The soluble or detergent soaps are prepared by the action of an alkali upon a fat or oil, the potash soaps being soft, and those of soda being hard. They contain glycerin unless this is removed by washing, are soluble in alcohol and water, and have an alkaline reaction.

Soap (sapo), Castile or hard soap, is prepared by the action of sodium hydroxide on olive oil. It is used in the manufacture of pills, soap liniment, chloroform liniment, and saponified tooth powders. (For the chemic reaction see above, under "Fixed Oils and Fats.") Some time ago a proprietary house put out a preparation described as acid sodium oleate. It was extensively prescribed by physicians, though it was nothing but Castile soap containing free fatty acid.

Soft soap or green soap (sapo mollis) is prepared from potassium hydroxide and linseed oil, without the removal of the developed glycerin, and is employed extensively for cleansing the hands and skin preparatory to operative work. A liquefied form of it is the liniment of soft soap (linimentum saponis mollis), commonly called the "tincture of green soap," made by dissolving soft soap in alcohol and adding oil of lavender flowers.