(from the same). Coagulation is when a fluid, or some part of it, is rendered more or less solid. This is variously effected, and from the different methods, as well as means, the appellations vary.
Heat and cold are the two principal natural agents for coagulating fluids. When heat is used by art, its effect is called coagulatio per separationem. When cold is thus made use of, its effect is called coagulatio per comprehensionem; implying that no part of the fluid is lost, but the whole brought into a solid state.
Different means coagulate different matters: thus heat coagulates salts by dissipating their moisture; cold coagulates water by freezing it; water coagulates camphor when dissolved in spirit of wine, by uniting with the spirit and rendering it a less perfect menstruum for the camphor; spirit of wine, if pure, coagulates the white of egg and other matters; and motion coagulates milk into butter, by uniting the particles of oil.
The coagulatio-continua is produced cither by im-pastation, that is, when powders, etc. are mixed with the fluid, or by condensation, that is, when coldness is applied to water so as to congeal it.
The coagulatio-partis is when one substance so adheres to another as to form a more solid body; for example, dry things with moist, oil with water, etc.
The coagulatio-totius is preternatural when heterogeneous matter is united; and natural, when homogeneous fluids are coagulated by way of generation.
Such are the logical distinctions of the Stahlians; but it is necessary to be more chemically accurate. Coagulation consists chiefly in precipitation, when the attraction of the fluid to the solid which keeps the latter suspended is weakened. Coagulations by heat are sometimes owing to evaporation, but more often to a chemical change, as in an instance soon to be mentioned. In many instances of apparent complete coagulation, the: fluid is only entangled between the spiculae of the crystals; and this is the case with the coagulation from cold or rapid crystallization. Sometimes it is occasioned by a chemical change. Albumen, for instance, when coagulated, is not again soluble in the same fluids as before; and butter will not again become milk with the albumen and whey. There are various instances of increased density, occasioned by different additions, which are not readily explained. The thickening of leather by astringent substances was one of these; but we now know it to proceed from precipitation of gelatine. The use of alum by the soap boilers, and in the coagulation of curd to produce whey, is not yet understood.