[AS., akin to L. sapo.] A mixture of oil or fat with soda or potash for washing. Since the cheapening of caustic soda by the Le Blanc process, soda is chiefly used instead of potash as the alkali of soap. Common soap is a compound of fat or oils (q.v.) and caustic soda. Many kinds of soap are made, but they all consist of some fatty substance (as tallow) boiled with an alkali - either caustic soda or caustic potash. Yellow soap is made from tallow and caustic soda colored by rosin ; mottled soap, from dripping, etc., boiled with caustic soda ; Castile soap, from olive oil and caustic soda; brown Windsor, from equal parts of tallow and olive oil boiled with caustic soda; whiteor curd soap, from tallow and caustic soda. Transparent soap, is made by dissolving curd soap in spirits of wine. Marine soap, is made from cocoa-nut oil and caustic soda; it will dissolve in salt water (which common soap will not do), and is therefore much used on board ships. Soft soap is made by boiling caustic potash with some fish-oil. - Soapstone, or talc, is a silicate of magnesia used to make stoves, hearths, crayons, etc.