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must. Vinegar is altered wine; it has an acid smell and taste, and has lost its spirituous flavour, as well as its exhilarating properties, its tendency being rather cooling and sedative. Search must be made in the air for the oil and wood which have disappeared during combustion; both these substances are converted into vapour or gas, and heat and light are thereupon evolved with the phenomenon of fire. Of a similar nature are the changes which animal and vegetable substances undergo if kept for a sufficient length of time; they are gradually converted, as they putrefy or decay, into various kinds of gas, some of which emit a very disagreeable odour.

Such processes, by which the weight, form, solidity, colour, taste, smell, and action of the substances become changed, so that new bodies with quite different properties are formed from the old, are called chemical processes, or chemical action.

Wherever we look upon our earth, chemical action is seen taking place, on the land, in the air, or in the depths of the sea. The hard basalt, the glass-like lava, become gradually soft, their dark colour passes into lighter, they crumble to smaller and smaller pieces, and are finally changed to earth. A potato placed in the earth grows soft, loses its mealy taste, becomes sweet and finally decays. The bud, that sends forth a sickly pale shoot in a dark cellar, when exposed to the light and air grows up a vigorous, firm, and green plant, which, imbibing its nourishment from the moist air and soil, forms from their elements new bodies, not to be found previously in the water or the air. A delicate network of cells and tubes pervades the whole plant, imparting to it firmness; these we call vegetable tissue, or woody fibre. We find in the sap, which passes up and down through these cells, albumin and other viscous substances; in the leaves and in the stalks, a green colouring matter-chlorophyll; and in the ripe tubers, a mealy substance-starch. None of these substances are injurious to health; but if the potatoes grow in the dark and without soil, for instance, in the cellar, there is produced in their long pale shoots a very poisonous body -solanine.

The potato forms one of our most important articles of food. The starch contained in it is not soluble in water, but when received into the stomach, quickly undergoes such a

change that it can be dissolved or digested, and then introduced as a liquid into the blood. The blood comes in contact in the lungs with the inhaled air; the blood changes its colour, the air changes its constitution, and the heat which we feel in our bodies is developed. We must conclude, from these changes, that chemical action is going on in our own bodies.

If a piece of iron be heated to redness, till a thick crust of scales is formed around it, and then weighed, it will be found to have increased in weight; consequently, it must have been supplied with something ponderable from the air. This ponderable substance is a species of gas, called oxygen; by its union with the iron it has become fixed, yet by other chemical processes it can be reconverted into its gaseous form. If this crust of iron is now exposed for a time to moist air, it will gradually become rust, and again weigh more than before; it has attracted and united to itself water, and more oxygen from the air. Accordingly, the crust consists of iron and oxygen; the rust, of iron, oxygen, and water, which have become most closely united with each other; they are chemically combined.

The force which produces such changes as these is called CHEMICAL FORCE. In the older works on chemistry it is described as 66 chemical affinity," because it is assumed that the tendency of substances to combine with one another is due to a kind of liking that they have for each other. Iron is so very fond of oxygen that it cannot help combining with it whenever it gets the chance. But it is unsafe in scientific matters to use names which assert more than we really know. Now we know that iron combines with oxygen, but we do not know that there is any "affinity" between the two. We know that force is exerted, but we do not know why. The conditions under which the force is exerted will be discussed in a future chapter.

Relation of the Forces to one another.-The forces we have been considering, unlike as they are in their manifestations, are very closely connected together. Almost any one of them may, under suitable circumstances, be made to produce another, and it has during the last thirty years been shown that the one is directly evolved from the other. It has long been perceived to be impossible, under the present

arrangement of the universe, for matter to be created or destroyed. It may change its form and pass ever so many times from one state of combination to another; but the total quantity remains always, as far as our means of observation enable us to judge, the same. The grandest generalization of modern science, a generalization which we may be proud to remember was born within our own day, is that which has taught us that force is equally indestructible; that it also, like matter, may undergo various and countless changes; may appear, now as heat, now as electricity, now as chemical action, and now as visible motion, and yet its sum will always remain fixed and unalterable. There is no new creation of force within the limits of our knowledge. It may lie hid for a time-for any time-but force is force for ever, and whenever we meet with any of its operations in nature we may confidently look for its cause in the disappearance of some previously active form of energy. The birth of one form of force is always the death of some other, and the two are equivalent to one another in quantity.

This grand doctrine, which is known as the doctrine of the conservation of force, requires complex apparatus, and great knowledge for its verification. The demonstration of it is indeed still incomplete, though it has gone far enough to satisfy every rational mind; but a few simple experiments will serve to illustrate the manner in which one force may give rise to others. In other words, we may prove the correlation of various forces, though the conservation of force must be taken on trust.

Experiment 1.-Rub a piece of copper wire briskly with sand-paper for a minute or two. It will become so hot that the hand cannot bear to touch it, and it will readily ignite phosphorus or the tip of a lucifer-match.

Similar illustrations will occur to all. When the axle of a railway carriage gets dry, so much heat is produced that the carriage is sometimes set on fire. A clever blacksmith can hammer a nail till it is red-hot, and so emits light as well as heat. During the boring of cannon the shavings are too hot to be touched, and, lastly, some savages are able to procure a light by rubbing two dry sticks together.

In all these cases there is a direct conversion of motion into heat, and even, in some cases, into light. Friction is

but arrested motion-motion hindered or reduced by an opposing force. If the sand-paper had not pressed on the wire, the same amount of force employed by the arms would have given rise to a much greater motion than that actually produced. The difference, though lost as motion, takes the new form of heat. When a pound weight falls to the earth from a height of 772 feet, its motion is lost, but the weight and the earth receive heat enough to raise the temperature of one pound of water one degree Fahrenheit.

We have already seen that friction may, under certain circumstances, produce electricity, so that we have gained illustrations of the conversion of motion into heat, light, and electricity. Let us now start from heat. The steam-engine affords the best possible illustration of the conversion of heat into motion. The work done by the engine is in direct proportion to the heat expended, and so ultimately to the coals burnt. When heat becomes sufficiently intense, it is always accompanied by light, and Tyndall has devised a direct experiment in which invisible heat becomes visible light. In fact, heat and light appear only to differ from one another as a low and high tone in music do. The conversion of heat into electricity requires for its demonstration a thermopile, a somewhat expensive piece of apparatus. It is doubtful whether heat is ever directly converted into chemical force, though the converse is one of the commonest of phenomena; but the liberation of the force of cohesion during the disappearance of heat can be readily demonstrated, as the following experiment will show.

Experiment 2.-Take a tumbler full of powdered crystals of sodium sulphate (Glauber's salt), and drench it with common hydrochloric acid (muriatic acid-spirit of salt). The salt will rapidly dissolve and become liquid, and the force of cohesion previously hidden or stored-up in the salt will become free and active. But at the same time heat will be taken so rapidly from the materials in the tumbler that the outside will become covered with hoar frost, and if a little water in a test tube be plunged into the mixture it will freeze in a minute or two. Most of the so-called freezing

mixtures depend upon this principle.

Experiment 3.-Now try the reverse of this experiment. Take a clean Florence oil flask, nearly fill it with hot water,

dissolve as much sodium sulphate in it as the water will dissolve (filtering the solution if it appears dirty) and then, while the solution is nearly boiling, cork the flask tightly and allow it to get quite cold. There is now a great deal more of the solid in the solution than cold water could dissolve, but for some curious reason the excess does not immediately separate out. But take out the cork, and in a minute beautiful feathery crystals appear at the upper surface and pass downwards until the whole mass is solid. If the crystallization does not begin at once, drop in a small crystal of the sulphate, which will immediately produce the desired effect. As soon as the liquid has changed to solid, feel the outside of the flask with your hand. It is sensibly warm, showing that during the exertion and disappearance of the cohesive force heat has been given out.

The conversion of electricity into other forms of force has already been illustrated to some extent in the battery. When the platinum becomes red-hot, it emits heat and light. When the poles of a powerful battery are connected with pieces of coke shaped like lead pencils, and the coke points after touching one another are separated by a short distance, a most intense light, called the "electric light," passes between them. The rays given out by it are similar to those of the sun, and, like the sun's rays, they consist of three kinds :

1. Heat rays.

2. Light rays.

3. Actinic or chemical rays.

The chemical rays are invisible, and are only to be recognised by their effect in producing chemical changes. The interesting art of photography entirely depends on the power of the rays to produce alterations of composition in certain chemical substances, and particularly the compounds of silver. The following experiment will illustrate this curious. property.

Experiment 4.-In a room from which daylight is entirely excluded, but which may be lighted by a candle, dissolve sixty grains of silver nitrate (lunar caustic) in an ounce of cold distilled water. Pin a sheet of smooth white paper on to a flat board, and, by means of a flat camel's hair brush, paint it all over uniformly with the colourless silver solution.

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