Now, although it is thus impossible that any force should be destroyed, or in any way cease to exist in one form without setting in action a precisely equal amount in some other form, it may, as it were, pass into a condition of restraint, and remain thus suspended and latent for an indefinite period—­ready, however, to break into action again the moment that the restraint is removed.  Thus a perfectly elastic spring may be bent by a certain force, and retained in the bent position a long time.  But the moment that it is released it will unbend itself, exercising in so doing precisely the degree of force expended in bending it.  In the same manner air may be compressed in an air-gun, and held thus, with the force, as it were, imprisoned, for any length of time, until at last, when the detent is released by the trigger, the elastic force comes into action, exercising in its action a power precisely the same as that with which it was compressed.

Force or power may be thus, as it were, stored up in a countless variety of ways, and reserved for future action; and, when finally released, the whole amount may be set free at once, so as to expend itself in a single impulse, as in case of the arrow or the bullet; or it may be partially restrained, so as to expend itself gradually, as in the case of a clock or watch.  In either case the total amount expended will be precisely the same—­namely, the exact equivalent of that which was placed in store.

Vegetable and Animal Life.

There are a vast number of mechanical contrivances in use among men for thus putting force in store, as it were, and then using it more or less gradually, as may be required.  And nature, moreover, does this on a scale so stupendous as to render all human contrivances for this purpose utterly insignificant in comparison.  The great agent which nature employs in this work is vegetation.  Indeed, it may truly be said that the great function of vegetable life, in all the infinitude of forms and characters which it assumes, is to receive and store up force derived from the emanations of the sun.

Animal life, on the other hand, exists and fulfills its functions by the expenditure of this force.  Animals receive vegetable productions containing these reserves of force into their systems, which systems contain arrangements for liberating the force, and employing it for the purposes it is intended to subserve in the animal economy.

The manner in which these processes are performed is in general terms as follows:  The vegetable absorbs from the earth and from the air substances existing in their natural condition—­that is, united according to their strongest affinities.  These substances are chiefly water, containing various mineral salts in solution, from the ground, and carbonic acid from the air.  These substances, after undergoing certain changes in the vessels of the plant, are exposed to the influence of the rays of the sun in the leaves.