ological activities were of purely
physical and chemical origin, and that there was no need to assume any
such thing as a vital force. Then came the discovery that chemical
force, or affinity, had only an adventitious existence, and that, at
absolute zero, there was no such activity. The discovery of, or rather
the appreciation of, what is implied by the term _absolute zero_, and
especially of the nature of heat itself, as expressed in the statement
that heat is a mode of motion, dismissed another of the so-called forces
as being a metaphysical agency having no real existence, though standing
for phenomena needing further attention and explanation; and by
explanation is meant _the presentation of the mechanical antecedents for
a phenomenon, in so complete a way that no supplementary or unknown
factors are necessary_. The train moves because the engine pulls it; the
engine pulls because the steam pushes it. There is no more necessity for
assuming a steam force between the steam and the engine, than for
assuming an engine force between the engine and the train. All the
processes are mechanical, and have to do only with ordinary matter and
its conditions, from the coal-pile to the moving freight, though there
are many transformations of the forms of motion and of energy between
the two extremes.

During the past thirty years there has come into common use another
term, unknown in any technical sense before that time, namely, _energy_.
What was once called the conservation of force is now called the
conservation of energy, and we now often hear of forms of energy. Thus,
heat is said to be a form of energy, and the forms of energy are
convertible into one another, as the so-called forces were formerly
supposed to be transformable into one another. We are asked to consider
gravitative energy, heat energy, mechanical energy, chemical energy, and
electrical energy. When we inquire what is meant by energy, we are
informed that it means ability to do work, and that work is measurable
as a pressure into a distance, and is specified as foot-pounds. A mass
of matter moves because energy has been spent upon it, and has acquired
energy equal to the work done on it, and this is believed to hold true,
no matter what the kind of energy was that moved it. If a body moves, it
moves because another body has exerted pressure upon it, and its energy
is called _kinetic energy_; but a body may be subject to pressure and
not move appreciably, an