AFFINITY. (1) Neighborhood; (2) relationship by marriage; (3) union, connection.
In ordinary language and law, literal.. ly, the relationship contracted by mar riage between a husband and his wife's kindred, or between a wife and her hus band's kindred. It is opposed to con sanguinity, or natural relationship by blood. It is of three kinds: (1) Direct, viz., that subsisting between a husband and his wife's blood relations, and vice versa; (2) secondary, or that which sub sists between a husband and his wife's relations by marriage; and (3) collateral, or that which subsists between a hus band and the relations of his wife's relations.
In biology, a resemblance, or resem blances, on essential points of structure, between species, genera, orders, classes, etc., really akin to each other, and which should be placed side by side in any natural system of classification. Affinity differs from analogy, the latter term be ing applied to resemblances between ani mals or plants not really akin, but which ought to be more or less widely separated in classifications. Thus the falcons, the hawks, the eagles, etc., are related to each other by genuine affinity; but the similarity on certain points, such as the possession of retractile claws, between the raptorial birds and the feline race of mammals, is one only of analogy.
In chemistry, chemical affinity, or chemical attraction, is the force by which union takes place between two or more elements to form a chemical compound.
According to another definition, it is a force exerted between two or more bodies at an infinitely minute distance apart, by which they give rise to a new sub stance, having different properties to those of its component parts. Elements have the greatest affinity for other ele rdents which differ most in their chemi cal properties. Thus, H has great affinity for CI and 0, but the affinity between 0 and Cl is much weaker. Acids unite readily with alkalies; most metals, with sulphur. A strong acid generally expels a weaker one. But when two salts are fused, if a more volatile compound is formed, it is driven off. The relative affinities between different substances varies with their temperature, insolu bility, and power of vaporization. Alter nation of temperature alters the affinity; thus, mercury heated to its boiling-point absorbs oxygen, which it liberates at a higher temperature.
Affinity of solution is such an affinity as exists between a soluble salt and the fluid in which it is dissolved. Till the liquid is saturated with the salt, the two can combine in an indefinite ratio, in stead of being limited to the fixed pro portions in which alone chemical affinity operates.