physical character, usually colour; thus one sometimes speaks of the brown oxide of chromium, the black, or the magnetic, oxide of iron.

The various systems of naming binary compounds are summarised in the following examples.

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Sulphur oxides.

SO Sulphurous oxide; Sulphur dioxide; Sulphurous anhydride.

SO, Sulphuric oxide; Sulphur trioxide; Sulphur peroxide; Sulphuric anhydride.

Chromium oxides.

CrO Chromous oxide.

Cr2O, Chromo-chromic oxide.

Cr.O, Chromic oxide; Chromium sesquioxide; Green oxide of chromium.

CrO, Chromium dioxide; Brown oxide of chromium.

CrO Chromium trioxide; Chromium peroxide; Chromic anhydride; Red oxide of chromium.

The nomenclature of many compounds of three or more 145 elements is based on the relations which exist between acids and salts. To each acid is given a name indicative, as far as possible, of its composition. Prefixes and terminations are used as in the naming of binary compounds. Thus all acids obtained by combining sulphur with hydrogen and oxygen are called sulphur acids; those formed by the combination of chlorine with hydrogen and oxygen are called chlorine acids ; those produced by uniting nitrogen with hydrogen and oxygen are called nitrogen acids; and so on.

The following examples shew how one acid is distinguished from others of the same series.

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The three acids H.PO, HPO,, H.PO, are all called phosphoric acid because they are all obtainable from the same oxide or anhydride, P,O,. The composition of the acid formed by the interaction of this oxide with water varies according to the relative masses of the interacting compounds, and the temperature; thus

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That acid of the three from which the greater number of well known and stable salts are obtained, viz. H,PO, is called orthophosphoric acid (opos = right or true); the acid HPO, may be obtained from HPO by removing water (H ̧PO-H ̧0=HPO2), it is generally called metaphosphoric (meta implies change of composition); the third acid HP,O, may be produced by heating HPO (2H,PO, heated = Í ̧P ̧0, + H ̧O), it is called pyrophosphoric acid.

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The names of the salts obtained from a given acid by causing it to interact with metals, basic oxides, or alkalis, are derived from the name of the acid; each salt is distinguished from others by the name of the metal or metals which form part of its composition. Thus the salts obtained from sulphurous acid are called sulphites, those from sulphuric acid are called sulphates, and so on.

Hypochlorous acid; HClO.
KCIO Potassium hypochlorite.
NaClO Sodium

Chlorous acid; HCIO,.

AgCIO Silver chlorite.
Pb2CIÓ, Lead

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Mercury sulphates.

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Hg,SO, Mercurous sulphate.
HgSO, Mercuric

Nitric acid; HNO,.

Iron nitrates.

Fe2NO, Ferrous nitrate.
Fe3NO Ferric

These examples shew the use of the adjectival form of the name of the metal, and the meaning of the terminations -ous

and -ic, in naming salts. A salt whose name ends in -ous is composed of less of the non-metallic elements, relatively to a fixed mass of the metal, than a salt of the same acid and the same metal whose name ends in -ic.

Ternary compounds (compounds of three elements) which 147 are not salts, as we are using this term, are generally named on the same principle as that which guides the nomenclature of binary compounds. Thus BiOCl is called bismuth oxychloride; BiSCI, bismuth sulphochloride; HgBrI, mercury bromoiodide, or iodobromide.

The nomenclature of carbon compounds cannot be discussed 148 here; suffice it to say that a name is usually given to each class of these compounds and that the individual members of this or that class are distinguished according to their composition. Thus, as we have a large class of acids, so we have a class of carbon compounds shewing certain common properties and certain well marked analogies in composition called alcohols; to another group of carbon compounds the name aldehydes is given; and so on.

M. E. C.

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CHAPTER XI.

149

CHEMICAL CLASSIFICATION.

WE have now gained some fairly clear notions of the methods adopted in chemistry for classifying elements and compounds. Similar elements are put into the same class. Similar elements are those which interact with other elements and with compounds under similar conditions to produce similar compounds.

Compounds again are said to be chemically similar or analogous when their compositions and their properties are similar.

The questions to be answered with regard to any element before its position in a scheme of chemical classification can be determined are such as these ;-Does the element combine with oxygen? Under what conditions are its oxides formed? What is the composition of each of these oxides? Are its oxides basic or acidic? Does it combine with hydrogen? What is the composition of its hydride or hydrides? Does it form any acids? Under what conditions are these acids produced, and what are their compositions and properties? Does it interact with acids to form salts? What are the products of its interaction with water? Or is it unchanged when brought into contact with water or steam? Does it form chlorides, bromides, oxychlorides, &c.? Under what conditions are these compounds produced? What are their compositions, and how do they interact with other bodies?

As chemistry is the study of the connexion between changes of composition and changes of properties, the subject of classification must be all important in chemistry. We must therefore proceed to examine a few classes of elements that we may learn how answers are gained to such questions as these

just proposed, and what kind of answers they are which are gained.

The three elements chlorine, bromine, iodine, are placed in 150

Occurrence.

table tissues.
Poisonous.

None of these elements is found uncombined 151 with others. The commoner compounds are those with sodium, potassium, calcium, and magnesium. Chloride, bromide, and iodide of potassium, sodium, calcium, or magnesium occur in sea water, in mineral waters, and in some rocks. Chlorides of some or all the metals named are widely distributed; bromides occur in smaller quantities; and iodides, with iodates (salts of the acid HIO), are found only in very small quantities.

Preparation. Sodium (or potassium) chloride, bromide, or 152 iodide, is mixed with manganese dioxide and sulphuric acid, and the mixture is heated; manganese sulphate, sodium sulphate, water, and chlorine, bromine, or iodine, are formed. Putting X as = Cl, Br, or I, the change may be thus expressed ;

2NaX + 2H2SO ̧ + MnO2 = MnSO, + Na ̧SO̟ ̧ + 2H ̧O + 2X.

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