ÆRU′GO (ē-). [L.] The rust of brass, bronze, or copper; verdigris.
ÆSCULIN. C21H24O13. A crystalline fluorescent substance existing in the bark of the horse-chestnut (æsculus hippocastanum) and of other trees of the genera Æsculus and Paria. In the above-named sources Æsculin is associated with another fluorescent body called Pariin.
Æ′′THER. See Ether.
ÆITHE′′REA (-thēré-). [L. pl.] Ethers.
ÆSTHET′ICS (ēz-). Syn. Æsthet′ica, L. Medicines or agents which affect sensation. See Anæsthetics and Hyperæsthetics.
ÆTHIOPS. See Ethiops.
AFFEC′TION. [Eng., Fr.] Syn. Affec′tio, L. In pathology, a term nearly synonymous with disease.
AFFINITY. Syn. Chemical affinity; Affinitas, L.; Affinité, Fr.; Verwandtschaft, Ger. If oil and water be shaken together they produce no change upon one another, as is proved by their separating into two layers with their properties unaltered, when the mixture is allowed to remain at rest for a short time. Such bodies are said, in chemical language, to have no affinity for one another. If iodine and metallic mercury be rubbed together in a mortar they will unite in definite proportions by weight, and form a combination possessing properties totally distinct from those of its constituents. Thus, iodine is a greyish, metallic-looking solid, convertible into a violet vapour by heat, perceptibly soluble in water, and capable of producing a blue compound with starch. Mercury is a metallic, silvery-looking liquid. The product of their union (biniodide of mercury) is a scarlet powder, destitute of metallic lustre, convertible into vapour by heat, without the production of violet fumes, insoluble in water, and incapable of developing a blue colour with starch. Again, the greenish-yellow and intensely poisonous gas, chlorine, unites in definite proportions by weight with the soft, wax-like, and highly poisonous metal sodium to produce the white crystalline solid chloride of sodium (common salt), a compound which, except in very large quantities, is not only not poisonous, but actually beneficial to health.
Such combinations are called chemical compounds, and the force which binds their constituents together is distinguished from all other attractive forces by the term affinity or chemical affinity. Bodies united by affinity are also said to have united chemically.
Affinity is in most cases exerted between different substances, in which respect it resembles adhesion; but bodies united by adhesion, e.g. ink to paper, paint to wood, &c., unlike those united by affinity, suffer no change of properties.