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.