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; 01 02. 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 H.SO, H2SO,, H,S,O,, and H.SO,, 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 H,PO,, 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.

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 CH1 is heated a portion of it is polymerised to diamylene C,,H20. Polymerism and allotropy are evidently more nearly allied than polymerism and isomerism.

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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 (CaCO) 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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We have also glanced at the changes of energy which accompany changes of composition, and we have seen that when equilibrium is attained in a chemical system there is a certain distribution of the members of the system, and also of the energy of the system.

We have arranged compounds in classes, such as acids, alkalis, salts, acidic oxides, &c. &c. We have found that all the members of each class shew certain similarities of composition. We have traced some of the connexions between the composition of each class and the typical property or properties of the class.

It has been found possible to assign to each member of certain classes of compounds a number, called the affinityconstant of the compound, which tells how much of a specified chemical change will be accomplished under definite conditions by that compound; and it has been found possible to establish the fact that there is a definite relation between this value and the composition of the compound, and, in some cases, to state the nature of this relation.

We have traced certain elements through the changes which they undergo by combining with other elements to form compounds belonging to different classes; we have seen the properties of the elements modified by the nature, and relative quantities, of the other elements with which they are combined; but we have not altogether lost sight of the original elements in these changes. Each element to some extent impresses its own properties on all the compounds of which it forms a part.

Is it possible then to connect the properties of the compounds of an element, using the term properties in its widest sense, with some one definite and measurable property of the element itself? Can it be shewn that the properties of the compounds vary with variations in the chosen property of the element?

If this can be done, we shall have the basis of a satisfactory method of chemical classification both of elements and compounds.

We have had instances of a regular variation of properties of similar compounds of a group of allied elements accompanying variations in the atomic weights of these elements (s. especially Chap. XI. par. 182).

Various chemists, among whom Newlands must be especially mentioned, from time to time have drawn attention