SOAP, strictly speaking, a salt con sisting of a fatty acid in combination with a metallic base. In common par lance, however, the term is applied to the soluble salts formed by the union of the fatty acids with the alkalies. If oil and water be shaken together mechanical union will take place; but on allowing the mixture to rest, the oil will gradually separate and float on the surface of the water. If a small quantity of caustic soda or potash be added to the mixture, and it be then agitated, union will take place between the three bodies, a milky fluid being formed. If a sufficient quan tity of alkali has been added, and the solution be boiled, it gradually becomes clear, giving rise to a soapy fluid, which froths strongly on agitation, presenting all the properties of a solution of soap. If to a portion of this clear liquid a strong solution of common salt is added, a peculiar curdling is produced. The liquid separates into a clear fluid, con taining glycerin, while the curdy portion rises to the surface. This substance is the fatty acid of the oil, in combination with the alkali used and a certain proportion of water, and if pressed and dried, ex hibits the properties of ordinary soap.
Ordinary soaps are of two kinds,—soft and hard. Soft soap is a combination of some fatty or oily substance with potash, and contains an excess of alkali; hence it is used for cleansing purposes where very highly detergent powers are re quired. The hard soaps are combinations
of the fatty acids with soda; the principal varieties being yellow soap, made from tallow and palm oil, and containing a certain proportion of resin to give it lathering properties; curd soap, which is made from tallow, only a small portion of olive oil or lard being added, to give it softness; mottled soap, which is pre pared from tallow, palm oil, and kitchen stuff, and contains a portion of insoluble iron soap giving it a marbled appearance. Marseilles and Castile soaps are made of olive oil and soda, a small quantity of sulphate of iron and sulphuretted lye being added to them while in a pasty con dition. The object of marbling soap with an insoluble matter is to show that they contain but little moisture, since, if too large a proportion of water were present, the coloring matter would sink to the bottom and remain there, instead of be ing diffused through the mass.
The United States stands at the head of all other countries in the use if not in the manufacture of soap. New York, Illinois, New Jersey, Ohio and Pennsyl vania lead, in the order named, all other states. Over 2 billion pounds of hard soap (including about 170,000,000 pounds of toilet-soap) are annually produced and 57,000,000 pounds of soft soap.