CITRIC ACID. 185 



calcium racemate is insoluble in acetic acid and am 

 monium chloride. 



When acid sodium racemate is saturated with am 

 monia, there are formed, when the crystallization takes 

 place slowly, two different salts of the same composi 

 tion and appearance C 4 H 4 6 NaNH 4 . These have, how 

 ever, such dissimilar and unsymmetrical hemihedral 

 faces, that the forms are not congruent, the form of 

 the one being a reflection of that of the other. The 

 crystals of the one salt rotate the plane of polariza 

 tion towards the right, those of the other towards the 

 left. The free acids, separated from the salts, show 

 the same difference in their form and their conduct to 

 wards polarized light. The acid, which rotates the plane 

 towards the righHs ordinary tartaric acid, the other is a 

 peculiar acid, isomeric with this, antitartaric acid. With 

 bases they form two series of salts, which differ from 

 each other in the same way. When both acids are 

 mixed together in a solution, there are formed crystals 

 of racemic acid, which are optically inactive, an evo 

 lution of heat accompanying the formation. 



The acids homologous with tartaric acid are only 

 very imperfectly known. 



D. TRIBASIC, TETRATOMIC ACIDS, C W H 2W - 4 7 . 



Citric Acid. 

 C 6 H 8 7 = C 3 H 4 .OH(CO.OH). 3 



Occurrence. Particularly in lemon-juice, in a free 

 state. Further, in currants and gooseberries, and many 

 other acidulous sweet fruits; in beet-juice; in the bark 

 of Pyrus mains and JEsculus hippocastanum. 



Preparation. Has not as yet been prepared artificially. 

 Lemon-juice, clarified by boiling with albumen, is 

 saturated while warm with chalk and milk of lime ; 

 the insoluble calcium citrate, which separates, filtered 

 oft , washed, and decomposed by means of dilute sul- 



16* 



