The exertion of force necessarily implies resistance, or reaction, in an opposite direction; and a force is effective only when the resistance is called into play. We are conscious of the ability to exert muscular force at our will; but we cannot conceive its exertion without opposition. So with a common bell-spring; we know from experience that it is capable of exerting the force of elasticity; but if we hold it by one end only, it is not exerted. If we hold one of its ends in each hand, and stretch it, we feel its elastic form, which we oppose by a muscular effort of one hand, necessarily accompanied by an equal effort, in the opposite direction, of the other; these efforts may be replaced by the gravity of a weight passing over a pulley at one end, balanced by an equal weight, similarly acting at the other end. A curious question might arise upon such an arrangement: Is the spring stretched by the sum of the forces of the two arms, or of the two weights, or by one only?

Let one end of the spring be attached to a fixed support, and it will be found that the same effort of one arm, or of one weight, applied to the other, will stretch it to the same amount as before; the force of the other arm, or of the other weight, only measured the amount of reaction which necessarily accompanies the action, and which is now borne by the support.

§14. There are several varieties of force, all of which may either mediately or immediately be referred to the standard of our own exertions. Some of these cause masses of matter to approach, and others to recede from each other, retaining them in their second position against an opposing force; the former are classed together under the name of ATTRACTION, the latter under that of REPULSION. The laws of these motions, and of the equilibrium of these forces, the intellect of man has been able to develope; but the origin of the forces themselves, though clearly perceived to be various, appears to be beyond his comprehension, even when that origin is in his own will. We cannot, at least, refer them to any secondary cause, and we must be content to know that they are powers conferred upon matter by the will of the CREATOR, for the maintenance of the order of his creation. Thus much the intellect of a child can comprehend, and to nothing more did that of a Newton attain. The effort which we feel that a mass of iron makes to fall to the ground, is due to an attractive force between it and the earth, which we denominate GRAVITY, and is an universal

1

power of nature; that which we feel the spring make to recover its first position when we press upon it with the hand, is due to the repulsive force of ELASTICITY, and is possessed in very different degrees by different kinds of matter.

Before we proceed to inquire into the laws which limit the action of these forces, it will be well to indicate certain other varieties both of repulsion and attraction, which will come within the scope of our inquiry.

§ 15. It was observed by the ancients, and the fact is mentioned by Theophrastus, that when a piece of amber is rubbed, it acquires the property of attracting light bodies. The Greek term for amber being electron, the unknown cause of this phenomenon has been called ELECTRICITY. It was not till the commencement of the seventeenth century, that the same property was found to be conferred, by friction, on various other bodies, as glass, sealing-wax, sulphur, &c.; and later still, that any two dissimilar substances, when rubbed together, with proper precautions, are capable of exerting the same attractive force. Further observation has shown, that after bodies have been thus attracted to the rubbed surface, they will be repelled from it by an equal but opposite force: and the phenomena will be presented to us in a striking light, if we suspend a feather from a piece of silk, and approximate to it a tube of glass, which has been rubbed with a silk handkerchief; the feather will be attracted to the tube from a considerable distance, and after attaching itself to it, will fly off, and cannot be made again to approach it. Both this attraction and repulsion will overcome the force of gravitation in the feather, and by its opposition we can estimate the amount of either.

§ 16. Again, there is an ore of iron which is common in Sweden, which has received the name of loadstone, from the property which it possesses of pointing, when freely suspended, to the polar star, or load-star, as it was formerly called; it has been known from remote antiquity to possess the property of attracting small pieces of iron: it is said to have abounded in Magnesia, a province of Lydia, and is mentioned by Pliny, under the name of magnes, hence the property derived the name of MAGNETISM. It is capable of being communicated to bars of steel by friction, which are also called magnets. If we take two bars so prepared, and balance one of them upon a

pivot, we may again have the opposite forces of attraction and repulsion exemplified from this new source; for upon approximating one end of one magnet to a certain end of the other, which is free to rotate, they will approach each other with considerable energy; if the other end be presented to it, they will separate with equal velocity. This attraction we can feel and estimate by the muscular force which we must exert to pull away a piece of iron which is under its influence. Artificial magnets, as well as the loadstone, when freely suspended, take up a definite position with regard to the poles of the earth, or the extremities of any other magnet: hence this property has been termed polarity; and forces which are conceived to act with equal intensity in opposite directions at the extremities of the axes of molecules, or of masses of matter, are denominated polar forces.

The forces which we have thus designated may be distinguished as EXTERNAL FORCES, for they act upon masses of matter at sensible distances; there are others which act only upon its constituent molecules, and at insensible distances, and may be called INTERNAL OF MOLECULAR FORCES.

§ 17. The first, and most general of these, is homogeneous attraction, or cohesion. If we take two masses of lead, on which two perfectly smooth surfaces have been cut,—and a cloven bullet will answer the purposes of illustration,—and bring these planes in contact, we shall find that they cohere, or attract each other with a force which we can appreciate by a muscular effort to separate them; or by opposing to them the gravitation or weight of another body, for we may suspend a heavy substance to them without forcing them asunder. In the same way, two pieces of plate-glass cohere strongly together when brought into contact, by their polished faces. It has often happened in manufactories of this article, that the plates, which after receiving their last polish are placed in cases, where they are arranged like books in a book-case, a little inclining upon each other, have been found to have contracted so strong a cohesion, as not to be separable without a fracture; and sometimes three or four plates have thus become so incorporated as to bear working together, and cutting with a diamond, like a single plate. Some specimens, which were selected from the manufacture of St. Gobin, were found to be thus united no less intimately than if they had been fused together; and when

a violent degree of force was applied to make them slide from each other, the experimenter was surprised to find that the substance of the glass was actually torn, so that the surface of one was covered with large flakes detached from the substance of the other.

Another example of this force is afforded by the running together of two globules of mercury, or two drops of water; the moment they appear to touch each other they coalesce, and become one. One of the most striking illustrations of homogeneous attraction is afforded by sifting some finely powdered resin upon a plate, and carefully scattering a few drops of water upon it; each of these will assume an almost perfect globular form, from the equal action of this force, in all directions from the centre upon the exterior film; in obedience to which the particles of each mass are free to move: these small globes may be made to roll about in all directions, like solid shot, by inclining the plate. When made gently to touch each other, they still continue separate, the liquid particles being defended from actual contact by the light dust with which they are powdered; but when by greater violence this is dispered, they immediately run together.

Some measure of the activity of this force in liquids is afforded by the contraction of the thin film which constitutes a soap-bubble. If such a bubble be blown with a pipe, and the open end of the tube be directed towards the flame of a taper, the flame may be blown out from the reaction of this contraction upon the air.

This is the force which connects the particles of bodies together in the solid form with greater or less energy, and which still prevails in liquids, though reduced to a less amount by an opposing force which we must presently indicate. We have seen from our experimental illustrations, how gravity, or muscular force, may be made to act in opposition to it, and the two may be combined to such a degree of intensity by the continuous action, for instance, of the sledge-hammer, that the cohesion of the hardest solid may be obliged to yield.

§ 18. And here it may be remarked, that time is an important element in the action of force. When a mass of matter moving with a certain momentum strikes against a substance whose cohesion is insufficient to resist the force which it conveys, the fracture of the latter is the consequence, and the momentum

of the former may be destroyed; but if a greater velocity be given to the mass, it may pass through the opposing substance with little comparative injury, and still continue to move. Thus a leaden bullet projected from the hand may demolish a plate of glass, while the same bullet impelled from a gun will pass through the plate, and pierce it with a small hole, without destroying the cohesion in any other part. In the practice of artillery, where it is required to batter down walls, or the sides of a ship, the charge of gunpowder is so regulated as that the projectile may not be carried through the object aimed at; as in that case the time of its passage is not sufficient for the transfer of the force from the moving body.

§ 19. Elasticity also may be made to act against cohesion; and a most beautiful instance of the equilibrium of these two forces is presented to us in the common schoolboy's plaything, called a Rupert's drop. This is a drop of glass suddenly solidified by dropping it into water: its cohesion is very considerable, as we may ascertain by the force which would be required to crush it; it will bear the pressure of half a hundred weight without injury. Glass is also a body endued with a high degree of elasticity, which is greatly increased by heat. When the exterior molecules are suddenly cooled they contract, and violently compress the still heated interior particles. That these retain their state of tension, and strive energetically to recover from it, even after they are cool, is proved experimentally. If we grasp the drop in one hand, and with the other break off the point, or merely scratch the surface, we diminish the force of cohesion, and the elasticity assumes the ascendency, breaking the mass to an almost impalpable powder, and giving a violent shock to the hand which grasps it. By holding it under water, in a stout glass bottle, or other vessel, the effect is rendered still more striking; for upon scratching the surface with a file, not only is the drop, but the containing vessel, shivered into a thousand fragments.

§ 20. But the real, and, but for one remarkable exception, universal antagonist of cohesion, is the repulsive force of HEAT: and of the meaning, again, of this term, our consciousness will afford the best explanation. The sensations of the most ignorant teach them to apply a meaning to the terms hot and cold, which the most philosophic definition would not

C