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pounds of non-metallic elements, as a class, interact with water; that the compounds of the more distinctly negative elements generally produce oxides (or oxyacids) and haloid acids; and that the compounds of the less negative non-metals generally produce oxyhaloid compounds and haloid acids. The oxyhaloid compounds of metals are usually very complex, and are produced either by heating the oxides with haloid compounds, or by passing halogen over a heated mixture of oxide and carbon.

A classification of the elements of the nitrogen group into 224 metals and non-metals based on the behaviour of their haloid compounds with water would place phosphorus and arsenic with the non-metals, antimony with the metalloids, and bismuth either with the metalloids or the metals. A classification of the same elements based on the properties of their oxides would lead to the same, or about the same, arrangement; bismuth would certainly be placed with the metals. A classification based on the existence and properties of hydrides would result in all the elements except bismuth being placed among the non-metals. A classification based on the occurrence or non-occurrence of allotropy would result in phosphorus and arsenic being placed with the non-metals, antimony with the metalloids, and bismuth with the metals.

The properties of the sulphides of a group of elements 225 sometimes afford class-marks by using which the elements may be subdivided into families.

All the elements of the nitrogen group form sulphides : the following are certainly known :-NS, PS, PS, and several other compounds of phosphorus and sulphur; As, Se, As, Sz; Sb S, Sb Sz; Bi S, and probably other sulphides of bismuth. “Nitrogen sulphide differs considerably from the others : it is very explosive ; it is decomposed by water or aqueous alkalis giving ammonia and ammonium salts of sulphur oxyacids. The sulphides of phosphorus react with aqueous solutions of potash to form potassium salts of sulpho(or thio-) acids of phosphorus; but these salts are unstable and easily decomposed. The sulphides of arsenic and antimony dissolve in aqueous solutions of potash or potassium sulphide (K,S) to form potassium sulpho- or thio-arsenite and antimonite, respectively (K, AsSg; KSbS,); these solutions are decomposed by boiling, or by interacting with dilute acids, giving arsenious, or antimonious, sulphide, and hydrogen sulphide. Bismuth sulphide does not interact either with caustic potash or potassium sulphide.

The sulphides of phosphorus, arsenic, and antimony, are therefore acidic in their reactions with alkalis and alkaline sulphides ; sulphide of bismuth is not acidic, it interacts with acids to form salts and hydrogen sulphide, just as oxide of bismuth interacts with acids to form salts and hydrogen oxide. The following formulae exhibit the relations of composition between (1) acidic oxides and their oxyacids, (2) basic oxides and their salts, (3) acidic sulphides and their sulphoacids, and (4) basic sulphides and the salts obtained by their reactions both with acids and with acidic sulphides.

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A classification of the elements of the nitrogen group based on the properties of their sulphides would place phosphorus, arsenic, and antimony, with the non-metals, and bismuth with the metals. Nitrogen would be placed with the non-metals, but to some extent apart from the other members of the group.

We have now learned something of the methods used in chemistry for classifying elements and compounds. We cannot classify elements without at the same time classifying compounds; we cannot arrange compounds in groups without at the same time arranging elements in groups.

As class-marks we have used, the properties and the conposition of oxides; the properties and the composition of sulphides; the formation, composition, and interaction with water, of haloid compounds; the formation, or non-formation, the properties, and the composition, of hydrides; the production of compounds with oxygen and hydrogen which are either

acids, alkalis, or intermediate between these; the occurrence or non-occurrence of allotropic change; the position of elements in the electrical series ; &c., &c.

We have traced the change from one class of compounds to another accompanying the increase or decrease in the relative mass of one of the elements in a series of binary or ternary compounds. We have seen basic oxides combining with oxygen and forming acidic oxides; we have compared basic hydroxides with acidic hydroxides of the same element. We have also traced the change from one class of compounds to another accompanying the substitution of a positive by a negative element in compounds of similar composition. We have compared acidic hydroxides of negative elements with alkaline hydroxides of positive elements, and both with neutral hydroxides of elements which were neither markedly positive nor strongly negative.

We have also learned something of the conditions under which classes of compounds are formed. We have seen some elements combining directly at ordinary temperatures with oxygen; others only at high temperatures; others only when they interact with compounds under such conditions that oxygen is produced in the interactions.

Of each compound we have asked ; of what elements, and of how much of each, is a reacting weight of this compound composed ? What can this compound do, and under what conditions does it perform its chemical functions? Composition, and properties, these have been our guides.

In examining the classification of elements and compounds we have travelled far from the starting point of our inquiries. We have not looked much to the general conditions of chemical change, or to the circumstances which modify the course and the final goal of chemical processes. We have been content to know that acids, for instance, are compounds of hydrogen with one or more negative elements, generally with oxygen and another more or less negative element, and that these compounds interact with metals, with basic oxides, and with alkalis, to produce salts. We have not asked ; under what conditions does this or that acid interact ? if two acids react with one alkali will one combine with the whole of the alkali, or will the alkali be divided between the acids ? But we must now come back to such inquiries as these.



227 BODIES interact chemically only when brought into very

close contact. The contact may be effected by dissolving, gasefying, or melting, one or more of the reacting substances; or, in some cases, by submitting the substances to very great pressure.

Barium chloride and sodium sulphate, for example, remain unchanged when mixed; but when aqueous solutions of these compounds are mixed, barium sulphate and sodium chloride are at once produced. Ammonia and hydrogen chloride gases combine to produce ammonium chloride. A mixture of iron and sulphur remains unchanged for an indefinite time ; but when iron filings are added to molten sulphur chemical change occurs and iron sulphide is formed. The same product is formed by exposing the mixture of finely divided iron and sulphur to a pressure of several thousand tons on the square

inch. 228 As a general rule chemical change proceeds more rapidly

between a solid and a liquid, or a solid and a gas, the more finely divided the solid is, that is, the greater the surface exposed by a given quantity of the solid. Thus a piece of iron oxidises slowly in ordinary air ; but very finely divided ironobtained by strongly heating iron tartrate in a glass tube and closing the tube while hot-oxidises so rapidly in ordinary air that the particles of the oxidising iron glow and emit light. Granulated zinc, that is zinc in thin irregular-shaped pieces, dissolves in dilute sulphuric acid much more quickly than a

compact mass of zinc. 229

If one or more of the possible products of a chemical interaction is gaseous under the experimental conditions, that

chemical interaction usually occurs readily. Thus calcium carbonate (CaCO,) rapidly interacts with dilute hydrochloric acid solution, to produce calcium chloride (CaCl,) which remains in solution, and carbon dioxide (CO) which escapes as a gas;

CaCO2 + 2HCIAq= CaCI, Aq +,0+CO, Calcium carbonate is decomposed by heat to solid calcium oxide and gaseous carbon dioxide : CaCO, (heated) = CaO + CO,. Zinc and dilute sulphuric acid interact at ordinary temperatures to produce zinc sulphate and gaseous hydrogen :

Zn +HSO, Aq= ZnSO, Aq + 2H. If one or more of the possible products of a chemical inter- 230 action between solutions is a solid under the experimental conditions, that chemical change usually occurs readily. Thus aqueous solutions of barium chloride and sodium sulphate interact when mixed to produce solid barium sulphate, and sodium chloride which remains in solution :

BaCl, Aq + Na SO, Aq = BaSO, + 2NaClAq. A solution of antimony chloride in hydrochloric acid interacts with water to produce solid antimony oxychloride, and hydrochloric acid which remains in solution : SbCl, (in HC1Aq) + H,0+ «H,0 = SbOCl + 2HClAq + xH,0.

Potassium acetate is soluble in alcohol, potassium carbonate is insoluble in alcohol; if carbon dioxide is passed into an alcoholic solution of potassium acetate, the formation of an insoluble compound becomes possible; this compound, potassium carbonate, is formed and precipitated. On the other hand, if acetic acid is added to an aqueous solution of potassium carbonate, the formation of a gas becomes possible ; this gas, carbon dioxide, is formed and the carbonate is decomposed. These reactions furnish an instance of the reversal of a chemical change by alterations in the experimental conditions such that in one case the formation of a solid, and in the other the formation of a gas, becomes possible.

The conditions which chiefly modify chemical changes, besides those already mentioned, are temperature, and the relative masses of the interacting substances.

Influence of temperature on chemical change. A 231 chemical change occurs within a definite range of temperature. Some changes take place readily and completely at ordinary temperatures; other changes begin only at higher temperatures.

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