Exp. 6. Dissolve a few lumps of sugar in warm water in a beaker: then evaporate the liquid to dryness on a waterbath. Notice the bubbles of air which rise to the surface of the water during solution; this air was imprisoned in the pores of the sugar, and as the solid sugar dissolved in the water it was set free. After the water has been removed by evaporation you obtain a white, sweet, substance, the properties of which indicate that it is sugar.

Place a few pieces of marble in a bottle with a cork and tubes arranged as shewn in Fig. 2. Pour a little water on

Fig. 2.

to the marble, then pour a little hydrochloric acid down the funnel tube into the bottle.

The marble gradually disappears in the water and acid, and a gas is produced. Allow this gas to pass into a dry bottle full of air as shewn in the figure. After 5 minutes or so, bring a lighted taper a little way into the bottle; the light goes out.

You thus prove that when marble interacts with hydrochloric acid and water, a new kind of matter, very unlike either the marble or the acid, is produced. This new kind of matter is a colourless, odourless, gas: you recognise its presence in a vessel by making use of the property it possesses of extinguishing a lighted taper.

This gas is called carbon dioxide or carbonic anhydride; it is heavier than air, and can therefore be collected, as you have collected it, by allowing it to pass to the bottom of a vessel full of air.

The method of collecting gases heavier than air which you have just used is called collection by downward displacement. It is often used, especially for collecting heavy gases which dissolve in water.

The change of sugar into solution of sugar was a physical change; no new kind of matter was produced. The change of marble into carbon dioxide was a chemical change; the properties of the matter produced differed in a very marked way from those of the marble.

These experiments illustrate the prominent differences between physical and chemical change. When a specified

substance is physically changed, it temporarily acquires a new property or new properties; but the substance is present, and can be recognised by its ordinary properties as being present, during and after the physical change. When a substance is chemically changed, the original substance disappears and at least one new substance is produced in its place. The differences between the properties of the original substance and the new substance are so marked that we do not hesitate to call each a different kind of matter from the other.

The experiments in this chapter have illustrated the meanings of the terms;-filtration, sublimation, solution, evaporation, precipitation. They have also shewn how to use a water-bath for evaporation, and how to collect a gas heavier than air by downward displacement.

Reference to "ELEMENTARY CHEMISTRY." Chapter I.

CHAPTER II.

ELEMENTS AND NOT-ELEMENTS.

Exp. 1. Place a piece of magnesium ribbon in a porcelain crucible (about 12 inches of ordinary magnesium ribbon

Fig. 3.

loosely wrapped together), put on the lid, and counterpoise the whole on a fairly good balance against shot or small pieces of metal. Set the crucible on a triangle standing on one of the rings of an iron stand (Fig. 3). Heat the crucible gently, then strongly, and then over a good blowpipe; raise the lid from time to time, for a few seconds, to admit air.

The magnesium is slowly burnt to magnesia (comp. Chap. I. Exp. 1). Do not remove the lid at any time for more than a second or so, else some of the magnesia will be volatilised and lost. When the burning is finished allow the crucible to cool, then place it on the balance and the counterpoise on the other pan.

The crucible and the magnesia together weigh more than the crucible and the magnesium.

The change of magnesium to magnesia is a chemical change: the product of this change, magnesia, weighs more than the magnesium.

Exp. 2. Place a little very finely divided iron in a crucible, and counterpoise the whole. Heat the crucible over a lamp until the iron glows throughout. Allow to cool, and counterpoise again. Compare the appearance and action towards a magnet of the substance in the crucible with the appearance and action towards a magnet of the original iron.

There has been an increase in weight and a new kind of matter, iron oxide, has been produced.

Magnesium and iron have been chemically changed, each into a kind of matter different from itself: the new matter produced in each case weighs more than the original matter. Therefore the change has consisted in the addition to, or combination with, the iron, and the magnesium, of some other kind, or other kinds, of matter.

Exp. 3. Place a little finely divided copper in a piece of hard glass tubing about 6 ins. long. Counterpoise the tube and its contents. Connect the tube, by means of a cork and glass tube, with a U tube containing calcium chloride, which is a substance that quickly absorbs moisture (Fig. 4). Gradu

Fig. 4.

ally heat the glass tube containing the copper until it is red-hot. By means of a pair of bellows pass a very slow stream of air through the U tube and over the hot copper. The copper is soon changed to a black solid quite unlike the original red copper; this solid is oxide of copper. Continue heating until the change seems to be complete.

Allow the tube and its contents to cool, and then counterpoise again; there has been an increase in weight.

Inasmuch as the copper, iron, and magnesium, have all been heated in air, and the new matter produced has in each case weighed more than the matter before heating, we may conclude, provisionally, that the three chemical changes are analogous; and that, probably, their causes are similar. In each case there has been a combination with the heated substance of some other kind, or other kinds, of matter. The most likely source of this other kind of matter, considering the conditions of the experiments, is the air.

We must now make two assumptions, which can be, and have been, proved by accurate experiments. We shall assume (1) that the air is a mixture of at least two gases called oxygen and nitrogen; (2) that water is a compound of two gases, hydrogen and oxygen. If then hydrogen is

brought into contact with a heated solid substance and water is produced, it follows that oxygen must have been taken away from the heated solid by the hydrogen.

Now remove the bellows, and in their place put a flask containing zinc and a little dilute sulphuric acid (Fig. 5). By

E

Fig. 5.

the interaction of these hydrogen is produced. Keep the flask in cold water. Let the hydrogen pass through the U tube containing calcium chloride, and then through the tube containing the oxide of copper. Let the end of this tube pass into a small dry flask. When the hydrogen has been passing for quite 5 minutes, but not before, begin gradually to heat the tube.

The flask is dried by rinsing it out with water, then with a little alcohol, allowing the alcohol to drain out of the flask, warming the flask in the flame of a Bunsen-lamp, blowing air into it by a bellows, and repeating the warming and blowing in air two or three times.

After a little you notice that red copper is produced in the tube, and that drops of water are formed in the small flask. Continue the passage of the hydrogen over the hot copper oxide so long as any water seems to be produced, or any change to occur in the appearance of the matter in the tube. Towards the close of the experiment heat the tube strongly, to make sure that no water remains in the tube but that all is