126 LACTIC ACIDS. 



1. Ethylidene-lactic acid. CH 3 . CH(OH) . COOH. 

 This is the ordinary form of the acid, obtained characteristically as 

 the chief product of the lactic fermentation of sugars (see p. 105). 



From this source it may be readily prepared by adding a little old cheese and 

 sour milk to a solution of cane sugar to which some carbonate of zinc is added. 

 The whole is kept warmed to 40 or 45 for ten days or a fortnight, being vigorously 

 stirred at frequent intervals. The lactic acid is fixed as a lactate by the zinc salt 

 as fast as it is formed, this removal of free acid being essential to the progress of 

 the fermentation which does not take place in an acid solution. The crusts of 

 zinc-lactate formed during the above process are purified by recrystallising, the acid 

 is then liberated from the compound by the action of sulphuretted hydrogen, and 

 extracted by shaking up with ether, in which it is soluble. By a similar process 

 lactic acid may be readily obtained from lactose. 



Lactic acid occurs in the contents of the stomach and intestine, 

 more particularly during a diet rich in carbohydrates, and may be 

 readily formed by the digestion of gastric mucous membrane with 

 solutions of dextrose or saccharose 1 . According to Heintz 2 it is found 

 also in muscles, and according to Gscheidlen 3 in the ganglionic cells of 

 the grey substance of the brain. 



The most important salts of this acid are those of zinc and calcium. 



Zinc lactate. Zn (C 8 H 5 3 ) 2 + 3H 2 0. Soluble in 53 parts of water at 

 15 ; in 6 parts at 100. Almost insoluble in alcohol. 



Calcium lactate. Ca (C 3 H 5 3 ) 2 + 5H 2 O. Soluble in 9 '5 parts of cold 

 water ; soluble in all proportions in boiling water. Insoluble in cold 

 alcohol. 



2. Sarcolactic acid. 



This form of the acid is isomeric with the preceding one. In its 

 general chemical behaviour as tested by the various decompositions it 

 can undergo it is found to be identical with ethylidene-lactic acid, the 

 sole observable difference being in the different solubility of its calcium 

 and zinc salts. But both sarcolactic acid and its salts differ strikingly 

 from the preceding acid and its salts as regards their physical properties, 

 for the former exert a distinct rotatory action on polarised light while 

 the latter do not. This peculiar kind of isomerism, chemical identity 

 with physical difference, has been called ' physical isomerism ' to distin- 

 guish it from the ordinary form of chemical isomerism. It is now more 

 usually and correctly called * stereochemical isomerism' in accordance 

 with the theory which is held as to the nature and cause of the pheno- 

 menon. (See below.) 



1 Maly, Ann. d. Chem. u. Phai-m. Bd. CLXXIII. (1874), S. 227. 



2 Ann. d. Cliem. u. Pharm. Bd. CLVII. (1871), S. 314. 



3 Pfliiger's Archiv, Bd. vm. (187374), S. 171. 



