Chymical attraction, or affinity, is that power by which dissimilar substances placed under certain circumstances are enabled to unite, and form new aggregates, in which the properties of the component particles are lost or changed. Its action is confined to the minute atoms or ultimate particles of bodies, and is exerted only at insensible distances or the closest proximity: not indifferently, however, but electively, between the particles of all bodies. The result of its operation is a combination of the constituent particles of the substances, so intimate that the components cannot be recognised, nor separated by any mechanical force. Thus, lime acts as a powerful caustic when applied to animal matter, and is partially soluble in water; phosphoric acid has an acid taste, and is very soluble in water; but phosphate of lime, the compound produced by the chymical combination of these substances, is inert when applied to animal matter, insipid, and insoluble in water; and cannot be again resolved into lime and phosphoric acid by any mechanical power.

Chymical combination, therefore, is the result of the affinity of two or more substances for each other. It differs from mixture, in which the substances are only blended without acquiring any new properties, and in which the dissimilar parts are easily discovered, and may be separated by mechanical powers. Chymical compounds can, however, be decomposed, either by exposure to a high temperature, which weakens the force of attraction existing between their principles; or by mixture, under favourable circumstances, with some other chymical agent, which has a more powerful affinity for one of the components than these have for each other; thus, if a solution of camphor in alcohol be poured into water, the alcohol, having a stronger affinity for water than for camphor, leaves the camphor to unite with the water, and thus sets the camphor free. In operating by such means, which constitute chymical analysis, the principles of a compound may be ascertained.

1 For many of the following observations on affinity I am indebted to the remarks of my late respected friend Dr. Murray: see his System of Chymistry, vol. i.

As Analysis separates compounds into their constituent principles, so Synthesis may re-produce them by re-combining these principles; and when this can be effected, it is the surest proof of the accuracy of any analysis. In many instances, however, this is impossible; and the evidence of the truth of an analysis is to be drawn from other sources.

It is an acknowledged law of chymical affinity, that a compound "does not possess properties merely intermediate between those of its component parts, but has acquired others more or less new." One of the most general changes is that of form. The combination of two gases, for example, muriatic gas and ammoniacal gas, forms a fluid; that of two fluids, again, may form a solid, as is demonstrated by dropping into a saturated solution of muriate of lime some concentrated sulphuric acid; and the common process of solution presents to us the fact, that, by the combination of a solid with a fluid, the solid assumes the fluid form. In the last-mentioned instance the fluid is generally regarded as the active substance; but, nevertheless, the attraction of affinity is reciprocal; and, thence, the general mode of expressing the fact, that the fluid dissolves the solid, or is the solvent or menstruum, is, in strict language, erroneous. These terms, however, are more correctly applied, when the properties of the solid, except form, are scarcely sensibly altered; as, for example, when common salt, chloride of sodium, is dissolved in water.

Chymical combination produces an alteration of density- that of the compound not being the mean of the components, but often different. In the greater number of cases the density is increased; and there is a diminution of volume, owing, probably, to the compound atom being of a form admitting a more compact aggregation than the component atoms in their separate state; but the specific gravity of a compound cannot be determined by calculation from the specific gravity of its ingredients. There are cases of combination, however, in which the density is diminished, and there is an increase of volume in the resulting compound : for instance, when a solid is dissolved by a liquid, the increase of volume acquired by the solid, in passing into the fluid state, may be greater than the condensation resulting from its union in that state with the liquid; and this happens from the solution of a considerable number of the salts in water.

"The exertion of chymical attraction is generally accompanied by a change of temperature" Thus, if four parts of sulphuric acid and one part of water, both at the temperature of 30°, be mixed together, the temperature of the mixture rises to 200°; and the density of the compound is much greater than the mean of the densities of the components. The heat also which is evolved by combustion, and in fermentation, is the direct consequence of chymical combination. In all cases the increase of temperature is accompanied with an increase of density, to which, and the change of form suffered by one or both of the components, its production may be ascribed. Thus, water, which is solidified, or loses its liquid state, by being mixed with quicklime, parts with a large portion of caloric; the contrary effect, however, or an absorption of caloric, is also produced by chymical combination, when the density of the compound is less than the mean; as, for instance, when by solutions of salts in water, or the conversion of some other solids into fluids, very intense cold, greater than any natural cold, is artificially produced.1

The exertion of chymical affinity is influenced by various circumstances : these, according to Berthollet, are, quantity, cohesion, insolubility, specific gravity, elasticity, and inflorescence.