Soap

(sõp.) [ Gr. sa pon. ] (Chem.) All soaps result from the action of the alkalies upon the oils and fats which action is called Saponification. In the manufacture of hard S., the alkali is prepared by boiling a solution of carbonate of soda (soda-ash) with lime to remove the carbonic acid, the clear solution of soda, or soda-ley, being drawn off from the insoluble carbonate of lime. The tallow is at first boiled with awe ak soda-ley, because the soap which is formed is insoluble in a strong alkaline solution, and would envelop and protect a quantity of undecomposed tallow; in proportion as the saponification proceeds, stronger leys are added, until the whole of the grease has disappeared. In order to separate the soap which is di olved, advantage is taken of the insolubility of soap in solution of salt; a quantity of common salt being thrown into the boiler, the soap rises to the surface, when the spent ley is drawn off from below, Social and the soap transferred to iron moulds that it may harden sufficiently to be cut up into bars. Perfumes are sometimes added; and marbling, when desirable, is effected by stirring into the S. during manufacture a solution of sulphate of iron, which is decomposed, black oxide separating in streaks and patches. Soft S. are generally made with potash and fish oil. Yellow S. contains resin. S. is soluble in pure water and alcohol; the solution in the latter gelatinizes when concentrated, forming what is called in medicine opodeldoc. If the S. solution is carefully evaporated, transparent S. is obtained. Earth and common metallic oxides form insoluble S.; and hence S. is wasted, and precipitates are formed with it, by hard water-that is, such as contains lime. S., if kept in a damp place, is capable of remaining com-Society, bi ned with a large quantity of water; this add sto its weight, and diminishes its utility, by rendering it liable to waste during use. [s. 834]