2. Basic oxides, which by combination with water yield bases.

3. Neutral oxides.

The connection that these oxides have with the acids and bases has already been described (page 97).

Combustion. The process which we call combustion, or burning, is nothing but rapid combination with oxygen, with evolution of light and intense heat. It is in fact only a particular case of oxidation. In all ordinary cases oxidation is attended with the production of heat, which we have before seen (pp. 13, 27) is a frequent result of the exertion and consequent loss of chemical force. But the oxidation of a certain weight of substance may take place rapidly or slowly, and the intensity of the heat will, of course, vary in like proportion, although the total quantity remains the same. The term combustion is generally limited to those cases of oxidation where light as well as intense heat is evolved. But the term may also be applied to similar combinations between other elements, as, for instance, between chlorine and hydrogen or metals.

Experiment 9.-Invert a tumbler over a small piece of lighted candle standing on a plate. In a short time the candle will go out, having removed a portion of the oxygen from the air of the tumbler. The inside of the tumbler will be seen to be covered with moisture, and if it is rapidly removed, and a little lime-water shaken in it, the latter will become milky, showing the formation of carbonic anhydride.

All ordinary kinds of fuel-coal, wood, gas, oil, &c.— contain carbon and hydrogen. This experiment shows the important fact that, when they burn, the carbon becomes carbonic anhydride, and the hydrogen water.

The phenomena of slow oxidation are very interesting and important.

Experiment 10.-Throw some iron filings, which have been moistened with water, into a bottle, so that they may adhere to its bottom. Invert the bottle with its mouth in a basin of water, and leave it in a warm place for a few days. The water will gradually ascend in the bottle, for the air becomes diminished in bulk to the extent of one-fifth; in fact, the whole of the oxygen is slowly absorbed by the iron and combines with it, forming ferric oxide, FeO. The gas remaining in the

bottle is nitrogen, as is shown by plunging a lighted taper into the gas; the taper will be extinguished (see Nitrogen). The rusting of iron is here seen to be its slow oxidation.

Experiment 11.-Bruise a little hay with water, stuff it in the bottom of another small phial, and proceed as in the former experiment. The same effect will be produced as with the iron filings; that is to say, the oxygen will be absorbed by the hay, and nitrogen will remain, mixed in this case with a little carbonic anhydride from the oxidation of the hay. The hay undergoes the process of decay, which is thus shown to be a slow oxidation. In both of these cases, heat is evolved, though it possesses but slight intensity. The heat of a hot-bed is due to the slow oxidation of the manure.

Respiration.-Experiment 12.-The heat of the animal body is maintained by a process of slow oxidation, analogous to that of the hay. Blow through a glass tube into a tumbler half full of lime-water. In a few minutes it will become turbid, showing that carbonic anhydride is contained in the expired air. Breathe into a cold and dry tumbler. It will become covered with moisture. Animals consume the oxygen of the air, and return carbonic anhydride and water to it.

Substances are more easily burnt when they are in a finely divided state than if they are solid; shavings are easier to burn than billets of wood, and iron is easily combustible when in the form of filings.

Experiment 13.—Hold a small bar of iron in the flame of a spirit lamp; it will not burn; but file it and let the filings fall into the flame; they will be immediately ignited. Iron filings when sprinkled in a flame burn brilliantly, and have a very pretty appearance.

Deflagration. The term deflagration is applied to those cases of combustion where the body is burnt, not by the agency of free oxygen as that of the air, but by the aid of combined oxygen as it exists in substances which are easily decomposed, such as potassium chlorate, KCIO,, and potassium nitrate, KNO. Deflagration may be exemplified as follows:

Experiment 14.-Heat 20 or 30 grains of nitre in a testtube with a spirit lamp, and when it is melted throw in some powdered charcoal or a fragment of sulphur; it will burn violently, being oxidized at the expense of the nitre.

OZONE, O3.

When certain kinds of electrical discharge occur in an atmosphere of pure oxygen, or even of air, a contraction of volume takes place, and some of the oxygen is converted into the curious gas called ozone (Gr., ow, to smell), on account of its peculiar odour. This odour can be perceived in the neighbourhood of an electrical machine during its action. The specific gravity of ozone is found to be 24 (H= 1), that of oxygen being 16; so that ozone is half as heavy again as oxygen. This fact is explained by the more than probable theory that every molecule of ozone contains three atoms of oxygen instead of two, as in ordinary oxygen gas.

Ozone has never been obtained free from oxygen. It is an amazingly powerful oxidizing, bleaching, and disinfecting agent, a property which is due to the ease with which it gives out its third atom of oxygen. At a temperature of about 250° C. (482° F.), it is reconverted into common oxygen, 20, 302. It can be formed by several methods besides the one above mentioned.

=

Experiment 1.-Take a stick of phosphorus, about an inch long, and scrape it with a knife, under water, so as to expose a new surface; then lay it in the bottom of a large wide-mouth bottle, pour in enough water to half cover it, and having loosely stoppered the bottle, set it aside in a warm place for about half an hour. White fumes of phosphorous acid will arise from the phosphorus, but will soon be absorbed by the water, and, simultaneously, ozone will be formed. The phosphorus combines gradually with part of the oxygen of the air in the bottle, while some of the remainder is converted into ozone, 302, becoming 203. Its presence may be shown by the use of what is called the ozone test," which is made by dissolving a few grains of potassium iodide in a table-spoonful of starch jelly, such as is prepared for use in the laundry, and smearing the latter on strips of paper. Introduce one of these test-papers in the bottle of ozone, and it will immediately become dark blue or violet, the colour being more or less intense according to the quantity of ozone. The iodine is liberated from the potassium iodide by the ozone, and it combines with the starch to form iodide of starch (Experiment 5, page 119).

Experiment 2.-Lay a piece of the test-paper in the bottom of a tumbler and gradually invert a bottle of ozone, prepared as in Experiment 1, as though in the act of pouring from the one vessel to the other. The ozone will stream out and occupy the tumbler, and come in contact with the test-paper, which will be immediately affected. This experiment shows that ozone is heavier than air.

Experiment 3.-Suspend a bright silver coin in a bottle of ozone. In a few minutes it will be covered with a grey deposit of silver oxide, thus showing the remarkable oxidizing power of ozone.

Experiment 4.-Place a few fragments of sulphide of iron in a glass or jar, and pour upon them a little water and a few drops of sulphuric acid; sulphuretted hydrogen, a gas of an extremely disagreeable odour, will be evolved. Moisten a strip of paper with a solution of lead acetate, and bring it in contact with the escaping sulphuretted hydrogen, and the paper will become dark brown or black, from the formation of lead sulphide, PbS. If the paper be then plunged into a bottle containing ozone it will again become colourless. The ozone oxidizes lead sulphide, which is black, into lead sulphate, which is white :

PbS+40, PbSO4 + 402.

Experiment 5.-Moisten a glass rod with a strong solution of ammonia, and introduce it into a bottle of ozone. White fumes will be abundantly formed, which consist of ammonium nitrite, NH,,NO2.

The

Experiment 6.-Moistened litmus-paper is immediately bleached when introduced into a bottle of ozonized air. bleaching and disinfecting of bodies by ozone are owing to their oxidation. Ozone is very commonly found in the atmosphere, formed perhaps by the passage of lightning through it. It cannot be doubted that this atmospheric ozone has important functions to fulfil in the economy of nature.

Experiment 7.-Strips of paper moistened with ozone test may be exposed to the air for a few hours. They will frequently be found to have turned blue, especially in country places.

K

WATER, H2O.

Water does not exist in nature in a perfectly pure condition, even fresh water contains small quantities of solid impurities, of which the carbonates and sulphates of calcium and magnesium are among the most important and general. Natural water also holds in solution small quantities of oxygen, nitrogen, and carbonic acid; but besides these general impurities, there are others which occur only in particular instances, such as iron, sulphuretted hydrogen, iodine, &c., which are present in certain springs, and which give to them a medicinal value, as is the case with the waters of Harrogate, of Cheltenham, of Tunbridge Wells, &c. Rain water, collected in a vessel placed in a large open space, such as a field, is the purest kind of natural water that can be obtained, and may be used by the student, for most purposes, in lieu of distilled water, in cases where the latter cannot easily be obtained.

The composition of water was first determined by Cavendish; he proved that it is the sole product of the combustion of hydrogen.

Experiment 1.-That water is really formed by the burning Fig. 48. of hydrogen can easily be shown by inverting a clear glass bottle over the hydrogen flame; the glass soon becomes clouded over, because the water, which in consequence of the heat is generated in the form of steam, condenses in small globules on the cold sides of the bottle.

Experiment 2.-The extraordinary degree of heat developed by the chemical union of oxygen and hydrogen may be shown by the following experiments. Insert into the opening of a large pig's bladder, which has been softened by soaking in water, the broken-off neck of a flask, and bind it firmly round with a string. Then select two perforated corks, fitting this neck. One cork is connected with a bent glass tube, conducting the oxygen from the apparatus in which it is evolved into the bladder, which soon becomes filled with it. When this operation is finished, replace the first cork by the second, having a glass tube adapted to it only a few inches long, and