PRACTICAL APPLICATIONS OF FORCE.

(1.) Introduction. When I want to speak to a man, I move my tongue, and so set the air in motion: the air in his ear is thus also set in motion, and by the action of this he is enabled to understand what I want to tell him. It is as real a motion, as real a blow upon his ear, as if I had thrown a stone at him, and the force exists as much after I have used it as it did before. The sound is not the end of the motion, but the accompaniment of it. Excess of sound frequently produces headache, as does excess of light, probably because the motion of the nerves is communicated to the brain in greater quantity, and more rapidly, than it can part with it, and it is therefore in a state of undue activity. Rest is usually the best cure for a headache-i.e., cessation of motion as far as possible, giving the brain time to get rid of its already too great amount.

(2.) Circulation of Force.-Whence do I derive the power to move my tongue when I speak? From my bodily strength. How is this recruited? By food. Whence the food? From the earth (for animal food is produced by vegetable), but not only. Light and heat are necessary for the growth of food. Whence come these? From the sun and now for the first time in the circle we get beyond the earth. Whence does the sun obtain the force which we call light and heat, and which it is continuously pouring upon us? This question it is quite out of our power to answer we usually talk of the sun as the origin of force; but it is not impossible to imagine that the force may be more or less returned to the sun from the planets. But this is a point far beyond the province of my book; and I only mention it to enable my readers to realise the circulation of force, from the sun to the earth, to be reproduced by its trees and grass, by these, used as food, to be placed within our power, not to increase or destroy, but to use (or waste) in our every action.

(3.) Media of Force.-I have a piece of iron to melt; I place

it in the fire, but the heat is insufficient. This means that I cannot accumulate upon the iron a sufficient amount of force, because the supply by the fire is at too small a rate; it radiates too rapidly to allow of a sufficient amount being collected. I use an oxygen and hydrogen blow-pipe, and I can succeed, because I can supply the force at a sufficient rate. It must be realised that heat is force, just as really as the blow of a blacksmith's hammer.

I want to melt some ordinary gum, which I can do by water; but I find that hot water will enable me to do this much more

readily than cold. Why? Because it contains more force. Whence did the water derive its force? From the fire. And the fire, whence obtained it the force to impart? From the combustion of the coals.

It is often said that force is stored up in coal, derived from the sunlight of past ages. And I once ventured to say to a wealthy farmer of a Yorkshire dale, that coal had been called "the sunlight of former ages, bottled for present use." The worthy farmer replied, that I, being a teacher, "ought to know better than to talk such nonsense.' I honour him for his frankness, and am not quite sure that he was totally wrong.

The point which I want to make clear is, that water is much used as a medium of force-that by means of it I can conveniently move force from one place to another. It is not the only medium, but it is one of the most practically useful.

(4.) The Steam-Engine. This is especially evident in what is called the steam-engine, which some writers now call the "heat engine." I want to move a train of carriages along a railway, and the driving-wheel of the engine seems the most suitable point to which to direct the action of the force I am to use. I might pull the wheel, or push it, or turn the axle. The last is evidently the best method. I attach to the axle a lever, so arranged that a small to-and-fro motion of the other end shall communicate a rotary motion to the axle. How shall I produce this to-and-fro motion? The method in use is to move a piston to and fro in a cylinder, and to connect the piston with the lever. To move the piston

steam is admitted first on one side and then on the other.

Steam is water of which the particles are urged apart by the force of heat. By cooling it may be reconverted into water. The weight of the train tends to keep the piston stationary; the force in the steam tends to move it. But the steam is enclosed in a metal cylinder, of which the piston is the only yielding part. The force cannot be destroyed, it must either move the piston or escape through the cylinder as heat. The piston is moved, the train also is moved, by the force of the heat in the steam. How was the force communicated to the water? By the burning of so much coal.

(5.) Combustion.-Coal burns: so does wood, paper, and in

COAL AS A MEDIUM OF FORCE.

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numerable other substances. What is burning? why does it give out heat? and why is coal the most convenient form of fuel? What we call burning is really chemical combination. If we burn coal, the carbon, which is its chief constituent, unites with the oxygen of the air, and forms carbonic gas. If we burn hydrogen, that also unites with the oxygen, and forms water, sometimes called the "enemy of combustion." The heat is probably derived from the force with which the atoms rush together when uniting. But whence is this derived? Why do they rush together and unite? The usual reply is, that they have a chemical affinity for each other; which, translated in plain language, means that they do so because they do so, and we call it "affinity" because we do not know how to explain it. It is often said that because the carbon in wood was derived from the decomposition of carbonic gas, the separation of oxygen and carbon set free the force which causes the reunion when combustion takes place. But the carbonic gas must have been obtained by some previous combination, unless we suppose the oxygen and carbon never previously to have existed separately.

(6.) Coal as a Medium of Force.-But why is coal the best form of fuel? A pound of hydrogen in burning will give out more than four times as much heat as a pound of carbon will; a pound of marsh gas will give out nearly twice as much, and a pound of olefiant gas nearly half as much again, as a pound of carbon. Also, a pound of phosphorus will give out nearly as much heat as carbon. Why then use coal so universally?

Because of its practical convenience. Coal is solid, and occupies but small space as compared with gas, to say nothing of the danger of explosion and the trouble of obtaining and conveying the gas. Phosphorus is also a solid, but ignites at a much lower temperature than coal, and is therefore much more dangerous. Coal is compact, easily procurable, does not ignite so easily as to be dangerous, nor with so much difficulty as to be troublesome.

(7.) Water as a Medium of Force. To revert to the steamengine, the one thing desired is to move the piston to and fro. The force which does this is derived from the combustion of coal; but we cannot light two fires, one each side of the piston, and make them play at ball with it. We require some medium to convey the force from the fire to the cylinder; a manageable medium which can be easily divided into small portions, and sent here or there as we desire. All this water gives us. It absorbs a large amount of force, which it conveys to the cylinder, and it can be divided and directed at will.

(8.) Electroplating.-We saw (p. 183), that a "Daniell's constant battery" derived its element of constancy from the use of a salt of the metal of which the negative plate was made. Thus

the negative plate being copper, a solution of sulphate of copper was used to excite the battery. We saw also that one result was the covering of the copper plate by a deposit of copper. The sulphate of copper was decomposed into its constituents of sulphuric acid, copper, and oxygen. The oxygen was used to replace other atoms of oxygen that combined with the zinc, and the copper was deposited upon the copper negative plate. That is, practically, the oxygen went to the positive pole of the battery, and the copper to the negative.

The chief purpose and advantage of this construction was the constancy of the current. In other batteries the action becomes, after a time, weakened by the coating of the negative plate with the hydrogen derived from the decomposition of water. But in the Daniell battery it is oxide of copper, not water, that is decomposed; and it is copper, not hydrogen, that goes to the negative plate. But this plate being itself copper is unaffected, except in size, by the copper deposited on it.

There were, however, results of another kind, quite unexpected, and which have proved of immense practical importance. It was found that the copper thus deposited was deposited with great regularity, and really formed another copper plate of uniform thickness throughout. This second plate could be detached from the first, upon which it was deposited. Upon being so detached it was found that the new plate, from having been built up, atom by atom, formed a mould or casting of the original plate. Wherever this had any slight projection or depression, its place was marked by a corresponding depression or projection.

It was soon seen that this power of obtaining an exact reproduction of a given surface might be utilised for the purpose of obtaining casts of medals, or other small bodies having raised surfaces. But the use of this battery extended beyond the reproduction of a reversed facsimile of a surface. The uniform thickness of the deposit made the outer surface of the new plate a second fac-simile, not reversed, of the surface of the original plate; so that the two plates might be left together to form one of greater thickness than the one, but preserving the irregularities of the original surface. This second surface is rough, but can easily be burnished.

The next extension of utility was when it was seen that the two plates need not be of the same metal. In the original battery of Daniell they were both of the same metal, because it was this which gave a continually pure surface of metal, and thus gave the continuous and constant action of the battery which is its especial distinction. But this is not necessary when the deposit is the end, and not a means, of the galvanic action.

This power of coating one metal with another and yet preserving its projections and depressions, however fine or intricate, soon suggested the idea of plating metal articles by this means. advantages are manifold: especially valuable was the power of

Its

easily regulating the thickness of the deposit and the deposition of an equal thickness on all parts of the surface, so that the depressions were as thickly coated as any other portion. If I place a thin plate of valuable metal over a thicker plate of commoner, and then engrave a pattern on it, I cut away the one plate only, leaving but a very thin substance beneath any deeply-cut part of the pattern; but by cutting out the pattern on the common metal, and then coating it by making it the negative plate of a battery in which another metal is being decomposed, I get a deposit of this second metal of the same thickness throughout.

In this way teapots, spoons, forks, and numberless other articles of domestic use, are made of a common metal coated with silver, of any degree of thickness, and presenting all the appearance of solid silver. It would be out of place here to describe the methods of this electro-plating otherwise than as they illustrate the principles of electricity.

It is necessary to attach a plate of some conducting substance to the positive as well as the one attached to the negative pole for the purpose of being covered. Why is this necessary? If I connect a wire of copper to each end of a battery, the second ends of these wires become the poles of the battery, and from the positive to the negative pole there passes a continual current of galvanic force. If these poles have termination of unequal size, this force will either diverge from the smaller to the larger, or converge from the larger upon the smaller, according as the positive or negative pole be the larger. In either case the force will be unequal in intensity. Thus, if I suspend a medal to the negative pole, while the positive pole terminates in a point, the current of force will tend to cross the medal in a line between the end of the connecting wire and the point nearest the opposite or positive wire. The deposit will thus be, comparatively, heaped on this line, and decrease in thickness towards the sides. But if I attach to the positive pole a plate of metal of a size corresponding to that of the medal at the negative pole, and bring the two surfaces face to face, within a short distance (say half an inch) of each other, each point of the positive plate will have a corresponding point in the negative plate, and thus the force will be generally and equally diffused over the whole surface.

Another point, of theoretic as well as practical importance, is that the substance to be copied, or coated, must either be a conducting substance or be artificially rendered such. Thus, if I desire to coat a wax model or a glass model, it will not be sufficient to suspend it by a copper wire from the negative pole, for the wax or glass being a non-conducting substance will break the electric circuit, and no force will be available for coating it. It is necessary to enclose the model in some conducting envelope, which shall take the shape of the model in all its nicety. This can be done by coating it with some finely powdered conductor (plumbago is generally used), so that every point is covered.