I do not claim to set forth here the complete history of this principle, but I will endeavour to show with what pains it was born, how it was kept back in its early days and then obstructed in its development by the unfavourable conditions of the surroundings in which it appeared.  It first of all came, in fact, to oppose itself to the reigning theories; but, little by little, it acted on these theories, and they were modified under its pressure; then, in their turn, these theories reacted on it and changed its primitive form.

It had to be made less wide in order to fit into the classic frame, and was absorbed by mechanics; and if it thus became less general, it gained in precision what it lost in extent.  When once definitely admitted and classed, as it were, in the official domain of science, it endeavoured to burst its bonds and return to a more independent and larger life.  The history of this principle is similar to that of all evolutions.

It is well known that the conservation of energy was, at first, regarded from the point of view of the reciprocal transformations between heat and work, and that the principle received its first clear enunciation in the particular case of the principle of equivalence.  It is, therefore, rightly considered that the scholars who were the first to doubt the material nature of caloric were the precursors of R. Mayer; their ideas, however, were the same as those of the celebrated German doctor, for they sought especially to demonstrate that heat was a mode of motion.

Without going back to early and isolated attempts like those of Daniel Bernoulli, who, in his hydrodynamics, propounded the basis of the kinetic theory of gases, or the researches of Boyle on friction, we may recall, to show how it was propounded in former times, a rather forgotten page of the Memoire sur la Chaleur, published in 1780 by Lavoisier and Laplace:  “Other physicists,” they wrote, after setting out the theory of caloric, “think that heat is nothing but the result of the insensible vibrations of matter....  In the system we are now examining, heat is the vis viva resulting from the insensible movements of the molecules of a body; it is the sum of the products of the mass of each molecule by the square of its velocity....  We shall not decide between the two preceding hypotheses; several phenomena seem to support the last mentioned—­for instance, that of the heat produced by the friction of two solid bodies.  But there are others which are more simply explained by the first, and perhaps they both operate at once.”  Most of the physicists of that period, however, did not share the prudent doubts of Lavoisier and Laplace.  They admitted, without hesitation, the first hypothesis; and, four years after the appearance of the Memoire sur la Chaleur, Sigaud de Lafond, a professor of physics of great reputation, wrote:  “Pure Fire, free from all state of combination, seems to be an assembly of