Part 6

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.

The cohesive power of water is beautifully illustrated. Have a small barrel or bucket so constructed as to be fitted with gauze at the top; immerse it exactly, so that the water may form a film between the meshes, and then open the tap at the bottom: the water will not flow till the meshes at the top are broken by blowing on their surface. The adhesion of the particles in a soap-bubble is another illustration, no less beautiful, as well as more familiar; for the soap, which might be supposed to be the cause of the phenomenon, serves only to prevent the intrusion of dust between the particles, but by no means to intensify their attractive power.

There are some liquids the adhesiveness of whose particles is so perfect as to bar out the access of air when we strew them on the surface of other liquids; and on the Continent it is not uncommon to protect wines against the action of the atmosphere by, instead of corking the bottle, simply pouring in a few drops of oil, which, being lighter than the wine, floats on the surface. It is parallel to the instance of the barrel with the gauze-wire top mentioned above, that if we loosely plug a bottle full of liquid with a piece of cotton-wool, and invert it, the particles in contact with the wool will cohere so closely that the fluid will not be able to escape. The adhesiveness of the particles of water to a solid surface can be exemplified by allowing one of the scales of a balance to float in water and leaving the other free; the one in contact with the water will refuse to yield after we have placed even a tolerable weight in the other which is suspended in the air.

The power of cohesion is more rigorous in some bodies than others. In some cases the body will rupture if it is interfered with ever so little; in others, the particles admit of a certain displacement, and if the limits are not transgressed, they return to their original position when the compressing or distending cause is removed. This rallying power in the cohesive force is called Elasticity, and it exists in no small degree in glass. The spaces between the particles can, within limits, be either lessened by compression or increased by distension, and the particles retain their power of recovering and maintaining the relation they stood in before they were disturbed. It is the power of cohesion or aggregation which resists any disturbance among the particles, and which restores order among them when once disturbance has taken place. And not only does nature resist directly any undue interference with the cohering force, but tampering with it even slightly has often a certain deteriorating effect upon the physical properties of bodies. A bell, for instance, loses its tone when heated, because by that means its particles are disturbed; though it recovers its tone-power as it cools, and as the particles return to their places.

In organic bodies, both during growth and decay, the particles are more or less in flux; but in feathers, after their formation, the attraction of aggregation remains constant, and by means of it their particles continue fixed in their places, not only with the life of the bird, but long after. Nay, you may even crumple them up, and toss them away as worthless, and yet if you expose them to the vapour of steam, they will not only recover their form, but they can be made to look as beautiful as ever.

Chemical Affinity.--The attraction of the particles of bodies of different kinds to each other is often striking and curious; as, for instance, those of salt to those of water. The salt attracts the water, and the water the salt, till at last, if there is a sufficient quantity of water, all the salt is attracted particle by particle from itself, and taken up and united to the water. The salt is no longer visible to the eye, and is said to be dissolved or in solution; but this change of form is due to its affinity for the water, and the resulting attraction of the one to the other. The same phenomena are observed, and they are due to the same cause, in other solutions; as when we infuse our tea or sweeten it with sugar. The attraction of water, or one of its elements rather, for other substances, sometimes shows itself in vehement forms. When a piece of potassium, for example, is thrown into a vessel of water, its attraction for the water is such, and of the water for it, that it instantly takes fire, and the two blaze away, particle violently seizing on particle until the elements of the water unite part for part with the metal. It is the mutually attractive force that causes the heat and flame which accompany the combination; and this force is most violently active in the union of dissimilar substances. Unions of a quieter kind, though not less thorough, occur even between solids when placed in contact. For instance, sulphate of soda and sulphate of ammonia, when placed side by side, will diliquesce, and in liquid form unite into a new combination. Sulphuric acid, when we mix it with water, generates great heat; and this is due to its attraction for the water. Sometimes two fluids unite together, and, in doing so, pass from the liquid into the solid form; as, e.g., sulphuric acid and chloride of calcium. Attraction of this nature is called chemical: it takes effect between dissimilar particles, and results in combinations with new properties. It operates not only between solid and solid, solid and liquid, and liquid and liquid, but between these and gases, and gases with one another; and these as well as those combine into new substances, and evince in the act not a little violent commotion. Thus, phosphorus catches fire in the atmosphere at a temperature of 140 degrees, and it goes on rapidly combining with the oxygen, burning with a dazzling white light, and producing phosphoric acid. Indeed, most metals have an affinity for the oxygen in the air, and oxydise in it with more or less facility; and a metal, as such, has more value than another according as it has less affinity for that element, and is less liable to oxydise or rust in it. This is one reason, among others, why gold is the most precious metal, and the conventional representative of highest worth in things.

There are some metals, such as lead, for instance, which oxydise readily, but this process stops short at the surface in contact with the air, and so forms a coating which prevents the metal from further oxydation; so that here, as in so many things else, strength is connected with weakness.

Electricity.--This, in the most elementary view of it, is a more or less attractive or repellant force latent in bodies, and which is capable of being roused into action by the application of friction. It is excited in a rod of glass by rubbing it with silk, and in a piece of sealing-wax by rubbing it with flannel, though the effect is different when we apply first the one and then the other to the same body. Thus, e.g., if we apply the excited sealing-wax to a paper ring, or a pith-ball, hung by a silk thread from a horizontal glass rod, it will, after contact, repel it; and if, thereafter, we apply to it the excited glass rod, it will attract it; or if we first apply the excited glass rod to the paper ring, or pith-ball, it will, after contact, repel it; and if thereafter we apply to it the excited sealing-wax, it will attract it. The reason is, that when we once charge a body by contact with either kind, it repels that kind, and attracts the opposite; if we charge it from the glass, i.e., with vitreous electricity, it refuses to have more, and is attracted to the sealing-wax; and if we charge it from the sealing-wax, i.e., with resinous electricity, it refuses to have more, and is attracted to the glass-rod; only it is to be observed that, till the body is charged by either, it has an equal attraction for both. From all which it appears that kindred electricities repel, and opposite attract, each other.

Two pieces of gold leaf suspended from a metal rod, inserted at the top of a glass shade full of perfectly pure, dry air, will separate if we rub our foot on the carpet, and touch the top of the rod with one of our fingers; for the motion of the body, as in walking, always excites electricity, and it is this which, as it passes through the finger, causes the phenomenon; though the least sensation of damp in the glass would, by instantly draining off the electricity, defeat the experiment. What happens in this case is, that one kind of electricity passes from the finger to the leaves, while another kind, to make room for it, passes from the leaf to the finger; and the leaves separate because they are both more or less charged with the same kind of electricity, and kindred electricities repel each other. Ribbons, particularly of white silk, when well washed, are similarly susceptible of electrical excitation; and they behave very much as the gold leaf does when they are rubbed sharply through a piece of flannel. Gutta-percha is another substance which, when similarly treated, is similarly affected.

This power is a very mysterious one, and of a nature to perplex even the philosophic observer. Certain bodies, such as the metals, convey it, and are called conductors; certain others, such as glass and porcelain, arrest it, and are called insulators. It is for this reason that the wires of the telegraph are supported by a non-conductor, for if not, the electric current would pass into the earth by the first post and never reach its final destination. Glass being an insulator, it was found that, if a glass bottle was filled with water, and then corked up with a cork, through which a nail was passed so that the top of it touched the water, it would receive and retain a charge as long as it was held in the hand; and this observation led to an invention of some account in the subsequent applications of electricity, known, from the place of its conception, as the Leyden jar. This is a glass jar, the inside of which is coated with tinfoil, and the outside as far as the neck, and into which, so as to touch the inside coating, a brass rod with a knob at the top is inserted through a cork, which closes its mouth. By means of this, in consequence of the isolation of the coatings by the glass, electricity can, in a dry atmosphere, be condensed, and stored up and husbanded till wanted.

A series of eggs, arranged in contact and in line, give occasion to a pretty experiment. In consequence of the shells being non-conductors, and the inside conducting, it happens that a current of electricity, applied to the first of the series, will pass from one to another in a succession of crackling sparks, in this way forcing itself through the obstructing walls. This effect of electricity in making its way through non-conducting obstructions accounts for the explosion which ensues when a current of it comes in contact with a quantity of gunpowder; as it also does for the fatal consequences which result when, on its way from the atmosphere to the earth, it rushes athwart any resisting organic or inorganic body.

Magnetism.--Unlike electricity, which acts with a shock and then expires, magnetism is a constant quantity, and constant in its action; and it has this singular property, that it can impart itself as a permanent force to bodies previously without it. Thus, there being natural magnets and artificial, we can, by passing a piece of steel over a magnet, turn it into a strong magnet itself; although we can also, when it is in the form of a horse-shoe, by a half turn round and then rubbing it on the magnet, take away what it has acquired, and bring it back to its original state. The magnetic property is very readily imparted (by induction, as it is called) to soft iron, but when the iron is removed from the magnetising body, it parts with the virtue as fast as it acquired it. To obtain a substance that will retain the power induced, we must make some other election; and hard steel is most serviceable for conversion into a permanent magnet.

The properties of the magnet are best observed in magnetised steel; and when we proceed to test its magnetic power, it will be found that it is most active at the extremities of the bar, which are hence called its poles, and hardly, if at all, at the centre; that while both poles attract certain substances and repel others, the one always points nearly north and the other nearly south when the bar is horizontally suspended; and that, when we break the bar into two or any number of pieces, however small, each part forms into a complete magnet with its virtue active at the poles, which, when suspended, preserves its original direction; so that of two particles one is, in that case, always north of the other; nay, it is probable that each of these has its north pole and its south, as constant as those of the earth itself, which, too, is a large magnet.

The magnet acts through media and at a distance, as well as in contact; and it has an especial attraction for iron, the more so when the conducting medium is solid, such as a table; and so when the magnet is horizontally suspended, or poised, in the vicinity of iron, its tendency to point north and south is seriously disturbed. The disturbance of the bar, or needle, in such a case, is called its deflection; and it is corrected by so placing a piece of soft iron or another magnet in its neighbourhood as to neutralise the effect, and leave said bar, or needle, free to obey the magnetism of the earth. The needle, it is to be remarked, does not point due north and south, neither, when poised freely on its centre, does it lie perfectly horizontal; in our latitude it points at present 20° west of north, which is called its declination, and its north pole slopes downwards at an angle of 68°, which is called its dip.

By holding a rod of iron, or a poker, for a length of time parallel to the direction of the needle, so as to have the same declination and the same dip, it will gradually assume and display magnetic virtue, and this will ere long become fixed and powerful under a succession of vibratory shocks. There is a beautiful experiment in which a needle, when magnetised, can be made to float on water, when it adjusts itself to the magnetic meridian, and will incline north and south the same as the needle of the compass.

The Chemical Action of Electricity and Magnetism.--These agents possess powers which develop wonderfully in connection with chemical combination. Thus, if we suspend a piece of iron in a vessel which contains oxygen gas, and apply to the metal an electric current, it will immediately begin to unite rapidly, and form an oxide with oxygen, emitting, during the process, intense heat and a bright flame. Zinc, too, when similarly acted on, will ignite in the common atmosphere and burn away, though with less intensity, till it also is, under the electric force, reduced to an oxide. It is presumed that many other chemical combinations take place because of the simultaneous joint development of electric agencies, as in copper, water, and aquafortis, nitrate of copper, &c. So also it happens that, when a plate of iron is for some time immersed in a copper solution, it comes out at length covered over with a coating of copper. And it is because there is electricity at work that a silver basin will be coated with copper when we pour into it a copper solution, and at the same time place in it a rod of zinc, so that it rests on the side and bottom, though no coating will form at all when there is no rod present to excite the electric current. The same phenomena will appear if we deposit a silver coin in the solution in question: the coin will come out unaffected, unless we excite affinity by means of a rod of iron. It is under the action of an electric current that one metal is coated with another. The metal, copper say, is steeped in a solution of the coating substance, and connected by means of wires with a galvanic battery, under the action of which the metal in solution unites with the surface of the plate immersed in it. Heat also is developed under magnetic influence, and that often of great intensity. Thus, if we connect the poles of a voltaic battery by means of a platinum wire, heat will develop to such a degree that the platinum will almost instantaneously become red hot and emit a bright light, and that along a wire of some considerable length. A similar effect is noticeable when we substitute other metals, such as silver or iron, for platinum. And the electric light, which flashes out rays of sunlike brilliance, is the result of placing a piece of compact charcoal between the separated but confronting poles of a powerful galvanic battery, light, developing more at the one pole and heat more at the other of the incandescent substance.

Kindred, though much milder, results will show themselves under simpler, though similar, contrivances. A flounder will jump and jerk about uneasily if we lay it upon a piece of tinfoil and place over it a thin plate of zinc, and then connect the two with a bent metal rod; which will happen to an eel also, if we expose it to a gentle current from a battery.

By means of electric or magnetic action we can separate bodies chemically combined, as well as unite them into chemical compounds; as will appear if we place a piece of blotting paper upon tinfoil, and this upon wool; if we then spread above these two pieces of test-paper, litmus and turmeric, the one the test of acids, and the other of alkalis, and saturate both with Glauber salt (which is by itself neither an acid nor an alkali, but a combination of the two), and, finally, connect each by means of a piece of zinc with the poles of a battery, the test-papers will immediately change colour, as they do the one in the presence of an acid simply, and the other of an alkali simply, but never in a compound where these are neutralised; thus proving that the compound has in this case been decomposed, and its elements disintegrated one from another.

A very powerful magnet can be produced by coiling a wire round a bar of soft iron, and attaching its extremities to the poles of a galvanic battery, when it will be found that its strength will be proportioned to the strength of the current and the turns of the coil. This is especially the case when the bar is bent into the form of a horse-shoe, and the wires are insulated and coiled round its limbs. The force communicated to a magnet of this kind, which is often immense, is the product of the chemical action which goes on in the battery, and, in a certain sense, the measure of it. How great that is we may judge when we consider that, evanescent as it is in itself, it has imparted a virtue which is both powerful and constant, and ever at our service.

Summary.--Thus, then, on a review of the whole, we find all things are endowed with attractive power, and that there is no particle which is not directly or indirectly related, in manifold ways, to the other particles of the universe. There is, first, the universal attraction of gravitation, under which every particle is, by a fixed law, drawn to every other within the sphere of existence. There is, secondly, the attraction of cohesion or aggregation, which acts at short distances, and unites the otherwise loose atoms of bodies into coherent masses. There is, thirdly, the power by which elements of different kinds combine into compounds with new and useful qualities, known by the name of chemical affinity. And, lastly, related to the action of affinity, aiding in it and resulting from it, there are those strange negative and positive, attractive and repellant polar forces which appear in the phenomena of electricity and magnetism, agencies of such potency and universal avail in modern civilisation.