And so _vital energies_ for this twofold reason, intrinsic and extrinsic, are not readily apparent. To reveal them, the careful analysis of the physiologists is required. They are acts, in most cases silent and invisible, which we should scarcely recognize but by their effects, after they have terminated in familiar, phenomenal forms. This is, for example, what goes on in the muscle in process of shortening, in the nerve carrying the nervous influx, in the secreting gland. And this is what constitutes the different forms of energy which we call _vital properties_. M. Chauveau and M. Laulanié use the phrase _physiological work_ to distinguish them. _Vital energy_ would be preferable. It better expresses the analogy of this special form with the other forms of universal energy; it helps us better to understand that we must henceforth consider it as exchangeable by means of equivalents with the energies of the physical world just as they are exchangeable one with another. § 2. FIRST LAW OF BIOLOGICAL ENERGETICS. It is easy to understand, after these remarks, the significance and the scope of this assertion which contains the first principle of biological energetics—namely, that the phenomena of life have the same claim to be called energetic metamorphoses as the other phenomena of nature. _Irreversibility of Vital Energies._—However, there is one characteristic of vital energies which deserves the closest attention. Their transformations have a direction which is in some measure inevitable. They descend a slope which they never re-ascend. They appear to be irreversible. Ostwald has rightly insisted on this fundamental characteristic, which no doubt is not that of all the phenomena of the living being without exception, but which is certainly that of the most essential phenomena. There are reversible phenomena in organisms; there are energetic transformations which may take place from one form of energy to another, or _vice versâ_. But the most characteristic phenomena of vitality do not act in this way. We shall presently see that most functional physiological acts begin with chemical and end with thermal action. The series of energetic transformations takes place in an inevitable direction, from chemical to thermal energy. The order of succession of ordinary energies is thus determined in the machine of the organism, and therefore by the conditions of the machine. The order of transformation of vital energies is still more rigorously regulated, and the phenomena of life evolve from childhood to ripened years, and thence to old age, without a possible return.