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 ENERGY

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ENERGY

reproduce the original amount of heat — assuming that in the actual process of transformation there were no waste. In other words, it is now accepted as estab- lished that, in any "conservative" or completely iso- lated system of energies, whatever changes or trans- formations take place among them, so long as no external agent intervenes, the sum of the energies will always remain constant. The Principle or Law of the Conservation of Energy has been thus formulated by Clerk Maxwell: "The total energy of any body or system of bodies is a quantity which can neither be increased nor diminished by any mutual action of these bodies, though it may be transformed into any of the forms of which energy is susceptible "(Theory of Heat, p. 93). Thus stated, the law may be admitted to hold the position of a fundamental axiom in modern physics; the nature of the evidence for it, we shall con- sider later. But there is a further generalization, ad- vancing a considerable way beyond the frontiers of positive science, which affirms that the total sum of such energy in the universe is afLxed amount " immuta- ble in quantity from eternity to eternity" (Von Helm- holtz). This is a proposition of a very different char- acter; and to it also we shall return. But first a brief historical account of the doctrine.

History. — The doctrine of the Conservation of Energy was long preceded by that of the Constancy of Matter. This was held vaguely as a metaphysical postulate by the ancient materialists and positively formulated as a philosophical principle by Telesius, Galileo, and Francis Bacon. Descartes assumed in a somewhat similar a priori fashion that the total amount of motion (MV) in the universe is fixed — certam tamen et determinatam habet quantitatem (Prin- cip. Philos., II, 36). But the effort to establish such assumptions by accurate experiment begins later. According to many we have the principle of the conser- vation of energy virtually formulated for the first time in Newton's Scholion developing his third law of motion (action and reaction are equal and opposite), though his participation in the current erroneous con- ception of heat as a "caloric", or independent sub- stance, prevented his clearly apprehending and expli- citly formulating the principle. Others would connect it with his second law. Huyghens, in the seventeenth century, seems to have grasped, though somewhat vaguely, the notion of momentum, or vis vica (MV^). This was clearly enunciated by Leibniz later. The fundamental obstacle, however, to the recognition of the constancy of energy lay in the prevalent " caloric theory ". Assuming heat to be some sort of substance, its origin and disappearance in connexion with fric- tion, percussion, and the like seemed a standing con- tradiction with any hjrpothesis of the constancy of energy. As early as 1780, Lavoisier and Laplace, in their " Memoire sur la chaleur", show signs of ap- proaching the modern doctrine, though Laplace sul> sequently committed him.self more deeply to the caloric theory. Count Rumford's famous experiments in measuring the amount of heat generated by the boring of cannon and Sir Humphry Davy's analogous observations (1790) on the heat caused by the friction of ice, proved the death-blow to the caloric theory. For the view was now beginning to receive wide ac- ceptance among scientists, that heat was " probably a vil)ration of the corpuscles of bodies tending to sep- arate them ". Dr. Thomas Young, in 1807, employed the term energy to designate the vis viva or active force of a moving body, which is measured by its mass or weight multiplied by the square of its velocity (MV^). Sadi Carnot (1824), though still labouring under the caloric theory, advanced the problem sub- stantially in his remarkable paper, " Reflexions sur la puissance motrice du f eu ", by considering the question of the relation of quantity of heat to amount of work done, and by introducing the conception of a machine with a reversible cycle of operations. The great

epoch, however, in the history of the doctrine occurred in 1842, when Julius Robert Mayer, a German physi- cian, published his "Remarks on the Forces of Inani- mate Nature", originally WTitten in a series of letters to a friend. In this little work, "contemptuously rejected by the leading journals of physics of that day" (Poincare), Mayer clearly enunciated the princi- ple of the conservation of energy in its widest gener- ality. His statement of the law was, however, in advance of the existing experimental evidence, and he was led to it partly by philosophical reasoning, partly by consideration of physiological questions. At the same time. Joule, in Manchester, was engaged in de- termining by accurate experiments the djmamical equivalent of heat — the amount of work a unit of heat could accomplish, and vice versa; and "Colding was contributing important papers on the same subject to the Royal Scientific Society of Copenhagen, so that no particular man can be describee! as the Father of the doctrine of the Conservation of Energy" (Preston). Between 1848 and 1851, Lord Kelvin (then Sir William Thomson), Clausius, and Rankine developed the ap- plication of the doctrine to sundry important problems in the science of heat. About the same time Helm- holtz, approaching the subject from the mathematical side, and starting from Newton's Laws of Motion, with certain other assumptions as to the constitution of matter, deduced the same principle, which he termed the " Conservation of Forces". Subsequently, Fara- day and Grove illustrated in greater detail the extent and variety of the transformation and correlation of forces, not only heat being changed into work, but light occasioning chemical action, and this generating heat, and heat producing electricity, capable of being again converted into motion, and so on round the cycle. But it further became evident that in such a series there inevitably occurs a waste in the usableness of energy. Though the total energy of a system may remain a constant quantity, since work can be done by heat only in its transition from a warmer to a cooler body, in proportion as such heat gets diffused through- out the whole system it becomes less utilizable, and the total capacity for work diminishes owing to this dissi- pation or degradation of energy. This general fact is formulated in what has been called the principle of Carnot or of Clausius. It is also styled the second law of therm odynamicsand has been made the basis of very important conclusions as to the finite duration of the universe by Lord Kelvin. He thus enunciates the law: " It is impossible by means of inanimate material agency to derive a mechanical effect from a portion of matter by cooling it below the temperature of the coldest surrounding bodies. "

Living Organisms. — The successful determination of the quantitative equivalent of one form of energ>' in some other form, obviously becomes a far more diffi- cult problem when the subject of tlie experiment is not inanimate matter in the chemical or physical labora- tory, but the consumption of substances in the living organism. Scientific research has, however, made some essays in this direction, endeavouring to estab- lish by experiment that the principle of the constancy of energy holds also in vital processes. By the nature of the case the experimental evidence is of a rougher and le.ss accurate character. Still it tends to show at all events approximate equivalence in the case of some organic functions. Among the best investiga- tions so far seem to be those of Rubner, who kept tlogs in a calorimeter, measuring carefully the quantity of food received and the heat developed by them. The chemical energy of the substances consumed manifests itself in heat and motion, and the heat generated in the consumption of different substances by the animals seems to have corresponded rather closely to that re- sulting in laboratory experiments; hence it is affirmed that the observations all point to the conclusion that "the sole cause of animal heat is a chemical process"