Stearic Acid (Gr. 'στέαρ, tallow), a fatty acid obtained from mutton suet, and other fats that contain stearine, by saponifying suet and decomposing the hot solution of the soap with hydrochloric, or still better with tartaric acid. The oily acids are next submitted to pressure between hot plates, by which means a large portion of the oleic acid is separated; the solid residue is then to be purified by recrystalliza-tion from alcohol three or four times. Its formula is HC18H36O2. When recrystallized from ether, until the fusing point becomes constant at 159°, and slowly cooled, the acid forms beautiful colorless, transparent,rhombic plates; these melt into a colorless oil, tasteless and without odor, and when quickly cooled the substance concretes in a white crystalline mass, which is insoluble in water, but readily forms with hot alcohol a solution having acid reaction. It is the material of stearine candles. Stearic acid exists in fats in combination with glycerine, forming stearine, from which it is separated by saponification. (See Glycerine.) It combines with numerous bases, and forms with them both normal and acid salts, called stearates.

Stearate of soda is the basis of ordinary hard soap; stearate of lead is a constituent of lead plaster.