CHAPTER XIV.

THE ACTIVITY OF CHILDREN.

IN order rightly to understand the true nature of that extraordinary activity which is so noticeable in all children that are in a state of health, so as to be able to deal with it on the right principles and in a proper manner, it is necessary to turn our attention somewhat carefully to certain scientific truths in respect to the nature and action of force in general which are now abundantly established, and which throw great light on the true character of that peculiar form of it which is so characteristic of childhood, and is, indeed, so abundantly developed by the vital functions of almost all young animals. One of the fundamental principles of this system of scientific truth is that which is called the persistence of force.

The Persistence of Force.

By the persistence of force is meant the principle—one now established with so much certainty as to command the assent of every thinking man who examines the subject— that in the ordinary course of nature no force is either ever originated or ever destroyed, but only changed in form. In other words, that all existing forces are but the continuation or prolongation of other forces preceding them, either of the same or other forms, but precisely equivalent in amount; and that no force can terminate its action in any other way than by being transmuted into some other force, either of the same or of some other form; but still, again, precisely equivalent in amount.

It was formerly believed that a force might under certain circumstances be originated-created, as it were—and hence the attempts to contrive machines for perpetual motion-that is, machines for the production of force. This idea is now wholly renounced by all well-informed men as utterly impossible in the nature of things. All that human mechanism can do is to provide modes for using advantageously a force previously existing, without the possibility of either increasing or diminishing it. No existing force can be destroyed. The only changes possible are changes of direction, changes in the relation of intensity to quantity, and changes of form.

The cases in which a force is apparently increased or diminished, as well as those in which it seems to disappear, are all found, on examination, to be illusive. For example, the apparent increase of a man's power by the use of a lever is really no increase at all. It is true that, by pressing upon the outer arm with his own weight, he can cause the much greater weight of the stone to rise; but then it will rise only a very little way in comparison with the distance through which his own weight descends. His own weight must, in fact, descend through a distance as much greater than that by which the stone ascends, as the weight of the stone is greater than his weight. In other words, so far as the balance of the forces is concerned, the whole amount of the downward motion consists of the smaller weight descending through a greater distance, which will be equal to the whole amount of that of the larger one ascending through a smaller distance; and, to produce a preponderance, the whole amount of the downward force must be somewhat greater. Thus the lever only gathers or concentrates force, as it were, but does not at all increase it.

It is so with all the other contrivances for managing force for the accomplishment of particular purposes. None

of them increase the force, but only alter its form and character, with a view to its better adaptation to the purpose in view.

When a bullet

Nor can any force be extinguished. strikes against a solid wall, the force of its movement, which seems to disappear, is not lost; it is converted into heat-the temperature of both the bullet and of that part of the wall on which it impinges being raised by the concussion. And it is found that the amount of the heat which is thus produced is always in exact proportion to the quantity of mechanical motion which is stopped; this quantity depending on the weight of the bullet, and on the velocity with which it was moving. And it has been ascertained, moreover, by the most careful, patient, and many times repeated experiments and calculations, that the quantity of this heat is exactly the same with that which, through the medium of steam, or by any other mode of applying it, may be made to produce the same quantity of mechanical motion that was extinguished in the bullet. Thus the force was not destroyed, but only converted into another form.

And if we should follow out the natural effects of this heat into which the motion of the bullet was transferred, we should find it rarefying the air around the place of concussion, and thus lifting the whole mass of the atmosphere above it, and producing currents of the nature of wind, and through these producing other effects, thus going on forever; the force changing its form, but neither increasing or diminishing its quantity through a series of changes without end.

The Arrest and temporary Reservation of Force.

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 nat ural 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. By the power of these rays-including the calorific, the luminous, and the actinic-the natural affinities by which the above-mentioned substances were united are overcome, and they are formed into new combinations, in which they are united by very weak affinities. Of course, they have a strong tendency to break away from the new unions, and fall back into the old. But, by some mysterious and incomprehensible means, the sun has power to lock them, so to speak, in their new forms, so as to require a special condition of things for the releasing of them. Thus they form a reserve of force, which can be held in restraint until the conditions required for their release are realized.

The process can be illustrated more particularly by a single case. Water, one of the substances absorbed by plants, is composed of oxygen and hydrogen, which are united by an affinity of prodigious force. It is the same