were suggested by them, because the theory did not explain how an atom is constituted. An instructive illustration of the practical divergence of view that can arise in this manner is afforded by the difference of opinion (cf. Appendix D infra) between Newton and Huygens regarding the scope of the law of gravitation.
The general conception of a kinetic molecular structure for matter invites a reconstruction of the theoretical basis of ordinary mechanics itself. The customary development of abstract dynamics rests on the foundation of forces acting between particles so as to satisfy Newton's three laws of motion. The only meaning that can here be assigned to such a particle is an unchanging element of mass whose volume is large enough to contain a vast assemblage of molecules. Even in the hands of Kirchhoff, who is considered to have in his treatment notably broadened the subject, the dynamics of finite bodies is derived through the principle of Least Action from the conception that they are constituted of particles acting on each other in this way; thus in fact adhering to Lagrange's original procedure developed in the year 1760. It seems not too much to say, in the light of molecular science, that such a constitution for a material body is purely imaginary, and even meaningless when applied to such subjects as stress and strain in elastic matter. The real foundation of general abstract mechanics is either the principle of Action in its general form, assumed as a descriptive analytical formulation of the course of phenomena, or else the principle of d'Alembert which is analytically equivalent to it. The latter is doubtless the simpler basis when we are dealing only with elements of mass in the ordinary sense; for it is merely the statement that the irregular or uncoordinated part of the internal motions and strains in a stable or permanently existing element of mass has nothing to do with the changes of configuration of that element considered as a whole; and the preparation for its application consists in the process of picking out and specifying the coordinated parts so far as is needed for
