454 Prof. Liebig 07i the Constitution of 



composition. It is a well-known fact, that when all the milk- 

 sugar in a given quantity of milk has become converted into 

 lactic acid, a fresh quantity of lactic acid may be produced by 

 repeated addition of milk-sugar; indeed, the common white 

 caseine in a state of decomposition yields, upon renewed ad- 

 ditions of milk-sugar, almost unlimited quantities of lactic 

 acid, provided the precaution be taken to neutralise the free 

 acid from time to time by means of an alkali. Grape-sugar 

 and cane-sugar, under the same conditions, yield the same 

 products as milk-sugar. The examinations and experiments 

 of Haidlen* have proved that milk in its fresh state contains 

 neither lactic acid nor lactates. 



So long as chemists were ignorant of the intimate relation 

 existing between lactic acid and the milk-sugar contained in 

 milk, there appeared to be some reasons for believing in the 

 presence of lactic acid in animal secretions generally, when 

 manifesting an acid reaction, or passing into a state of acidity. 

 It thus happened, without any real and positive proof, that 

 lactic acid was considered to be a constituent of urine ; the 

 presence of lactates in the blood was assumed, and these salts 

 were even supposed to perform a highly important part in the 

 process of respiration. 



Lactic acid is a non-nitrogenous substance. Nothing has 

 hitherto been observed tending to show that it may be pro- 

 duced from the elements of a nitrogenous substance, by the 

 decomposition of such a substance and the transposition of its 

 elements. In every instance where the formation of lactic 

 acid has been observed, the result of careful examination has 

 proved the presence of a non-nitrogenous substance of an 

 identical or, at least, similar composition with lactic acid. 



These observations would seem to render the formation of 

 lactic acid in the body of the herbivorous and graminivorous 

 animals, which take starch and sugar in their food (substances 

 from which lactic acid may be formed), not merely possible, 

 but in many cases highly probable ; and yet, strange to say, 

 chemists have hitherto attempted in vain to detect lactic acid 

 in the urine of the cow and of the horse. The urine of the 

 cow or horse has no acid reaction ; on the contrary, its reac- 

 tion is strongly alkaline; it contains carbonated, hippurated, 

 or benzbated alkali, or alkalies combined with mineral acids, 

 but no trace of any lactate. 



In contrast with this, the urine of man, and of carnivorous 



animals, manifests, when in a healthy state, a strongly acid 



reaction. Now, it is precisely in analyses of the blood and 



urine of man, and of carnivorous animals, that we find lac- 



* Annalcn der Chemie und Pharmacie, vol. xi. p. 263. 



