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Having now gained a fairly clear notion of the kind of 116 material phenomena which form the subject matter of chemistry, and of the methods by which the chemical aspects of these phenomena are investigated ; and having arrived at certain fundamental generalisations from facts established by quantitative experiments and quantitative reasoning, we are in a position to proceed with the main subject of our inquiry, which is to establish the relations which exist between changes of composition and changes of properties of the definite kinds of matter we call compounds, and the relations which exist between the properties of the elementary constituents of compounds and those of the compounds themselves. This inquiry branches out in two directions; it requires us to study (1) the properties of compounds, and the properties of elements as exhibited in their compounds; and (2) the composition of compounds. To do this we must classify; we must group together those compounds which have similar properties, and those which have similar compositions.

We shall begin our attempt to learn how elements and 117 compounds are classified, and to become acquainted with the more important results of this classification, by considering the two elements hydrogen and oxygen and some of the compounds of these elements.

Occurrence. Oxygen, as we know, forms about of the 118 atmosphere. Hydrogen is sometimes found in small quantities in volcanic gases. Numerous compounds of each element occur in nature; of these water (H0) is the most abundant. Oxides of aluminium, iron, calcium, magnesium, silicon, and many other elements, are found widely distributed and in

large quantities. Ammonia—a compound of hydrogen and nitrogen--and compounds of ammonia, exist in the air and in the soil; and most of the substances which form the parts of plants and softer tissues of animals are compounds of hydrogen, with carbon, oxygen, and nitrogen.

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Chemical properties. Hydrogen and oxygen readily combine to form water. If a stream of hydrogen is allowed to flow into oxygen, or into air, and a light is brought to the jet the hydrogen takes fire and burns in the oxygen, and water is produced. If a stream of oxygen is allowed to flow into hydrogen from a narrow tube, and a light is brought to the jet the oxygen takes fire and burns in the hydrogen, and water is produced. In each case the chemical change is the same; 2H+ 0 = H,0.

Fig. 18 shews a simple arrangement for exhibiting these reactions. A and B are stoppered glass jars; each is fitted with a cork through which passes a tube narrowed at the end which is to go into the jar; A is filled with oxygen, B with hydrogen ; each stands in a little water whereby the

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Fig. 18. gas inside the jar is isolated from the air outside; the tube passing through the cork which fits jar A is connected with a gasholder containing hydrogen, the other tube is connected with a gasholder containing oxygen. Hydrogen is caused to pass slowly through one tube and oxygen slowly through the other; after a minute or so (when the air is all driven out of these tubes) the hydrogen jet is lighted, the stopper of A is withdrawn and the cork with its tube is quickly inserted; the hydrogen burns brilliantly; the stopper of B is withdrawn and a light is brought near the opening of B, the hydrogen in Bburns; the cork is now very quickly pressed into its place, and the jet of oxygen is seen to burn in the atmosphere of hydrogen.

A little consideration shews that the chemical reaction 2H + 0 = H,0 must occur, for the most part, at or near the surface of that gas which is flowing into the other, which other is, comparatively, at rest. If the inflowing gas is hydrogen, then, as the flame is visible along the surface of the inflowing gas, we say that the hydrogen burns in the oxygen; that the hydrogen is burnt and the oxygen supports the combustion. If the inflowing gas is oxygen, the flame being as before visible along the surface of the inflowing gas, we say that the oxygen is burnt, and the hydrogen supports the combustion.

Oxygen combines directly with many elements; compounds 121 of oxygen with every other element, except bromine and fluorine, have been prepared, either by direct combination, or as the results of several chemical changes.

I. The elements sodium, potassium, lithium, thallium, phosphorus, and some others, combine with oxygen more or less rapidly at ordinary temperatures. II. Antimony, arsenic, carbon, lead, sulphur, and many other elements, combine with oxygen at temperatures above the ordinary. III. Oxides of calcium, bismuth, chromium, copper, &c. &c. are usually prepared by (i) obtaining compounds of these metals with oxygen and hydrogen, and (ii) heating these hydroxides, and so decomposing them into oxides and water. IV. Oxides of lead, manganese, bismuth, and some other metals---composed of much oxygen relatively to the mass of lead &c.— are obtained by bringing these metals, or oxides of them composed of the metal united with relatively small masses of oxygen, in contact with two or more compounds which interact to produce oxygen. V. Oxides of nitrogen, sulphur, tellurium, &c. are obtained by decomposing, by heat or otherwise, compounds of these elements with oxygen and some other element or elements.

The following equations present examples of each of the
foregoing methods of preparing oxides :-
I. 2Na+ 0 = Na,0; 2P+50=P,0,

at ordinary temps.
II. 2Sb + 30 = Sb,03; C+ 20 =COZ.

at higher temps.
III. CaO,H, = CaO + H20; Bi 0 H. = Bi, 0, + 3H,0.

by action of heat.
IV. KCIOAq + PbO (heated) = Pb0, +KCIAq;

Sb,03 + 2HNO, (heated) = Sb, 0, +4,0 + 2NO,
2HNO, + P,0g (heated)=N, O + 2HPOz;

H, Teo, (heated) = H,0 + Teo.
Many elements form more than one compound with oxygen.
Thus, five oxides of nitrogen are known, viz. NO, NO, NO,
NO, N,0.; four oxides of lead have been prepared, viz. Phở,
6, Pb, PbO

Hydrogen combines directly with a few elements; the combination usually occurs at moderately high temperatures : thus, 2H+ S (molten) = H,S; H+ Br (heated) = HBr; 2C+2H (by passing electric sparks) = C,H,; &c.

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Compounds of hydrogen with other elements are sometimes formed in chemical interactions between several elements or compounds; thus when phosphorus is heated with an aqueous solution of caustic potash, phosphorus hydride (PH) is one of the products of the reaction ; when a solution of arsenic oxide in water is brought into contact with dilute sulphuric acid and zinc, arsenic hydride (AsH), zinc sulphate, hydrogen, and water are formed.

Very many compounds of oxygen and hydrogen, each with two or more other elements have been prepared.

Compounds of oxygen and one other element are called 123 oxides ; compounds of hydrogen with one other element are called hydrides.

The physical properties of oxides and hydrides vary much; some are solids, others are liquids, others are gases, at ordinary temperatures and pressures. The chemical properties of these compounds also vary much, but a great many oxides may be placed in one or other of two classes.

As representatives of these classes let us take the oxides of 124 sulphur and magnesium whose compositions are expressed by the formulae So, and MgO, respectively (S = 32, Mg = 24, 0=16).

Sulphur trioxide (S03) is a white, crystalline, solid ; it dissolves very easily in water forming a colourless solution. This solution has the following among other) properties (1) a sour taste; (2) it turns blue litmus solution bright red; (3) it dissolves many metals-e.g. zinc, iron, aluminium, magnesium, cadmium, barium, &c. &c.—with production of

more substances, one of which is hydrogen, and another is a compound of sulphur, oxygen, and the metal used; (4) it interacts with oxides of many metals—e.g. oxide of zinc, iron, aluminium, magnesium, cadmium, barium, &c. &c.—to produce two or more substances one of which is the same compound of sulphur, oxygen, and the metal of the oxide used, which was produced in reaction (3), and another of which is water. Further, if the solution of sulphur trioxide in water is evaporated considerably a thick oily liquid is obtained ; if this liquid is cooled below 0° crystals separate having the composition H SO, This compound is called sulphuric acid. If sulphuric acid is dissolved in water the solution exhibits the same properties as a solution in water of sulphur trioxide. It seems therefore fair to conclude that an aqueous solution of this oxide contains the compound H,SO,



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