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

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

driven over into the small flask. Then allow the tube to cool, and counterpoise again.

The weight is the same as it was at the beginning of this series of experiments.

You have therefore proved, on the basis of certain assumptions, that when copper is heated in air it combines with oxygen in the air to produce a new kind of matter called copper oxide; and that the weight of the copper oxide thus produced is greater than that of the copper from which it has been produced.

By experiments too difficult to be performed at present it can be proved that the difference between these weights is the weight of the oxygen which has combined with the copper.

The change which occurs when magnesium, or iron, is burnt, also consists in combination of the magnesium, or iron, with oxygen in the air.

Exp. 4. Powder some crystals of potassium chlorate in a dry mortar; dry the powder by pressing, not rubbing, it between filter paper, and place a little of it in a dry test tube.

Counterpoise the tube with its contents; heat gently until the potassium chlorate melts and evolves a gas, then raise the temperature a little. Prove that the gas which is evolved is not air, by bringing into the tube a chip of wood which is just glowing; the wood bursts into flame. The gas evolved is oxygen. Continue heating for a little, then allow the tube to cool, and counterpoise again.

The contents of the tube weigh less than the potassium chlorate did. But if this is so, it is probable, although not yet experimentally proved, that the quantity of gas which was evolved also weighs less than the potassium chlorate.

Prove that the solid residue in the tube is not potassium chlorate, by dissolving it, and a little potassium chlorate, separately, in water, and adding to each solution a few drops of solution of silver nitrate; in the case of the chlorate no visible change occurs, in the other case a white pp. is produced (this pp. is silver chloride: the solid obtained by heating potassium chlorate is called potassium chloride).

The substance potassium chlorate has been changed, by the action of heat, into at least two different kinds of matter, oxygen and potassium chloride; the mass of each of these is less than the mass of the potassium chlorate changed.

Exp. 5. Place a little dry cuprous oxide in a dry tube of

glass; counterpoise; heat; notice the change which occurs; cool; counterpoise again. A new kind of matter different from, and weighing more than, the original cuprous oxide, has been produced. This new kind of matter is called cupric oxide. In Exps. 1, 2, and 3 you changed specified masses of certain kinds of matter each into another kind of matter; the mass of the new kind of matter was in each case greater than the mass of the original kind of matter. The changes which magnesium, iron, and copper underwent are representative of the kind of chemical changes which Elements undergo. An Element is chemically changed by adding to, or combining with, it some kind, or kinds, of matter different from itself; the product, or the sum of the products if there are more than one, of such a change weighs more than the element weighed.

In Exp. 4 you changed a specified mass of potassium chlorate into two different kinds of matter, each unlike, and each weighing less than, the original matter. But in Exp. 5, you changed a specified mass of a certain kind of matter, cuprous oxide, into another kind of matter different from, and weighing more than, itself. The change which potassium chlorate underwent, and also the change which cuprous oxide underwent, are representative of the chemical changes which Not-elements undergo. A Not-element is chemically changed, either, as an element is, by adding to, or combining with, it some other kind or other kinds of matter, or by separating it into two or more different kinds of matter each unlike the other and unlike the original substance, and each weighing less than the original substance did before the change.

Exp. 6. Carefully counterpoise a small piece of tinfoil in a crucible; place the crucible on a sand-bath in the draught cupboard; heat the sand gently; let a little nitric acid fall on to the tin drop by drop. When the contents of the crucible have turned to a white powdery solid cease to add acid, but continue heating, and raising the temperature, until fumes of acid are no longer given off. Then cool, and counterpoise; the solid in the crucible is different from the tin used and it weighs more than the tin.

Reasoning solely on the results of this experiment and of Exps. 1 to 5, would you be inclined to place tin in the class of elements or in that of not-elements?

Do you think that tin can be placed in one or other of these classes solely on the evidence given by this experiment?

Reference to "ELEMENTARY CHEMISTRY." Chap. II.

CHAPTER III.

NOT-ELEMENTS DIVIDED INTO MIXTURES AND

COMPOUNDS.

Exp. 1. You are given some finely powdered sulphur and some very finely divided iron. Each of these is an

element.

(a) Place a little of each in a test tube, pour some carbon disulphide into each tube, shake up, and gently warm by placing the tubes in hot water. The sulphur slowly dissolves; the iron is unchanged. To prove that sulphur has dissolved, filter, and evaporate off the carbon disulphide by placing the liquid in a watch-glass on a water-bath.

Never warm carbon disulphide over a flame; a mixture of air with carbon disulphide vapour is very explosive.

(b) Shake up a little of the iron and the sulphur, separately, with water; the iron quickly sinks to the bottom of the tube, part of the sulphur floats in and on the surface of the water.

(c) Place a little of the iron and sulphur, separately, on clean sheets of paper; bring a magnet under each, touching the under side of the paper; send a stream of air from the mouth over the surface of the iron and the sulphur. The sulphur is quickly blown away, but the iron is more or less firmly held by the magnet.

Now make a mixture of the iron and sulphur in the ratio of 1 part of sulphur to 12 parts of iron, by weight. Examine this mixture by (a) placing a little of it in warm carbon disulphide; (b) shaking a little with water; (c) placing a little on paper, bringing a magnet under the paper, and blowing a rapid air-stream over the substance. Each substance in the mixture behaves in the same way as it did when unmixed with the other substance.

Place a little of the mixture of iron and sulphur in a clean dry test-tube, and heat by a Bunsen-lamp until the whole mass glows strongly; allow to cool; break the tube in a mortar, and pick out the broken glass. Examine the black solid thus formed by action on it of (a) carbon disulphide, (b) water, (c) a magnet. The substance appears to be homogeneous; it is not separated into unlike parts by any of the methods by which the mixture of iron and sulphur was separated into unlike parts.

A Compound of iron and sulphur, iron sulphide, is formed by heating these two elements together in the ratio of 1 part by weight of sulphur to 1 parts of iron. The properties of the compound are very different from those of either of the elements which have combined to form it. The compound cannot be separated into unlike parts by methods which succeed in separating a Mixture of the constituents of the compound, i.e. a mixture of iron and sulphur, into unlike parts.

Exp. 2. Place a little black copper oxide in a tube with some water; warm; no visible change occurs. Separate the liquid from the solid by filtration, and evaporate the filtrate to dryness; no solid remains; therefore none of the copper oxide dissolved in the warm water.

Place a little yellow potassium chromate in water, and warm; the yellow solid dissolves, forming a yellow coloured liquid. Evaporate this to dryness; the original yellow potassium chromate remains.

The

Make a mixture of copper oxide and potassium chromate by pounding the two substances together in a mortar. mixture is yellowish-black. Examine it by a magnifying glass; you can distinguish black particles (copper oxide), and yellow particles (potassium chromate). Place some of the mixture in water; warm; filter; add more water to the remaining solid, again warm, and filter. Dry the black solid which remains by placing it in a steam-bath. Evaporate the yellow filtrate to dryness. Mix the yellow potassium chromate thus obtained with the black copper oxide also obtained from the mixture; a yellowish black substance, similar to the original mixture, is produced.

You have thus separated a Mixture into its constituent parts by making use of a certain physical property of each constituent; viz. solubility in water of one constituent, and insolubility of the other.