AFFINITY is a term used to express the tendency of different substances to unite che mically. The expressions chemical affinity and chemical attraction are applied indifferently to that power by which bodies combine and form compounds always possessing some pro perties very different from those of their con stituents, and frequently diametrically oppo site to them. It differs from the attraction of gravitation in not acting on masses, and only at sensible distances. In this last pro perty it resembles cohesive affinity, but is dis tinguished from it by occurring only between the particles of dissimilar bodies. Thus, the particles of a mass of sulphur are held toga tiler by cohesive affinity, and so also are those of a mass of copper ; but if a particle of sul phur be brought into contact with a particle of copper (under a particular application of heat), the two particles being different, and possessing chemical affinity for each other, unite by this power, and form sulphuret of copper.
In all mere mixtures, such as alcohol and water, or salt and water, the two may act to gether in almost any proportions; hut in the chemical compounds brought about by affinity, the proportions are always definite. Some liquids, such as mercury, water, and oil, have no tendency to unite in any proportion what ever, unless by indirect means.
The simplest eases of chemical affinity are these in which two bodies unite into a binary compound. This is the result of what is termed single affinity, and this power may be exerted between two elementary or two com pound bodies ; and also, though it occurs more rarely, between an elementary and a compound substance. Sulphur and copper for example, both elementary bodies, readily unite when heated; and sulphuric acid and oxide of copper, both compounds, combine with great readiness.
The effects of chemical combination, thus induced, are very remarkable. For instance, sulphur is yellow and copper is red, but a compound of the two is black. Acids give a red tint to vegetable blues, and alkalies give them a green tint ; but a salt compounded of an acid and an alkali seldom affects the co lour. There are numerous other cases in which the form, colour, smell, taste, density, and other physical qualities, and the chemical properties of fusibility, volatility, solubility, and tendency to combination, in the com pound, bear no resemblance to those of its constituent parts.
The force with which bodies chemically unite arises from mutual and equal affinity : thus, sulphuric acid and potash combine, not merely on account of the affinity of the acid for the alkali, but of the alkali equally for the acid. Each substance, too, has a power of unity with many others; and as this power or tendency is stronger in some instances than in others, there exists what is termed elective affinity, or a sort of preference (so to speak) in the combining action. An example will illustrate this ;—Nitric acid is capable of com bining by single affinity with lime or with magnesia ; and if some dilute nitric acid, con taining 54 parts of real acid, be mixed with 28 parts of lime, the earth will be dissolved in the acid, and a neutral solution of nitrate of lime is obtained. A similar quantity of this acid forms a neutral solution of nitrate of magnesia by combining with 20 parts of that earth. Now, if we mix together 54 parts of nitric acid, 28 of lime, and 20 of magne sia, it might be supposed that the acid—which is of course incapable of dissolving the whole of both of the earths—would dissolve them in the proportions of 14 of lime and 10 of mag nesia : it is found, however, that this is not the case, for the whole of the lime is dis solved, and the magnesia entirely left. This arises from the nitric acid having an elective affinity for lime rather than for magnesia.
To exhibit the degrees of elective affinity, tables are constructed, in which the substance whose affinities are to be expressed is placed at the head of a column, and is separated from the rest by a horizontal line; beneath this line are arranged the bodies with which it is capable of uniting, in the order of their respective forces of affinity ; the substance which it attracts most strongly being placed nearest to it, while that for which it has the least affinity is placed at the bottom of the column. Such a table is useful, but not always accurate.