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and communicating heat to colder bodies brought into contact with it. But the withdrawal of these properties did not leave the magnesium as it was before heating ; accompanying the exhibition of these properties there was a gradual change of the magnesium into a substance so unlike magnesium as to be at once recognised as a different kind of matter.

If a thin copper wire, covered with cotton or silk, is 3 wrapped many times round a piece of soft-iron and an electric current is then passed through the wire, the iron will acquire the property of attracting iron-filings; if the electric current is stopped the iron-filings cease to be attracted ; if the current is again passed through the wire the soft-iron at once acquires the attractive power. Before, during, and after, the passage of the electric current, the soft-iron exhibits all those properties—colour, relative density, tenacity, malleability, texture, &c.—which mark it off from other kinds of matter; while the current is passing there is added to the iron the property of attracting iron-filings. If the same piece of soft-iron is exposed to damp air for a considerable time, a portion of it, or if sufficient time be given the whole of it, will be changed into iron-rust, which is a reddish powder unlike the iron in texture, colour, lustre, tenacity, malleability, and many other properties.

The change of iron to iron-rust resembles the change of magnesium to magnesia, in that in both cases there is produced a new kind of matter. The temporary addition to iron of the property of attracting iron-filings resembles the temporary addition to platinum of the property of glowing and communicating heat to colder bodies, in that in both cases the change consists in the addition of a property which does not destroy or mask the original properties, and which can be withdrawn by reverting to the conditions existent before this property was added. Let a rod of

and an electric bell be introduced into 4 the circuit of a galvanic battery (s. fig. 1); the ringing of the bell shews that the electric current is passing through the rod of copper. The moment the current is broken the bell ceases to ring. The characteristic properties of the copper are not modified by the passage through it of the electric current. Let the rod of copper be now removed; let a piece of sheetplatinum be attached to the end of the wire from the battery, and also to the end of the wire from the electric bell, and let these pieces of platinum be placed, side by side but not touching, in a dilute aqueous solution of copper sulphate to

copper

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Fig. 1. which some sulphuric acid has been added (s. fig. 2). The ringing of the bell shews that the electric current is passing through the solution of copper sulphate ; but the passage of the current is accompanied by the deposition on one of the

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platinum plates of a reddish solid which may be proved to be copper, and by the gradual disappearance of the copper sulphate from the solution. When the current is stopped there remains the new kind of matter, copper, which has been formed by the action of the electric current on the kind of matter originally present, copper sulphate; a certain amount of one kind of matter has disappeared and a certain amount of another kind of matter has been formed. The original matter is not reproduced by reverting to the conditions which existed before the change began; that is to say, by stopping the passage

of the electric current: The three kinds of matter, platinum, soft-iron, and copper,

5 have been changed by temporarily adding to each a property which does not belong to it under ordinary conditions; this property existed only so long as the special conditions which caused its existence continued; the withdrawal of these conditions was accompanied by the withdrawal of the special property; when this property was withdrawn the platinum, soft-iron, or copper, remained exactly as it was before the change had been effected. On the other hand, the three kinds of matter, magnesium, iron, and copper sulphate, have been changed by each permanently losing certain properties which characterise it, and at the same time permanently acquiring new properties which characterise other kinds of matter. Now we recognise different substances by their properties.

6 One kind of matter is recognised, and distinguished from other kinds of matter, by its colour, texture, brittleness, opacity, relative density, hardness, &c.; also by its behaviour when heated, when submitted to the action of electricity, placed in direct sunlight, mixed with water, brought into contact with other substances at high and low temperatures, &c. Substances which have markedly different properties are said to differ in kind, or to be different kinds of matter. Thus we say that iron is a kind of matter different from glass; that sand is a kind of matter different from wood, &c.

The prominent feature of the change undergone by the 7 magnesium when heated, by the iron when allowed to remain in damp air, and by the copper sulphate when the electric current was passed through it, is, that in each case a kind of matter has been produced different from, and in the place of, that which existed before the change began. The prominent feature of the change undergone by the platinum when heated, by the iron when the current was passed round it, and by the

8

copper when the current was passed through it, is, that no new kind of matter has been produced, but that the kind of matter which existed before the change began existed also while the change lasted and after the change ceased.

The first kind of change is called chemical change; the second is called physical change. The differences and resemblances between these two kinds of change must be more fully illustrated.

Iodine is a definite kind of matter, distinguished from other kinds by its lustre, greyish purple colour, opacity, easy solubility in alcohol with production of a reddish brown liquid, and by the fact that when a drop or two of this liquid is added to a very little starch paste a substance is formed which colours the liquid deep blue. Lead nitrate is a heavy, white, crystalline, solid ; it dissolves in a little hot water and separates from this solution, as it cools, in white, lustrous, crystals.

Two retorts are arranged with the beaks passing into small dry flasks, as shewn in fig. 3; a little iodine is placed in one retort, and a little lead nitrate in the other; each retort is heated by

Fig. 3. a Bunsen-lamp. The iodine soon changes to a dark purple, almost opaque, gas;

but this condenses on the cooler parts of the retort and in the small flask, to a solid, which presents the same appearance, and is possessed of the same properties, as the iodine originally used. The lead nitrate is also changed; a brownish red gas is produced which does not condense to a liquid or solid; if the heating is continued so long as this gas is produced, a yellowish coloured solid remains in the retort; this solid is a different kind of matter from the lead nitrate originally used. The change of solid iodine to gaseous iodine, and of gaseous iodine to solid iodine, is a physical change; the change of lead nitrate into two new kinds of matter—a brownish red gas called nitrogen oxide, and a yellowish solid called lead oxideis a chemical change.

The change which water undergoes when it is boiled is a 9 physical change; if the water is placed in a retort arranged as shewn in fig. 3, the water-gas (or steam) produced by heating the water is condensed to liquid water which is found in the small flask. But water may also be chemically changed. An electric current is passed through water to which some sulphuric acid has been added. The current passes from one plate of platinum to another; these plates are placed each within an inverted tube full of water and standing in a vessel of water (s. fig. 4). Bubbles of gas rise from each platinum plate and collect in the inverted tubes.

If the process is continued the water will at last entirely disappear and in

Fig. 4. place of it we shall have two colourless

gases. The

gas

in each tube is examined as regards its behaviour towards a burning splint of wood: one of the gases takes fire, it is hydrogen; the other does not, but the splint of wood burns in the gas very rapidly and brilliantly, this

gas
is
oxygen.

These

gases are definite kinds of matter; each is evidently very different from the water from which both have been produced. The change of water into the gases hydrogen and oxygen is a chemical change.

A few pieces of loaf-sugar are placed in a little water in a 10 porcelain dish; the sugar slowly disappears; some change has

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