SOAP, a chemical compound, or mixture of chemical com pounds, resulting from the interaction of fatty oils and fats with alkalis, i.e., the salts of the fatty acids. In a scientific definition, the compounds of fatty acids with basic metallic oxides, lime, magnesia, lead oxide, etc., 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 customary to use the term "soap" for the compounds of fatty acids with potash, soda and ammonia ; the water-insoluble soaps of the other bases may be termed "metallic soaps" or "plasters." Soap appears to have been first made by boiling goat's tallow and causticised beech ashes (Pliny) ; the resulting soft potash soap was converted into hard soda soap by treating the paste repeat edly with salt. Later wood ashes were replaced by soda ashes from sea-plants, kelp, barilla, etc. In the i3th century the in dustry was introduced from Italy and Germany into France, and the manufacture of olive oil soap established at Marseilles. In England the trade flourished from the 14th century; during the reign of Charles I. a monopoly of soap-making was farmed to a corporation of soap-boilers in London. From 1712 to 1853 an excise duty ranging from id. to 3d. per lb. was levied on soap made in the United Kingdom; despite this heavy impost (equal, when 3d., to more than the cost of the soap) the industry ex panded with considerable rapidity. In 1793, when the duty was 2,1d. on hard, and lid. on soft soap, the revenue yielded was a little over L400,000; in 1815 it was almost .1750,000; in 1835, when the duty was levied at 12d. and id. respectively (and a drawback allowed for soap used in manufactures) the revenue approached ir,000,000. In 1852, the last year in which soap was taxed, it amounted to L1,126,046, with a drawback on ex portation of £271,000.
The processes and extent of the manufacture were revolu tionized during the first half of the 18th century as a result of Chevreul's classical researches on the constitution of oils and fats, and by the introduction of the Leblanc process (invented 1791) for the manufacture of soda from brine. It was some time, however, before manufacturers could be induced to sub stitute the new article for natural barilla soda; in fact, Muspratt, the founder of the English alkali industry, had to distribute gratis scores of tons of the manufactured soda in order to popu larise the material. The progress of the industry was assisted by further improved processes of caustic alkali manufacture, and by the utilisation of coconut, palm and other oils to sup plement the tallow and olive oil exclusively used in older times.
in 1741 pointed out that the "fat" (actually fatty acids) recov ered from a solution of soap by neutralisation with mineral acid, differed from the original fatty substance by dissolving readily in alcohol, in which ordinary fats were insoluble. The significance of this fact was overlooked, and equally unheeded was a not less important discovery by Scheele in 1783. In pre paring lead plaster by boiling olive oil with oxide of lead and a little water—a process palpably analogous to that of the soap boiler—he obtained a sweet substance which, termed by him "Olsiiss" (principium dulce oleorum), is now known as glycerin. These discoveries of Geoffroy 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, i.e., of compounds (esters) of the trihydric alcohol glycerol (glycerin) and some fatty acid such as palmitic acid, etc. Under suitable conditions, the decomposition of a glyceride into acid and glyc erin ("hydrolysis" or "saponification") takes place when the glyceride is distilled in superheated steam, or boiled in water with a certain proportion of caustic alkali. In the latter case the alkali combines with the fatty acid to form its potash or soda salt (soap), thus: (It may be remarked that the term "saponification" is used scientifically to denote the decomposition by water—hydrolysis of an ester, in this case a glyceride, into its constituent alcohol and acid ; it does not necessarily connote the formation of "soap") The most important constituent fatty acids of the natural fats are palmitic, stearic and oleic acids, and it is sufficient to regard these as the principal fatty bodies concerned in soap-making. The general characters of a soap are a certain greasiness to the touch, ready solubility in water, with formation of more or less viscid solutions, which on agitation yield a tenacious froth or "lather," and an indisposition to crystallize : hot concentrated solutions are slimy and, on cooling, set to jellies. Soaps give an alkaline reaction in water solution (due to hydrolysis) and have an acrid taste: in the pure condition—a state never reached in commercial practice—they have neither smell nor colour. Al most without exception potash soaps, even if made from the solid fatty acids are "soft" (glutinous jelly) and soda soaps, includ ing those made from the fluid olein are hard; there are, how ever, considerable variations in consistency according to the prevailing fatty acids. Almost all soda soaps are precipitated from aqueous solution by the addition of sufficient common salt. Potash soap, with the same reagent, undergoes double decom position—a proportion being converted into the soda soap with the formation of potassium chloride.