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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

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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

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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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occurred. The water is now removed by boiling; the solid residue in the dish is sugar, it is characterised by all those properties which mark off sugar from other kinds of matter. The change of solid sugar into a solution of sugar, and of sugar-solution into solid sugar, is a physical change.

Into a small quantity of water are thrown one or two pieces of the metal sodium; the metal swims on the surface of the water; a gas is produced which may be collected and examined; when the sodium has disappeared the water is boiled off; there remains a white solid, which dissolves in water without formation of any gas, and which is evidently very different from either the sodium or the water by the mutual action of which it has been formed. This change of

. water and sodium into a solid body, caustic soda, and a gas, hydrogen, is a chemical change.

A little hot concentrated sulphuric acid is poured on to some pieces of loaf-sugar; much heat is produced, steam is given off, and a black, charcoal-like, solid remains. Sugar was only physically changed when it was brought into contact with water : but the mutual action of sugar and hot sulphuric acid is a chemical change; both the visible products of this change, steam and carbon, are different kinds of matter from the bodies, sugar and sulphuric acid, by the interaction of which they have been produced.

All those changes which we have classed together as chemical have this in common, that one, or more than one, kind of matter has disappeared, and another kind, or other kinds, of matter has been formed. So far as our experiments could tell us, the new matter formed did not exist as a part of the material system before the change began.

Those changes which we have classed together as physical have been characterised by the continued existence, during and after each process, of the same kind of matter which was present before the change began. This matter temporarily acquired a new property, or new properties; but the new property did not prevent us from recognising the other properties by which the special kind of matter was marked off from other kinds. In both classes of occurrences the matter experimented with was subjected to new conditions different from those which existed before the experiments began. When these conditions were removed, in one class of phenomenathe physical—we had a return to the state of things which prevailed at the beginning of the experiments; we had the

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same kind or kinds of matter exhibiting the same properties : in the other class of phenomena—the chemical- --we had not a return to the original state of things; we had new kinds of matter exhibiting new properties.

If the occurrences we have been considering were very 14 closely and accurately examined it would be found that those we have called chemical include changes which belong to the physical class. The emission of light by the burning magnesium, the conduction of heat through the mass of lead nitrate, the heating and volatilisation of water in the interaction of sugar and sulphuric acid ; these are physical rather than chemical changes. The physical, and the chemical, are different aspects of the complete phenomenon. We try to separate them as far as we can that we may study each more accurately, and so find the general laws which hold good for each ; for the more we understand natural events the more are we convinced that one law of nature is never suspended or stopped by another law, however the effects of one may be modified by the effects of another.

But we cannot at present attempt minutely to analyse the phenomena we have to study into chemical and physical parts; we must be content to learn the broad features of the two classes of occurrences.

In reasoning on the data obtained in the experiments 15 already described, certain assumptions have been made, and certain expressions have been used somewhat vaguely. It was asserted that when magnesium was burnt in air, the matter called magnesium disappeared and its place was taken by a new kind of matter called magnesia; that when lead nitrate was heated the matter characterised by the properties summed up in the name lead nitrate disappeared and in its place there were formed two other kinds of matter,-nitrogen oxide, and lead oxide; that the passage of the electric current through copper sulphate solution was accompanied by the disappearance of one kind of matter,—copper sulphate, and the formation of another kind of matter,—-copper; and similarly with the other experiments. Now one may well ask: how can it be proved that the magnesium, or the lead nitrate, or the copper sulphate, really disappeared ? how can it be proved that the place of the magnesium was taken by magnesia, or of lead nitrate by lead oxide and nitrogen oxide, or of copper sulphate by copper? And questions such as these must also be asked : what exactly is meant by saying that the

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magnesium, or the lead nitrate, disappeared ? : magnesia took the place of magnesium ; copper, the place of the copper sulphate; what is meant by saying one substance takes the place of another? An electric current was passed through water, some of the water disappeared and in its place two gases were produced ; but may not these gases have come from the platinum plates, or from the glass of which the vessel was made? Or, assuming that the water was indeed changed in this experiment into the two gases called hydrogen and oxygen, we ask; what definite meaning is to be put upon the statement water can be changed into hydrogen and oxygen ?

The history of alchemy, and of the transition from alchemy to chemistry, teaches the necessity of putting, and of answering, such questions as these. The alchemists not only thought that they could, but asserted that they did, change water into earth or into fire, lead into silver, and copper into iron. Their conception of nature led them to regard all things as undergoing continual change ; but they were not able so accurately to study these changes as to discern the unchanging sequences in which they occurred, and to grasp the unchangeable parts of the phenomena they observed.

The assertion that water could be changed into earth, or into fire, was based upon such experimental evidence as this. A quantity of water was heated in an open glass vessel; the water slowly disappeared, and a little white earthy solid matter remained in the vessel. The water had disappeared and an earthy solid had been produced in its place. A piece of red hot iron was plunged into water contained in a bellshaped glass vessel ; bubbles of gas rose through the water and collected in the vessel ; this gas took fire when a lighted taper was brought into it. The water had been changed into the matter of fire'. To prove that copper could be changed into iron, the alchemist placed a piece of copper in aqua fortis (nitric acid); the copper slowly disappeared; in the blue-green liquid thus formed he placed a piece of iron; the iron disappeared, and copper was produced in its place. The conclusion which the alchemist drew from such experiments as these was that one kind of matter could be changed into other kinds of matter. But if this were so, why should not any kind of matter be changeable into any other? Heat brought about the change of water to earth; hot iron, the change of water into the matter of fire'; aqua fortis, the change of iron into copper. There must surely be some one thing which would

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