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CHAPTER IV.

Table of Contents

THE SPECIES OF FORCE.

Forces may be most conveniently divided according to the nature of the particles or bodies in which they initiate and accelerate aggregative motion or resist and retard separative motion. Of these, there are four principal kinds known to us or conjectured by us. The first kind is the Mass or visible aggregation of particles, which admits of mechanical separation into minor masses. The second kind is the Molecule, or ultimate mechanical unit, which does not admit of subdivision, except by resolution into its chemical components. The third kind is the Atom, or ultimate chemical unit, which does not admit of subdivision by any known means, though it may perhaps be resoluble hereafter into some simpler and more primitive units. The fourth is the Electrical Unit,[3] whose nature is very inadequately known to us, but which must be considered for our present purpose as in some way the analogue of the others, though we have no sufficient warrant for giving it any material properties.

The Force which aggregates Masses and resists the separation of Masses is known as Gravitation. When any two Masses are left free to act upon one another without the counteracting influence of an Energy, they aggregate in obedience to this Power. When the cannon ball falls upon the earth, it is Gravitation which draws them together. When an aërolite comes within the circle of the earth’s attraction, it is Gravitation which makes them leap towards one another. If the moon were to lose its orbital Energy, Gravitation would pull it to the earth; and if our planet in her turn were suddenly checked in her course, Gravitation would cause her to plunge into the sun, while the sun in return would make a slight bound to meet her. Again, when any two Masses are in a state of aggregation, the Force of Gravitation resists any attempt to sever them. If the cannon ball lies upon the ground, it cannot be raised without an expenditure of Energy, and the amount of the Energy required to lift it to a given height (or distance from the surface of the earth) is the measure of the resistance offered by Gravitation. Similarly, when the Masses are not in actual contact owing to the existence of an Energy which keeps them apart, as in the case of the earth and her satellite, or the sun and the planets, Gravitation resists any attempt to sever them beyond their actual distances. It would be impossible to remove the moon a hundred miles from the earth, or the earth a hundred miles from the sun, except by the employment of an adequate Energy; and, as in the simpler case, the amount of Energy required would be the measure of resistance offered by Gravitation.

The Force which aggregates Molecules and resists the separation of Molecules is known as Cohesion. When any two Molecules are left free to act upon one another without the counteracting influence of an Energy, they aggregate in obedience to this Power. But the cases are much more difficult to illustrate than those of gravitation, because while masses attract one another powerfully at very conspicuous distances, Molecules (practically speaking) only attract one another at infinitesimal distances. The difference, however, which is purely relative, may thus be illustrated and explained. An aërolite is not drawn on to the earth unless it approaches the earth very closely, because otherwise the earth’s attraction, though causing a deviation in its course, does not suffice to overcome the aërolite’s energy and the combined attractions of surrounding bodies. But if it be near enough to be more powerful than all of them put together,[4] the aërolite either circles round the earth as a satellite or even falls at once upon its surface. Similarly with Cohesion. If two pieces of uneven iron be laid upon one another, the molecules do not approach near enough to exert any conspicuous mutual influence: but if the two pieces be planed to an absolute smoothness, so that the several molecules can come within the sphere of their mutual attraction, they will cohere perfectly, and it will be impossible to tear them asunder. Again, in other cases, Cohesion can only be effected by such a molecular motion (or heat) as will cause the Molecules to approach one another closer than they can be induced to do by mechanical means: just as an aërolite which would not under ordinary circumstances come (practically speaking) within the sphere of the earth’s attraction, might do so if it were given an oscillating motion from side to side, so as to cross or closely approach some portion of the earth’s orbit. Thus, two pieces of iron, if heated, will cohere with one another. Furthermore, the molecular motion inherent in the liquid form is often sufficient for this purpose: thus, two masses of dough, which will not cohere in the dry condition, can be made to do so by the addition of moisture. In the practice of gumming and glueing, we make use of this device in everyday life. A further account of these phenomena will be given in the chapter on Liberating Energies. The second property of Cohesion, that of resisting the separation of Molecules actually aggregated, is much more familiar to us. If two Molecules or bodies of Molecules are in an aggregated condition—that is, are not rendered plastic or liquid or gaseous by some form of Energy—we cannot separate them without a considerable expenditure of Energy. The Energy may be in the form of a mechanical action, as when we tear or break a cohering substance; or of heat, as when we melt lead; or of the contained motion of liquids, as when we dissolve a lump of sugar. But in any case Energy must be expended to counteract the aggregative Force of Cohesion in solid bodies.

A qualification must be added to prevent misconception. The cohering Molecules need not be supposed to be in actual physical contact with one another. It is sufficient that they should be within the sphere of one another’s attraction; just as the moon is kept in its place by the earth, and the planets by the sun, in spite of the intervening space. Theoretically, of course, every body in the universe attracts every other; but as the attraction decreases as the squares of the distance, at practically infinite distances it becomes practically infinitesimal and can be overcome by an infinitesimal Energy. This is the case ordinarily with Cohesion: at very slight distances its Force is so diminished that only an imperceptible amount of Energy is required to counteract it. But there is no reason to doubt that when the two rough pieces of iron are laid upon one another, the supporting points, so to speak, come within the sphere of mutual attraction, though their number and area are so small that we cannot perceive the resistance resulting from their Cohesion when we separate the pieces. In short, Cohesion always tends to act between all Molecules, but its effects may be disguised either by distance or by counteracting Energies. Other cases will be treated in the chapter on Mutual Interference of Forces. Adhesion and Capillarity are only forms of Cohesion.

The Force which aggregates Atoms and resists the separation of Atoms is known as Chemical Affinity. As here employed it will be understood to mean not merely the Force which unites the Atoms of two or more elements into a compound molecule, but also the identical Force which unites two or more Atoms of the same element into a molecule such as that of ozone. When any two or more Atoms (or equivalents in combining proportions) are left free to act upon one another without the counteracting influence of an Energy, they aggregate in obedience to this Power. As in the case of cohesion, however, the Atoms must be brought into close contact with one another. When phosphorus is exposed to oxygen the aggregation is immediate. But in other cases a certain amount of molecular or Atomic motion is needed in order to bring the Atoms within the sphere of their mutual attractions. Thus heat is necessary to make carbon combine with oxygen, as in the ordinary phenomenon of combustion: while the more subtle motion of light suffices to effect a union between hydrogen and chlorine. But we may broadly assert that whenever free Atoms find themselves in the presence of a free Atom for which they have affinities (the proper proportions being of course supposed), and are brought within the sphere of their mutual attraction, the two Atoms or sets of Atoms aggregate under the influence of Chemical Attraction. Here, again, a qualification is needed. The above rule holds only for free Atoms. Just as a ball suspended by a rope from the ceiling does not fall to the ground, because the Force of cohesion outbalances the Force of gravitation, so, when two or more Atoms, united in stable combination, are brought into contact with other Atoms for which they have affinities less strong than those of their existing combination, they will not yield up their stronger to their weaker affinity. (See the subsequent chapter on Mutual Interference of Forces.) And again, just as the ball will break the rope, if gravitation outbalances cohesion; so, if the new affinities are stronger than the old ones, the Atoms will yield up their previous combination and enter into that to which they are most powerfully attracted. The second mode in which Chemical Affinity acts is in resisting the attempt to separate the component Atoms of a compound body. Setting aside for the present certain very abnormal cases in which ‘unstable’ bodies spontaneously decompose—cases which can only be explained at a very late stage of our exposition—all ordinary ’stable’ compounds require an Energy to separate their Atoms. Thus heat is needed to divide the Atoms of oxygen from those of iron in ferric oxide: while electricity is necessary to sever the Atoms of hydrogen from those of oxygen in water. This statement must be understood as applying only to the separation of free elements, not the formation of new compounds. Mere juxtaposition is sufficient to make certain compound bodies yield up their weaker affinities in the presence of stronger ones: but (with the special exception noted above, chiefly referring to organic compounds) an Energy is required to separate any compound into its component Atoms in a free state, without the aid of stronger antagonistic affinities.

The Force which aggregates Electrical Units and resists the separation of Electrical Units is known as Electrical Affinity. This Force is little understood, and can only be treated in a very symbolical manner. What few points can be formulated are briefly these. When Positive and Negative Electricities are left free to act within the sphere of their mutual attractions, they are aggregated by this Force, as in the discharge of a Leyden jar. In saying this, no implication of materiality is meant to be conveyed. In our present ignorance on the subject, Electrical Affinity must be placed in the same category as other Forces; though further researches will doubtless enable us to give a better account of its real nature. Similarly, an Energy is necessary to separate the Positive and Negative Electricities which subsist in combination in every material body. In the case of a glass rod or an electrical machine this Energy is that of mechanical motion: in certain other cases it is of thermal or chemical origin. These points will receive further consideration in the chapter on Electrical Phenomena.

A table will put in a clearer light the classification here adopted.

Forces or Aggregative Powers.
Molar	Molecular	Atomic	Electric
Gravitation	Cohesion	Chemical Affinity	Electrical Affinity