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usefully employed in discussions regarding the structure of the reacting weights of solid compounds.

Our conception of chemical composition has been widened 376 by the examination of the phenomena of isomerism.

A statement of the composition of a compound should tell the percentage composition of the compound; it should also tell the composition of a reacting weight stated in numbers of combining weights of each element, and the composition of a gaseous molecule stated in numbers of atomic weights of each element; if the compound is gasifiable, it should also give such an indication of the arrangement of the parts of the molecule relatively to each other as can be gained by studying the interactions of the compound and expressing these in a structural formula based on the hypothesis of atomic valency.

The formula which best expresses the composition of a compound also tells a great deal about the properties of the compound. A satisfactory structural formula suggests many of the characteristic reactions of the compound the composition of which it expresses.

The structural formulae of many compounds of carbon 377 which are acids have been determined; we are therefore able to trace some of the connexions between the properties of this class of compounds and their composition, using composition in the widest sense we have given to the term.

In the molecules of the greater number of the carbon acids an atom of carbon probably directly interacts with an atom of oxygen and with the atomic group OH; this statement is usually expressed by saying that these molecules contain the

group CO. OH (CO_H).

The following structural

formulae illustrate the statement concerning the composition of many carbon acids which has just been expressed in the symbolic language of the hypothesis of valency.

[blocks in formation]

The basicity of these acids is connected with, and is measured by, the number of CO.OH groups in the molecule; thus acetic, acrylic, and benzoic, acids are monobasic, succinic and phthalic acids are dibasic, and mellitic acid is hexabasic.

We can thus, in a great many cases, connect the group of properties connoted by the term acid with a certain arrangement of the parts of the molecules of the compounds which exhibit these properties.

In Chap. XIII. we found that a number can be assigned to each acid, called the relative affinity of the acid, which tells how much of a definite chemical change can be accomplished by that acid under defined conditions. An acid with a large affinity-constant is called a strong acid; an acid with a small affinity-constant is called a weak acid. If definite relations can be established between the values of the affinity-constants, and the compositions, of acids, a great advance will be made in solving the essential problem of chemistry, which is to connect changes of composition with changes of properties. Investigations have been made of late years in this direction, and many results have been obtained. Thus the relative affinities of the acids HCl, HBr, HI, HF, H,S, HCN, in aqueous solutions, are approximately in the ratio 89 89: 89: 30; 0.1 0.2. The change of composition from HCl to HBr or HI is attended with practically no change in the strength of the acid; but when an atom of fluorine is put in the place of an atom of chlorine, bromine, or iodine, in the molecule HX (X=Cl, Br, or I) this change is attended with a great decrease in the strength of the acid. Change of composition from HCl, &c. to HS or HCN is accompanied by a very great decrease in the strength of the acid.

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The ratio of the affinities of the three acids, of similar composition, H.PO,, H.PO,, and H.PO,, is approximately 77.8 74.5 : 61.8. The ratio of the affinities of the four similar acids H2SO,, H2SO,, H ̧S2O., and H2SO, is approximately 66.5 150·5: 178: 181·5; and the ratio for the pair of acids H.SEO, and H,SeO, is 45: 158. The basicities of the acids H.PO,, H PO, and H PO, are 1, 2, and 3, respectively; but all the acids of sulphur or selenion enumerated above are dibasic. In one series, H.PO, to HPO4, a decrease in the value of the affinity-constant is accompanied by an increase of the basicity, and also by an increase in the number of oxygen atoms in the reacting weights, of the acids. In the other series, the basicity remains constant, and an increase in the number of oxygen atoms is accompanied by an increase in the value of the affinity-constant.

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Relations have also been traced between the structural formulae of isomeric acids and the values of their affinity

constants. But enough has been said to shew the lines on which investigation is proceeding, and the importance of the results which are likely to be obtained.

The phenomena of isomerism exhibited by compounds are 378 more or less similar to those of allotropy exhibited by elements. The only instance of allotropy of elements to which the molecular and atomic theory can at present be applied in detail is that of oxygen and ozone (s. Chap. XVI., par. 323); in the other cases, allotropy is exhibited by elements in the solid state.

The application of the theory of atoms and molecules to the case of oxygen and ozone led to the conclusion, that the properties of gaseous molecules composed of 3 atoms of oxygen are different from the properties of gaseous molecules composed of 2 atoms of oxygen: the theory led us to associate change of properties with variations in the numbers, unaccompanied by variations in the nature, of the atoms constituting a gaseous molecule. The application of the theory to the cases of isomerism examined led to the conclusion that two gaseous molecules may be composed of the same number of the same atoms and yet differ in properties: the theory led us to associate change of properties with variations in the arrangement, unaccompanied by variations in the nature or number, of the atoms constituting a gaseous molecule.

The term polymerism is used to indicate the existence of 379 two or more different compounds, having the same composition but different molecular weights, and each capable of being formed from the other by some simple reaction, usually by raising or lowering the temperature. Thus, the molecular weight and composition of ethylic aldehyde are expressed by the formula CHO; when a little sulphuric acid is added to this compound, much heat is produced and the polymeride (para-ethylic aldehyde), CH,,O,, is produced; when this compound is distilled with a little acid, ethylic aldehyde, CHO, is obtained. Again, when amylene C.H1, is heated a portion of it is polymerised to diamylene C, H,

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Polymerism and allotropy are evidently more nearly allied than polymerism and isomerism.

The term allotropy is sometimes applied to compounds; it then signifies the existence of two or more varieties, or forms, of the same compound in the solid or liquid state. The differences between the various forms are physical rather than chemical; e.g. differences in specific gravity, melting

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point, crystalline form, &c. Thus calcium carbonate (CaCO2) crystallises in rhombic, and also in rhombohedral, forms; the rhombic crystals separate from hot solutions, the rhombohedral from cold solutions; when the rhombic crystals are heated they change to the rhombohedral forms.

The molecular and atomic theory cannot as yet be applied to give a detailed explanation of the allotropy of solid elements or compounds.

The molecular and atomic theory gives then a partial explanation of the phenomena of isomerism; it represents the change from one isomeride to another as a change in the configuration of a system of atoms; and in the hypothesis of valency it supplies a means whereby these changes may be pictured to the mind with some degree of clearness, and may be expressed in a consistent language. Structural formulae are the most developed forms of chemical language; they express much but not all; of them it may emphatically be said "they are wise men's counters but the money of fools."

CHAPTER XVIII.

THE PERIODIC LAW.

We have now to some extent studied the more important 381 relations which exist between changes of composition and of properties of compounds, and we have seen that there are definite points of connexion between the properties of a compound and the properties of the elements which compose it.

The meaning of the term composition widened as we proceeded. At first the composition of a compound was a statement of the quantity of each element combined in a definite quantity of the compound; then it was a statement of the number of combining weights of each element combined in one reacting weight of the compound; then it was a statement of the number of atoms of each element combined in a molecule of the compound; then it was not only a statement of the number, but also an attempted representation of the arrangement, of the atoms which formed the molecule of the compound.

As the meaning of composition has become wider, so have the terms in which composition is expressed become more symbolic.

The properties of elements and compounds studied have been the properties exhibited in those actions and reactions which result in the production of new elements and compounds. Chemistry considers elements and compounds when they form members of systems undergoing change, rather than when they are isolated from other kinds of matter.

Attempts have been made to indicate the methods by which chemists will probably be able to find, and state in quantitative terms, the connexions which certainly exist between the compositions and the chemical functions of compounds.

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