Lectures on popular and scientific subjects

by 14th earl of James Sinclair Caithness

It is to be especially noticed that attraction is no lopsided affair; that it is mutual; that, while the larger body attracts the less, the less also attracts and moves the larger in proportion; and that, indeed, every body and every particle attracts every other, far as well as near, to the utmost verge of the universe of matter. Under it the moon maintains its place with reference to the earth, the planets with reference to the sun, and the solar system with reference to the stellar. As for the moon, it maintains its orbit and revolves round the earth under the action of two forces, the one akin to that by which a ball is projected from the mouth of a cannon, and the other the attraction of the earth, which, by its constant and equal operation, bends its otherwise rectilineal track into a circular one, as we might show if we could only project a ball with such a force as exactly to balance the power of gravity, so that it would at no point in its course be drawn nearer the earth than at starting.

That the force we are considering pervades the solar system is demonstrable, for it is on the supposition of it and the laws it is known to obey that all the calculations of astronomy—and they never miscarry—are grounded; and it is by noticing disturbances in the otherwise regular movements of certain planets that astronomers have been led more than once to infer and discover the presence of some hitherto unknown body in the neighbourhood. It was actually thus the planet Neptune was discovered in 1846. Certain irregularities had been observed in the movements of Uranus, which could not be accounted for by the influence of any other bodies known to be near it; and these irregularities, being carefully watched and studied, gradually led more than one astronomer first to the whereabouts, and then to the vision of the disturbing planet.

Notwithstanding what we said about the universality of this force, and how it affects all forms of matter, it may still appear as if the air were an exception. But it is not so; the air also gravitates. The fact that it gravitates is proved in various ways. First, if it did not, it would not accompany the earth in its movements round the sun; the earth would sweep along into space, and leave it behind it. Secondly, if we place a bottle from which the air is exhausted in a balance and exactly poise it with a counter-weight, and then open it and let in the air, it will show at once that the air has weight or gravitates by immediately descending. Thirdly, if we extend a piece of india-rubber over the end of a vessel and begin to withdraw the air from it, we shall see the india-rubber sink in, under the pressure of the air outside, to fill up the space left vacant by the removal of the included air. The fact that air gravitates we have already taken for granted in explaining the ascent of a balloon; and the proofs now given are enough to show that the cause assumed is a real one. The lighter gas rises and the heavier sinks by law of gravitation.

Gravitation and Cohesion.—Unlike the attraction of aggregation, or cohesion, which acts only between particles separated from each other by spaces that are imperceptible, gravitation takes effect at distances which transcend conception, but it diminishes in force as the distance increases. The law according to which it does so is expressed thus; its intensity decreases with the square of the distance; that is to say, at twice the original distance it is 1-4th; at thrice, 1-9th; at four times, 1-16th, for 4, 9, 16 are the squares respectively of 2, 3, and 4. To take an instance, a ball which weighs 144 lb. at the surface of the earth will weigh 1-4th of that, or 36 lb., when it is twice as far from the centre as it is at the surface; and 1-9th, or 16 lb. when it is thrice as far; and 1-16th, or 9 lb. when it is four times as far. The attraction of cohesion, on the other hand, as we say, acts only when the particles seem almost in contact, and it ceases altogether when once, by mechanical or other means, the bond is broken, in consequence of the particles being forced too near, or sundered too far from, one another.

One distinguishing difference between the attraction of gravitation and that of cohesion is, that whereas the former is uniform, the latter is variable; that is, under gravitation the attraction of any one particle to any other is the same, but under cohesion, some sets of particles are more forcibly drawn together than others. For instance, a particle of iron and a particle of cork gravitate equally, but particles of iron and particles of cork among themselves do not cohere equally. And it is just because those of the former cohere more than those of the latter, that a piece of iron feels harder and weighs heavier than a piece of cork.

Further, the attraction of gravitation is unaffected by change in the condition of bodies, while that of cohesion is. It makes nothing to gravitation whether a piece of metal is as cold as ice, or heated with a sevenfold heat. Not so to the power of cohesion; withdraw heat, and the particles under cohesion cling closer; add it, and both the spaces grow wider and the attraction feebler. Thus, for example, you may suspend a weight by a piece of copper-wire, and the wire not break. But apply heat to the wire, and its cohesion will be lessened; the force of gravitation will overpower it, rupture the wire, and cause the weight to fall.

Cohesion.—That the action of the attraction of cohesion depends on the contiguity of the particles in the cohering body, may be shown by an illustration. Take a ball of lead, divide it into two hemispheres, smooth the surfaces of section, then press them together, and you will find it requires some force to separate them; thus proving the dependence of cohesion on contiguity, although the effect in this case may be due in some degree to the pressure of the atmosphere as well as the power of cohesion.

Heat is the principal agent in inducing cohesion, as well as in relaxing its energy; for by means of it you can weld the hardest as well as the softest substances into one, and two pieces of iron together, no less than two pieces of wax. It is possible, indeed, by heat to unite two sufficient waxed corks to one another, so as to be able by means of the one to draw the other out of a bottle: such, in this case, is the force of cohesion induced by heat.

The power of cohesion exists between the particles of liquids as well as those of solids, the only difference being that in solids the particles are relatively fixed, while in liquids they move freely about one another, unless indeed when they are attracted to the surface of a solid—a fact we are familiar with when we dip our finger into a vessel of water. The cohesive power of liquids is overcome by heat as well as that of solids, only to a much greater degree, for under it they assume a new form, acquire new properties, and expand immensely in volume. They pass into the form of vapour, occupy a thousand times larger area, and possess an elasticity of compressibility and expansibility they were destitute of before.

There is a beautiful phenomenon which accompanies the expansion of ether under the influence of heat. Placed in a flask to which heat is applied, the ether will go off in vapour; and as the heat increases, the vapour will gradually light up into a lovely flame. The expansibility of air, which is vapour in a permanent form, can be shown by experiment. If we tie up an empty or collapsed bladder, and place it in a vessel over an air-pump, we may see, as we withdraw the air from the vessel, and so diminish its pressure, the bladder gradually expand and swell as it does under inflation.