According to the first principle of energetics, for the vital fact to be derived from the thermal fact, the heat must be preliminarily transformed into chemical energy, since chemical energy is necessarily an antecedent and generating form of vital energy. Now this regressive transformation is impossible according to the current theories of general physics. The part played by heat in the act of chemical combination is that of a primer to the reaction. It consists in placing the reacting bodies, by changing their state or by modifying their temperature, in the condition in which they ought to be for the chemical forces to come into play. For example, in the combination of hydrogen and oxygen by setting light to an explosive mixture, heat only acts as a primer to the phenomenon, because the two gases which are passive at ordinary temperatures, require to be raised to 400° C. before chemical affinity comes into play. And so it is with the reactions which go on in the organism. They have a maximum temperature, and the part played by animal heat is to furnish them with it. It follows that heat intervenes in animal life in two capacities—first and foremost as _excretum_, or end of the vital phenomenon, of _physiological work_; and on the other hand, as a _condition_ or _primer_ of the chemical reactions of the organism; and generally, as a favourable condition for the appearance of the physiological manifestations of living matter. Thus, it is not dissipated in sheer waste. I was led to adopt these views some years ago from certain experiments on the rôle played in food by alcohol. I did not then know that they had already been expressed by one of the masters of contemporary physiology, M. A. Chauveau, and that they were related in his mind to a series of conceptions and of researches of great interest, in the development of which I have since then taken a share. _Two Forms of Energy supplied to Animals by Food._—To say that food is simultaneously a supply of energy and a supply of matter, is really to express in a single sentence the fundamental conception of biology, in virtue of which life brings into play no substratum or characteristic dynamism. According to this, the living being appears to us as the seat of an incessant circulation of matter and energy, starting from the external world and returning to it. All food is nothing but this matter and this energy. All its characteristics, our views as to its rôle, its evolution, all the rules of alimentation are simple consequences of this principle, interpreted by the light of energetics.