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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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effect all transmutations. The pursuit of this One Thing became the central quest of alchemy. “There abides in nature” we read in an alchemical treatise “a certain pure matter, which, being discovered and brought by art to perfection, converts to itself proportionately all imperfect bodies that it touches.”

Alchemy was a fascinating dream; but chemistry is a more satisfying reality.

Let us return to experiment. Let a small weighed quantity 17 of magnesium be burnt in air under conditions such that the whole of the magnesia produced in the burning remains in the vessel in which the burning proceeds. The apparatus shewn in fig. 5 is a simple one for the purpose.

Some magnesium ribbon is placed on a piece of wiregauze and covered with an inverted funnel, the stem of which is connected by caoutchouc tubing with another funnel ; the

upper

funnel
is covered with filter-paper. The
whole apparatus is counterpoised;
the funnels are removed; the mag-
nesium is ignited by allowing a
Bunsen-flame to play on to it from
above; the funnels are then re-
placed. When the burning is com-
plete the apparatus is allowed to
cool and is then counterpoised.

It is found that the magnesia pro-
Fig. 5.

duced weighs more than the magnesium before burning. We therefore conclude that the magnesia is produced by adding to, or combining with, the magnesium, some other kind of matter. As the change from magnesium to magnesia proceeded in air, it is probable that the new kind of matter, which, by our hypothesis, has combined with magnesium and so produced magnesia, is derived from the air. To find whether this conclusion is correct or not, it would be necessary, (1) to burn a known weight of magnesium in a known quantity of air; (2) to determine the weight of magnesia produced, and the diminution in the quantity of air which accompanied the production of this weight of magnesia; (3) to change the magnesia back to magnesium and air, and to determine the weight of each of these obtained. If the difference between the weight of the magnesia and that of

the magnesium, by burning which the magnesia was formed, was equal to the weight of air which disappeared during the burning; and if the magnesium obtained from the magnesia weighed the same as the magnesium originally burnt; and if the weight of air obtained from the magnesia was the same as the weight of air which disappeared during burning; then we should be justified in concluding that the chemical change which occurs when magnesium is burnt consists in the addition to, or combination with, magnesium, of a portion of the surrounding air, and that the new kind of matter produced is composed of two kinds of matter, viz. magnesium and air. We should further have learned that although the magnesium has disappeared it has not been destroyed ; and that although magnesia has been produced it has not been produced from previously non-existent matter.

We should have given a definite meaning to the terms produced' and 'disappeared', as regards the change of magnesium to magnesia at any rate; and we should, to some extent at least, understand what is meant by saying the magnesia has taken the place of the magnesium which was burnt.

It is not easy to arrange quantitative experiments by which the chemical change of magnesium to magnesia may thus be examined. But if we use mercury in place of magnesium, we can arrange an experiment which will enable us to find an answer to each of the three questions stated in par. 15.

The change of mercury to burnt mercury, or as we now call it oxide of mercury, was examined quantitatively by Lavoisier. A sketch of the essential parts of his apparatus is shewn in fig. 6. Lavoisier placed 4 oz. of mercury in a glass balloon, the neck of which, drawn out and bent, passed under mercury and then into the air contained in a bell-jar; the bell-jar contained 50 cub. inches of air. The mercury was heated nearly to its boiling point by means of a furnace; red specks appeared on the surface of the mercury and the volume of air in the bell-jar slowly decreased.

After some days the production of red solid matter on the surface of the mercury seemed to have ceased; heating was continued for a few days more (12 days in all), and was then stopped. The air in the bell-jar now measured between 42 and 43 cub. inches, the diminution in volume was therefore between 7 and 8 cub. inches; the red solid was collected, and was found to weigh 45 grains. These 45 grains of the red solid produced by slowly burning mercury in air were placed in a glass tube

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