SOAP  , a chemical

compound or mixture of chemical compounds resulting from the interaction of fatty oils and fats with alkalis . In a scientific definition the compounds of fatty acids with basic metallic oxides, lime, magnesia, lead oxide, &c., should also be included under soap; but, as these compounds are insoluble in water, while the very essence of a soap in its industrial relations is solubility, it is better to speak of the insoluble compounds as " plasters, " limiting the name " soap " as the compounds of fatty acids with soda and potash . Soap both as a medicinal and as a cleansing agent was known to Pliny (H.N. xxviii . 51), who speaks of two kinds—hard and soft—as used by the Germans . He mentions it as originally a Gallic invention for giving a bright hue to the hair (" rutilandis capillis ") . There is reason to believe that soap came to the Romans from Germany, and that the detergents in use in earlier times and mentioned as soap in the Old Testament (Jer. ii . 22; Mal. iii . 2, &c.) refer to the ashes of plants and other such purifying agents (comp. vol. x . P . 697) . Soap appears to have been first made from goat's tallow and beech ash; in the 13th century the manufacture was established at Marseilles from olive oil, and in England during the next century . The processes and extent of the manufacture were revolutionized at about the beginning of the 19th century by Chevreul's classical investigations on the fats and oils, and by Leblanc's process for the manufacture of caustic soda from common salt .

Previous to Chevreul's researches on the fats (1811–1823) it was believed that soap consisted simply of a binary compound of

fat and alkali . Claude J . Geoffroy in 1741 pointed out that the fat or oil recovered from a soap solution by neutralization with a mineral acid differs from the original fatty substance by dissolving readily in alcohol, which is not the case with ordinary fats and oils . The significance of this observation was overlooked; and equally unheeded was a not less important discovery by Scheele in 1783 . In preparing lead plaster by boiling olive oil with oxide of lead and a little water—a process palpably analogous to that of the soap-boilerhe obtained a sweet substance which, called by himself " Olsiiss " (" principium dulce bleorum "), is now known as " glycerin." These discoveries of Geoff roy and Scheele formed the basis of Chevreul's researches by which he established the constitution of oils and the true nature of soap . In the article OILS it is pointed out that all fatty oils and fats are mixtures of glycerides, that is, of bodies related to the alcohol glycerin C,H6(OH)3, and some fatty acid such as palmitic acid (C16H31O2)H .