and are ultimately traceable to the "molecular" movements of the conservative system of which our planet is a part. It would be a work of supererogation to attempt to prove this, or to illustrate it in detail; it is sufficient to refer the reader to the eloquent exposition in Tyndall's "Heat as a Mode of Motion," p. 447, et seq. (Appletons' edition).
I need not say that I use the term "molecular force" simply because it is a generally-received term, and because I am constrained to use it in order to be intelligible, but that I do not intend to commit myself by this use to the theory of the constitution of matter which it implies. In like manner I use the term "force" with the reservation that it rightfully denotes, not a substantive entity distinct from matter, but the relation of at least two particular states of matter at a given moment.
The molecular character of molar motion is evinced in numerous ways, which are almost wholly neglected and ignored by the modern physicist. To take the simplest instance: when two solids impinge, so that an exchange or distribution of their motions takes place, they contract and immediately expand again, according to the degree of their elasticity. It is unnecessary to inquire whether or not a communication of motion between two absolutely rigid bodies is possible; all bodies, of which we know any thing, are more or less elastic, and therefore contract and expand at the instant of impact. And their contraction is accompanied by the evolution of heat, by the conversion of molar into molecular motion, while in the expansion we have a reconversion of molecular into molar motion. No transfer of molar motion ever takes place without this momentary transition through the molecular phase.
Since the establishment of the doctrine of the conservation of energy and the correlation and mutual convertibility of forces, physicists have repeatedly called attention to the fact that the old interpretation of the phenomenon of an apparent destruction of force is inaccurate, and that the true interpretation of this phenomenon consists in the tracing of the evanescent molar motion into resulting molecular motion. But they fail to observe that the old notions respecting the transfer of molar motion, when there is no loss, are in similar need of rectification.
Now, what is this molecular motion, in the light of the insight which, as I hope, has been gained in the foregoing discussion? Simply an exhibition of the struggle involved in the formation or constitution of a body as a distinct conservative system. All molecular energy is in its nature constitutive, formative, or structural. All kinetic energy, or actual motion, represents the progress of morphological action in periodical alternations of advancing and retrograde metamorphosis. And the main problem of physical science is, not to calculate the play of atomic motions, under the sway of their constant
