Affinity, (from affinis, connected). Attractio, chemical affinity, also called elective attraction, may be defined the superior attraction evinced by all bodies for some particular substance; an attraction so great, that the component parts of a body are separated to enable the ingredients to form a new union.
Attraction is of different kinds in nature, though probably they all depend ultimately on the same principles: they are, 1. The attraction of gravitation. 2. The magnetic attraction. 3. The attraction of electricity. 4. The attraction of cohesion or of aggregation. 5. Chemical attraction, which is that tendency which bodies have, however different, to unite together, and to remain in union: e. gr. an acid unites with a metal, an earth, or an alkaline salt, and with either of these the acid forms one body; which body does not consist of a combination of the properties of the acid and the metal, etc.; but these losing their original properties on their union, a new body, different from either, is formed. 6. Elective attraction, or affinity, as already explained.
Chemical attraction does not usually take place but when the respective bodies, or one of them, are in a fluid state. Before chemical attraction can take place betwixt two or more bodies, it is necessary to destroy their attraction of aggregation or cohesion; this is effected by dissolving them. The component parts of bodies cannot come into the necessary contact with each other until the integrant parts of the bodies, which are to act and be acted on, are separated by a solution of them. Dry bodies, however finely powdered, sometimes unite chemically; and sometimes elective attractions take place when both the bodies are in a dry powder. Attraction of aggregation requires only the application of surfaces; but chemical attraction usually requires fluidity.
The power in bodies on which their various transpositions and combinations depend, and which is called their affinity, is a term like the Newtonian attraction, designed to express not the cause but the effect. When an acid spontaneously quits a metal to unite with an alkali, it is said that it hath a greater affinity to the alkali than to the metal; this is only to say. in other words, that it will unite with the alkali in preference to the metal.
The doctrine of the affinities of bodies is of very extensive use in chemical pharmacy; for as several processes are founded on it, so if an error happens, and the medicine proves unfit for its intended use, it may be rendered applicable to other purposes, by such transpositions of their component parts as are pointed out by the knowledge of their affinities. Combinations and separations that are chemical depend on elective attraction.
Tables of elective attraction include every chemical fact of importance; as the whole science consists only of the union of bodies, and the separation of the component parts of such as are not simple. The whole doctrine would however be misplaced in a work of this kind; yet, as the subject is so closely connected with chemical pharmacy, we must add the outlines. Chemical affinity or attraction was first spoken of by Barchusen, from whom Boerhaave adopted this or similar language. Geoffroy however first collected all the known facts of this kind in the form of a table, in the year 1718. The term elective attraction - perhaps not strictly proper, as implying a choice, yet not ill applied, as chemical attractions are not indiscriminate - was, we believe, first introduced by Dr. Cullen, who made some important advances in giving chemistry a scientific form. He was followed by Dr. Black, and afterwards by Bergman. Yet Geoffroy's table was published, with improvements by Grosse, in 1730; Gel-lert, in 1750; Andiger, in 1756; Marchen, in 1762; De Fouchy, in 1773; Machy, Erxleben, Viegel, and Bergman, in 1775. Rouelle's, Limbourg's, and Sage's tables appeared between 1760 and 1775; but the dates we cannot now ascertain. Bergman's tables are usually preferred, though that annexed to the translation of Gren's chemistry we think superior. We shall however give that of Bergman, omitting the affinities of platina, and with some of the modern improvements; as that of Gren is so minutely exact, as to confuse, in some degree, the less experienced chemist; and unnecessarily so for our present purpose. Those who would engage in this subject at greater length will find excellent assistance in the new French Encyclopedia, or in the Annales de Chimie, vols. 36, 37, 38 and 39, where M. Berthollet has given some excellent papers on the laws of affinity. M. Morveau has also published Prin-ripestheoriques and Pratiques des Affinitesou Attractions Chimiques.
The papers have been translated in some of the earlier volumes of the Philosophical Magazine, and the later work on this subject has been lately translated in two volumes 8vo. We may just add, that the symbolical characters of chemical substances, Used in the earlier tables, are now generally disused, as they are so numerous as to be with difficulty learnt or recollected.
It is no part of our present object to follow minutely the disquisitions of Morveau. It is sufficient for our purpose to point out, by the tables, the comparative force of attraction of a given substance for the various other chemical bodies. It has however been a desideratum to ascertain the precise numerical degree of attraction of these different bodies; and Mr. Kirwan has laboured with great success in this field. We need only refer to his labours in the Philosophical Transactions for 1780 and 1781, as well as the fourth and subsequent volumes of the Irish Transactions. M. M. Wenzel, A chard, Morveau, and Berthollet, preceded him with unequal success.
To understand the following table, it is only necessary to remark, that the substance at the top in capital letters has the greatest affinity with that immediately under it, and a less, in succession, with those which follow. Of consequence, the substance most remote from the uppermost will be separated from an union with it by one nearer it in the series. Where the spaces are different it shews an unusual difference in the powers.