And first of all, let us ask what forms of energy are afforded by food? It is easy to see that there are two—food is essentially a source of chemical energy; and secondarily and accessorily, it is a source of heat. Chemical energy is the only energy, according to the second law of energetics, which may be transformed into vital energy. It is true at any rate for animals; for in plants it is otherwise. There the vital cycle has neither the same point of departure nor the same final position. The circulation of energy does not take place in the same manner. On the other hand, and this we are taught by the third law, energy brought into play in vital phenomena is finally liberated and restored to the physical world in the form of heat. We have just said that this release of heat is employed in raising the temperature of the living being. It is animal heat. Thus there are two forms of energy supplied by food, chemical and thermal. It must be added that these are not the only forms, but the principal, and by far the most important. It is not absolutely true that heat is the only outcome of the vital cycle. It is only so in the subject in repose, contented to live idly without doing external mechanical work, without lifting a tool or a weight, even that of its own body. And again, speaking in this way, we neglect all the movements and all the mechanical work which is done without exercise of the volition, by the beating of the heart and of the arteries, the movements of respiration, and the contractions of the digestive tube. Mechanical work is, in fact, another possible termination of the cycle of energy. But there is no longer anything necessary or inevitable in this, since motion and the use of force are in a certain measure subordinated to the capricious volition of the animal.[11] [11] There is another reason why the rôle of mechanical energy, compared with that of thermal energy, is reduced, in the partition of afferent, alimentary energy—at least, in animals which have not to do excessive work. The unit of heat, the Calorie, is equivalent to 425 units of work—_i.e._, to 425 kilogrammetres. In the animal at rest, the number of kilogrammetres representing the different quantities of work done is small, the number of corresponding Calories is 425 times smaller. It becomes almost negligeable in comparison with the considerable number of Calories dissipated in the form of heat. At other times, again, it is an electrical phenomenon which terminates the vital cycle, and it is, in fact, in this way that things happen in the functional activity of the nerves and muscles in all animals, and in the functional activity of the electrical organ in fish, such as the ray and the torpedo. Finally, the termination may be a photic phenomenon, and this is what happens in phosphorescent animals.