126 LACTIC ACIDS. 



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

 calcium. 



Zinc lactate. Zn (C 3 H 5 Os)2 + 3H 2 O. Soluble in 53 parts of 

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



Calcium lactate. CA (CsHjOs)" + 5H 2 O. Soluble in 9'5 parts 

 of cold water ; soluble in all proportions in boiling water. In- 

 soluble 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 decom- 

 positions 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 distinguish it 

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

 usually and correctly called ' stereochemical isomerism ' in accord- 

 ance with the theory which is held as to the nature, and cause of 

 the phenomenon. (See below.) 



This acid has not yet been prepared synthetically and is only 

 known as occurring characteristically in muscles 1 to which it 

 gives their acid reaction, 2 and in blood. 3 In the latter it is found 

 more particularly, as might be expected, after the muscles have 

 been in a state of contracting activity. 4 It is also found in urine, 

 very markedly in cases of phosphorus poisoning, and in the same 

 excretion after violent muscular exertion, 5 or artificial stimulation 

 of groups of muscles, 6 and very strikingly after extirpation of the 

 liver in birds, 7 and frogs. 8 It is also stated to be formed in vari- 

 able and slight amount during the lactic fermentation of dextrose. 9 

 Lactic acid has also been frequently described as a constituent of 

 various pathological fluids ; in these cases it is probable that the 

 acid is often the sarcolactic acid. 10 



As occurring characteristically in muscles it is hence found in 



1 Wislicenus, Ann. d. Chem. u. Pharm. Bd. CLXVII. (1873), S. 302. 



2 Liebig, Ann. d. Chem. u. Pharm. Bd. LXII. (1847), S. 326. 

 8 Gaglio, Arch.f. Physiol. Jahrg. 1886, S. 400. 



4 Spiro, Zt. f. physiol. Chem. Bd. i. (1877), S. 111. Cf. Von Frey, Arch. J 

 Physiol. Jahrg. 1885, S. 557. Also Marcuse, loc. cit. below. 



Colasanti and Moscatelli. See ref. in Maly's Bericht. 1887, S. 212. 



6 Marcuse, Pfluger's Arch. Bd. xxxix. (1886), S. 425. 



7 Minkowski, Centralb. f. d. med. Wiss. 1885, No. 2. Arch.f. exp. Path. u. Phar- 

 makol. Bd. xxi. (1886), S.40. 



8 Marcuse, loc. cit. But see Nebelthau, Zt.f. Biol. Bd. xxv. (1889), S. 123. 



9 Maly, Her. d. d. chem. Gesell. Jahrg. 1874, S. 1567. 



10 Cf. Maly. Abst. in Maly's Jahresb. 1871, S. 333. Fluid from ovarial cyst. 



