LACTIC ACID. 35 



are produced by certain micro-organisms. It has a remarkable 

 tendency to form compounds which contain less water. 



On evaporating aqueous solutions of lactic acid, dehydrolactic 

 acid is formed, C 6 H 10 5 , which, by further evaporation (especially 

 at a high temperature), gives lactide, C 6 H 8 4 . 



Lactic acid acts as a monobasic acid ; while dehydrolactic 

 acid behaves as a monobasic acid, monohydric alcohol and 

 an ethereal salt at the same time ; lactide is a neutral sub- 

 stance. 



Sarcolactic acid gives the same lactide, which, on boiling with 

 water, is converted into the inactive modification. 



The so-called syrupy lactic acid is a mixture of lactic and 

 dehydrolactic acids with probably a little lactide. Wislicenus 

 has shown that by direct titration with alkali lactic, and dehydro- 

 lactic acids are estimated, while by further boiling with excess 

 of alkali one molecule of lactic acid is produced for each molecule 

 of dehydrolactic acid, and two for each molecule of lactide. 

 Dehydrolactic acid has not been obtained pure, but appears to 

 be amorphous and nearly insoluble in water. 



Lactide can be prepared by subliming syrupy lactic acid at 

 150 in a current of dry. air. It is insoluble in water, but can 

 be crystallised from alcohol in colourless rhombic plates meitine 

 at 124-5 C. It boils at 255 C. 



Syrupy lactic acid is said to have a specific gravity of 1-2485. 

 Lactic acid is not 'appreciably volatile in dilute solution, but 

 passes over with water to a slight extent as the solution becomes 

 concentrated. 



Lactic acid is soluble in and miscible in all proportions with 

 water, alcohol, ether, and glycerol. It is insoluble in petroleum 

 ether. Fats also dissolve it. It is probable that the lactic acid 

 present in sour milk is, partially at all events, dissolved in the 

 fat. 



As milk almost immediately after milking contains organisms 

 which produce lactic acid, it may be considered as a normal 

 constituent of milk ; indeed Bechamp has held that it is produced 

 from milk by organisms (micro-zymes) derived from the udder 

 itself. That this view is erroneous is shown by the fact that 

 Lister, Pohl, Warington, and others have succeeded in preserving 

 milk, drawn direct into sterilised vessels, for a considerable 

 length of time without the development of acidity. 



Lactic acid probably exists in milk, not in the free state, bufc 

 as a salt, at all events until the acidity is sufficient to curdle the 

 milk on boiling. 



Mineral Constituents. On burning milk a white ash is left ; 

 this contains the mineral constituents of milk, altered, however, 

 to some extent by the oxidation of some of the compounds 



