FAT OF COCHINEAL INSECT. 143 



These two acids have an obvious relation to each other. We 

 may consider them as consisting of a common radical, C 34 H 33 , 

 united to oxygen. 



Margaric acid will be . C 34 H 33 + O 3 

 Stearic acid, , . 2 (C 34 H 33 ) + O 5 

 The margaric acid is a compound of one integrant particle of 

 the radical united to 3 atoms oxygen, while the stearic acid con- 

 tains two integrant particles of the radical united to five atoms 

 oxygen. 



Stearic acid was discovered by Chevreul in 1816, and called 

 at first margarous acid ; but he afterwards adopted the term stea- 

 ric acid) * as more proper. To obtain it we must saponify mut- 

 ton suet, ox's fat, or hog's lard. The soap must be dissolved in 

 a weak solution of caustic potash. The solution is to be mixed 

 with a quantity of water forty-five times as great as that of 

 the tallow saponified. The mixture being left at rest in a tem- 

 perature between 50 and 60, there gradually falls to the 

 bottom a pearly looking substance, which is a mixture of bistea- 

 rate, bimargarate, and superoleate of potash. This substance 

 is allowed to dry. It is then washed three times successively in 

 eight times its weight of boiling alcohol of 0-820. The first of 

 these, on cooling, deposites a great quantity of crystals consist- 

 ing chiefly of bistearate of potash. It is rendered quite pure by 

 repeated solutions in alcohol, and crystallizations. It is then 

 decomposed by tartaric or muriatic acid. 



Thus prepared, stearic acid melts when heated to 158. Red- 

 tenbacher found the melting point of the stearic acid which he 

 prepared 160. On cooling, it crystallizes in fine brilliant 

 needles, interlaced with each other. It is tasteless, destitute of 

 smell, and insoluble in water. While in a liquid state, it com- 

 bines with alcohol in all proportions. If at the temperature of 

 167, we mix equal weights of stearic acid and alcohol, we ob- 

 tain a solution which, when cooled down to 122, crystallizes in 

 brilliant plates. At 113 the whole congeals into a solid mass. 

 Ether of 0-725 density dissolves its own weight of stearic acid 

 when assisted by heat. The solution is limpid at 140. At 

 1344 it concretes into a mass formed of beautiful plates. This 

 acid readily combines with the alkalies, and forms a soap. One 



* From rrttt, tallow. 



