Compounds of the elements may be divided conveniently into six classes:-

The Hydrides; The Halides;

The Oxides and Sulphides (with Selenides and Tellurides) ;

The Nitrides and Phosphides (with Arsenides and

Antimonides) ; The Borides, Carbides, and Silicides; The Alloys.

Compounds can be prepared by many methods ; it is not so easy to classify them as it is to arrange into classes the methods of preparation of elements. As a rule, the preparation is carried out by one of the following methods :-

(a) The interaction of elements ;

(b) The action of an element on a compound; \c) The action of heat on a compound ; la) The interaction of compounds \

\e) The addition of one compound to another.

These methods shall be considered in relation to each of the groups of compounds named above. The Hydrides.

(a) The Interaction of Elements-Lithium, sodium, potassium, rubidium, and caesium, when heated to about 3600 in an iron tube in a current of hydrogen, form white crystalline compounds ; their formula is MH ; they are stable within narrow limits of temperature,'for above 5000 they give metal and hydrogen.

Iron, nickel, palladium, and platinum, when heated gently in hydrogen, absorb the gas. Meteoric iron, indeed, has been known to give off, on heating, 2.85 times its volume of gas. This natural variety of iron contains about 6 per cent, of nickel. Palladium, gently warmed in an atmosphere of hydrogen, absorbs over 900 times its volume of that gas, corresponding to 4.68 per cent, of the weight of the body produced. It is difficult to determine whether or not the palladium is in chemical combination with the hydrogen, or whether the hydrogen is in a state analogous to solution, for it is known that a solid can exert solvent power. There is a considerable rise of temperature accompanying the absorption ; and if palladium, in a state of sponge, is placed in contact with a mixture of oxygen and hydrogen, the mixture may be made to explode. A thermometer-bulb coated with palladium sponge is a good test for the presence of an explosive mixture of marsh-gas and air in mines, for the rise of temperature produced is an indication of danger. These metals absorb hydrogen more readily if they are made the negative electrodes of a battery with which dilute sulphuric acid is electrolysed. Iron shows a very curious behaviour under these circumstances. If a thin plate of iron is made to close the top of a barometer-tube full of mercury and a small cell be constructed on it, hydrogen will pass through the iron, when the plate is made the kathode, and will depress the mercury in the tube. No other metal, so far as is known, shows this peculiarity ; it would appear that the hydrogen in the ionic state can penetrate the iron.

Carbon, heated to 12000 in an atmosphere of hydrogen, unites with it to form marsh-gas (methane), CH4, and acetylene, C2H2. Only a small percentage of the hydrogen, however, enters into combination; a balance soon establishes itself between the number of molecules of methane and acetylene being formed and decomposed in unit time. At a higher temperature, that of the electric arc, acetylene, C2H2, is the main product, owing to the decomposition of the methane into that gas and free hydrogen:-2CH4 = C2H2 + 3H2. Other compounds of carbon and hydrogen are formed simultaneously, and there again appears to be a state of equilibrium produced between the various hydrocarbons formed. With nitrogen, it appears to be impossible to induce hydrogen to enter into direct combination at such temperatures ; but if electric sparks be passed through a mixture of hydrogen and nitrogen, combination to a limited extent ensues. Should the ammonia, NH3, be removed by having water, or, better, dilute sulphuric acid, present, the combination proceeds until all the gases, if they were originally present in the correct proportion-one volume of nitrogen to three volumes of hydrogen-have combined. Conversely, if sparks be passed through ammonia gas, there is nearly, but not quite, complete decomposition into its constituents. This enables the volume relations of ammonia to be demonstrated ; for it is found that two volumes of ammonia gas can be decomposed into two volumes of nitrogen and six volumes of hydrogen. This is symbolised by the equation-

2NH3 = N2 + 3H Weight 2(14 + 3) 28 3(2) grams. Volume 2(22.4) 22.4 3(22.4) litres.

The hydrogen can be nearly completely removed by absorption with palladium-sponge, and the nitrogen remains.

Water, H2O, is more completely formed than any one of the previously mentioned compounds by the interaction of its elements. A mixture of oxygen and hydrogen, in the proportion of one volume of oxygen to two of hydrogen, is exploded by heat; this is most easily done by passing an electric spark through the mixture. While the position of equilibrium for a mixture of nitrogen, hydrogen, and ammonia lies at such a point that very little of the compound is present, but chiefly the uncombined gases, the contrary is the case with hydrogen and oxygen. Here nearly all the oxygen and hydrogen combine, and only a trace remains uncombined. Combination may be made to take place slowly at much lower temperatures; even at 3CO0 slow combination occurs. Colloidal platinum, prepared by making an electric arc between poles of platinum under pure water, which appears to consist of very finely divided platinum disseminated through the water, has the power of causing union of oxygen and hydrogen left standing in contact with it, even at the temperature of the atmosphere. On the other hand, if water-vapour be raised to a very high temperature, above 1800°, decomposition into its constituents takes place with considerable rapidity ; so that it is possible to obtain a mixture of oxygen and hydrogen by passing steam through a tube in which a spiral of platinum wire is kept at a white heat by means of an electric current. These actions are therefore termed * reversible," and they are expressed by such equations as-