They hold that the molecules of a gas fly in straight lines
by virtue of their inertia, quite independently of one another, except
at times of collision, from which they rebound by virtue of their
elasticity; or on an approach to collision, in which latter case, coming
within the