§ 4 The Third Law of Biological Energetics
The third law of biological energetics is also drawn from experiment. It relates no longer to the point of departure of the cycle of animal energy, but to its final position. The energetic transformations of the animal end in thermal energy.
This is the most novel part of the theory, and, if we may say so, that least understood by physiologists themselves. The energy resulting from the chemical potential of food, having passed through the organism (or simply through the organ which we are considering in action), and having given rise to phenomenal appearances more or less diversified, more or less dim or clear, obscure or obvious, which are the characteristic or still irreducible manifestations of vitality, finally returns to the physical world. This return takes place (with certain exceptions which will be presently indicated) under the ultimate form of thermal energy. This we are taught by experiment. The phenomena of functional activity are exothermal.
Real vital phenomena thus lie between the chemical energy which gives rise to them, and the thermal phenomena to which they in their turn give rise. The place of the vital fact in the cycle of universal energy is therefore completely determined. This conclusion is of the utmost importance to biology. It may be expressed in a concise formula which sums up in a few words all that natural philosophy can teach as to energetics applied to living beings. “Vital energy is a transformation of chemical energy into thermal energy.”
Exceptions.—There are some exceptions to the rigour of this statement, but they are not many in number. We must first of all remark that it applies to animal life alone.
In the case of vegetables, looked at as a whole, the law must be modified. Their vital energy has another origin, and another final form. Instead of being the destroyers of chemical potential energy, they are its creators. They build up by means of the inert and simple materials afforded them by the atmosphere and the soil, the immediate principles by which their cells are filled. Their vital functional activity forms by synthesis of the reserves, carbo-hydrates (sugars and starches), fats, albuminoid nitrogenous materials—that is to say, the same three principal categories of foods as those used by animals.
And to return to the latter, it should be observed that thermal energy is not the only final form of vital energy, as this dogmatic statement would have it supposed. It is only the principle of the final forms. The cycle of energy occasionally terminates in mechanical energy (phenomena of motion) and in a less degree in other energies; such as, for example, the electrical energy produced by the functional activity of the nerves and muscles in all animals, or in the functional activity of special organs in rays, torpedo-fish, and the malapterurus electricus, or finally, in the photic energy of phosphorescent animals. But these are secondary facts.
Heat is an Excretum.-The third principle of biological energetics may be therefore thus enunciated:—Vital energy in its final form becomes thermal energy. This principle teaches us that if chemical energy is the primitive generating form of vital energies, thermal energy is the form of waste, of emunctory, the degraded form as the physicists would say. Heat is in the dynamical order an excretion of animal life, as urea, carbonic acid and water, are excreta in the substantial order. By a false interpretation of the principle of the mechanical equivalence of heat, or through ignorance of Carnot’s principle, certain physiologists have fallen into error when they still speak of the transformation of heat into motion or into into electricity in the animal organism. Heat is transformed into nothing in the animal organism. It is dissipated. Its utility arises not from its energetic value, but from the part it plays as a primer in the chemical reactions, as has been explained with reference to the general characteristics of chemical energy.
The Effect of Energetics on our Knowledge of the Relations of the Universe.—The consequences of these principles of energetic physiology, which give us so much and which are so clear, are of the greatest importance from the practical as well as from the theoretical point of view.
In the first place, they show us the position and the rank of the phenomena of life in the universe as a whole. They throw fresh light on the noble harmony of the animal and vegetable kingdoms which Priestley, Ingenhousz, Senebier, and the chemical school of the beginning of the nineteenth century discovered, and which was expounded by Dumas with incomparable lucidity and brilliance. Energetics is expressed in a line. “The animal world expends the energy accumulated by the vegetable world.” It extends these views beyond the living kingdoms. It shows how the vegetable world itself draws its activity from the energy radiated by the sun, and how animals restore it again, in dissipated heat, to the cosmic medium. It extends the harmony of the two kingdoms to the whole of nature. The new science makes of the whole universe one connected system.
From a more limited point of view, and so that we may not restrict ourselves to a consideration of the domain of animal physiology, the laws of energetics sum up and explain a multitude of facts and of experimental laws—for example, the law of the intermittence of physiological activity, the facts of fatigue, the rôle and the general principles of alimentation, and the conditions of muscular contraction.