338 CONSIDERATION OF CARBON COMPOUNDS. 



citrate, iron and strychnine citrate, soluble ferric phosphate, and soluble ferric pyro- 

 phosphate. 



Lactic acid, Acidum lacticum, HC 3 H 5 O 3 = 9O. This acid is 

 the second member of a group of monobasic, diatomic acids which 

 have the general composition C n H 2D O 3 , and which contain two 

 hydroxyl groups, the hydrogen of one being capable of replacement 

 by metals, the other by alcohols. The first member of this series is 

 glycoliv acid, HC 2 H 3 O 3 , a white, deliquescent, crystalline substance 

 which is found in unripe grapes and in the leaves of the wild grape. 

 Glycolic acid has been shown to be acetic acid, C 2 H 4 O 2 , in which one 

 atom of hydrogen has been replaced by hydroxyl. The name 

 hydroxy-acetic acid, has, therefore, been given to this compound. 



Lactic acid occurs in many plant-juices ; it is formed, from sugar 

 by a peculiar fermentation known as " lactic fermentation," which 

 causes the presence of this acid in sour milk and in many sour, fer- 

 mented substances, as in ensilage, sauer-kraut, etc. The formation 

 of lactic acid from sugar may be expressed by the equation : 



C 6 H 12 6 = 2(HC 3 H 5 3 ). 

 Sugar. Lactic acid. 



For practical purposes lactic acid is made by mixing a solution of 

 sugar with milk, putrid cheese, and chalk, and digesting this mixture 

 for several weeks at a temperature of about 30 C. (86 F.). The 

 bacteria in the cheese act as a ferment, and the chalk neutralizes the 

 acid generated during the fermentation. The calcium lactate thus 

 obtained is purified by crystallization and decomposed by oxalic acid, 

 which forms insoluble calcium oxalate. 



Lactic acid is a colorless, syrupy liquid, of strongly acid properties ; 

 it mixes in all proportions with water and alcohol. The official 

 lactic acid contains 75 per cent, of absolute acid. 



A lactic acid called sarco-lactic acid is found in meat-juice, and, therefore, as 

 a constituent of meat-extract. This acid has the composition and all the 

 properties of the above ordinary lactic acid, with the exception that it acts 

 differently on polarized light. 



Ferrous lactate, Ferri lactas, Fe(C 3 H 5 3 ) 2 .3H 2 = 287.9. Made by dis- 

 solving iron filings in diluted lactic acid ; hydrogen is liberated and the salt 

 formed. It is a pale, greenish-white, crystalline substance, soluble in water. 



QUESTIONS. 431. Name the more common organic acids found in vegeta- 

 bles and especially in sour fruits. 432. What is the composition of oxalic acid, 

 how is it manufactured, and what are its properties ? 433. Explain the forma- 

 tion of crude tartar during the fermentation of grape-juice, and how is tartaric 



