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1858.] by the chemical action within it, or the quasi combustion of its food. Its will does but direct the application of its mechanical power. It creates none. You may weigh the animal and all the food it is to consume, and thence calculate the utmost ounce of work, of a given kind, which it can thereafter perform. It may do less, but cannot do more. Having consumed all of its food and part of itself, it dies. Its chemical organs have oxydated or burned up all the combustibles submitted to them, thus developing a definite amount of heat, a part of which, at the dictation of the will, by the mechanism of nerves and muscles, has been converted into mechanical motion. When the chemical function ceases, for the want of materials to act upon, the development of heat ceases. There is no more either to be converted into motion or to maintain the temperature of the body; and self-consumption having already taken the place of self-repair, there is no article left but the articulus mortis.

But of all the force or motion produced by, or rather passing through, a living animal, or any other organism, none is ever, so far as we know, annihilated. The motion which has apparently ceased or been destroyed has in reality passed into heat, light, electricity, magnetism, or other effect,—itself, perhaps, nothing but motion, to keep on, in one form or another, indefinitely. The fuel which we put into the stomach of the horse, of iron or of flesh, first by its oxydation raises heat, a part of which it is the function of the individual to convert into motion, to be expended on friction and resistance, or, in other words, to be reconverted into heat. What becomes of this heat, then? If the fuel were to be replaced or deoxydated, the heat that originally came from the oxydation would be precisely reabsorbed. But this heat of itself cannot overcome the stronger affinity which now chains the fuel to the oxygen. It must go forward, not backward, about its business, forever and ever. It may pass, but not cease. The sharp-eyed Faraday has been following far away this Proteus, with a strong suspicion that it changes at last into gravity, in which shape it returns straight to the sun, carrying down with it, probably, those flinty showers of meteors which, striking fire in the atmosphere of the prime luminary, replenish its overflowing fountain of life. But we are not aware that he has yet discovered the anastomosis of this conversion, or quite established the fact. We are therefore not yet quite ready to resolve the universe of physical forces into the similitude of the mythical mill-stream, which, flowing round a little hill, came back and fed its own pond. Nevertheless, we believe the physicists have pretty generally agreed to assume as a law of Nature what they call the conservation of force, the principle we have been endeavoring to explain.

Under the lead of this law, theory, or assumption, discoveries have been made that deeply and practically interest the most abject mortal who anywhere swings a hoe or shoulders a hod, as well as the lords of the land. For example, it has been ascertained that heat is converted into motion, or motion into heat, according to a fixed or constant ratio or equivalent. To be more particular, the heat which will raise the temperature of a pound of water one degree of Fahrenheit's scale, when converted into mechanical motion, is equivalent to the force which a weight of seven hundred and seventy-two pounds would exert by falling one foot. This is a wonderfully small quantity of heat to balance so heavy a blow, but the careful experiments of Mr. Joule of Manchester, the discoverer, confirmed by Regnault, Thomson, Rankine, Clausius, Mayer, Rennie, and others, have, we believe, satisfied scientific men that it is not far from the correct measure. Were the same, or a far less amount of heat, concentrated on a minute chip of steel struck off by collision with a flint, it would be visible to the eye as a spark, and show us how motion is converted into light as well as heat.

It is not our vocation to dive into the infinities, either upward or downward, in