Affinity is exerted at immeasurable distances, therefore substances to be submitted to its influence must be brought into (apparently) actual contact. This condition is frequently fulfilled by the vaporisation, fusion, or solution of one or more of the bodies to be submitted to its action.
In many instances substances which have no affinity for one another at ordinary temperatures manifest this power when heated.
Whenever chemical union takes place, heat is invariably evolved; conversely, the decomposition of a chemical compound is always accompanied by an apparent loss of heat or reduction of temperature.
Finally, the most striking phenomena characteristic of, and accompanying, chemical affinity are, development of heat, change of properties, and union in definite or constant proportions by weight.
AFFUSION. In chemistry, the washing of a precipitate, &c., for the purpose of removing soluble matters. In medicine, affusion is of three kinds:—
1. Lotions, which consist in washing a part of the body with a sponge or rag soaked in a liquid.
2. Aspersions, which consist in throwing a liquid drop by drop, like rain, upon the body.
3. Shower baths, which consist in allowing a number of small streams of water to fall from a height upon the surface of the body. If the water fall from a considerable height, affusion is then termed douche by the French.
AFT′ER-DAMP. Syn. Choke-damp. Carbonic acid gas resulting from explosion of air and fire-damp (light carbonetted hydrogen) in coal mines.
AFT′ER-PAINS. Those following childbirth. The only remedy is patience; they may, however, be frequently alleviated by small doses of morphia or liquor opii sedativus. Heated cloths and warm fomentations are sometimes useful, particularly if assisted by moderate but sufficient pressure on the abdomen, by means of a broad bandage. They seldom follow with severity the first birth.