THE CHEMISTRY OE THE ANIMAL BODY. 545 



konmi is a modification of this. Here a drop of a substance containing biliary acids is 

 placed on a small white porcelain cover, with a drop id' dilute cane-sugar solution, and one 

 of dilute sulphuric acid; the mixture is then very carefully evaporated over a flame and 

 leaves a brilliant violet stain. 



Oxy- Fatty Acids, Lactic-acid Group. 



These are diatomic monobasic acids of the glycols. A glycol is a diatomic 

 alcohol. The oxy- fatty acids have the general formula C n H 2I1 3 , and include : 



Carbonic acid, CH 2 3 . Oxy-butyric acid, C 4 H 8 3 . 



Glycollic acid, C 2 IT 4 3 . Oxy-valerianic acid, C 5 H 10 O 3 . 



Lactic acid, C 3 H 6 3 . etc. 



Carbonic Acid, or Oxy-formic Acid, HO.CO.OH. — This is, in reality, 

 a dibasic acid on account of the symmetric structure of the two — OH radicals. 

 It has already been considered. 



Lactic Acids, or Oxy-propionic Acids. — Of these there are two isomeres, 

 which vary in the position of their — OH group, the a- and ft- lactic acids. 

 Physiology is concerned only with the first. 



a-Lactic Acid, or Ethidene Lactic Acid, CH 3 .CHOH.COOH. — This 

 is called fermentation lactic acid, being a product of the fermentation of carbo- 

 hydrates (see p. 539) : 



C 6 H 12 6 = 2C 3 H 6 3 - 



On lactic fermentation of milk-sugar depends the souring of milk. This fer- 

 mentation does not take place in the presence of sufficiently acid gastric juice, 

 but maybe more active in the alkaline intestine. It has been noticed that the 

 fecal excrements after a carbohydrate diet react acid, after proteid diet alkaline. 

 The acid reaction is due chiefly if not wholly to acetic acid, since lactic acid, 

 being the stronger acid, is first neutralized by the intestinal alkali. Lactic 

 acid, when absorbed, is completely burned in the body. Lactic-acid fermenta- 

 tion between the teeth dissolves the enamel, and gives bacteria access to the 

 interior. The fermentation lactic acid is inactive to polarized light, and, since 

 it has in its formula an asymmetric carbon atom,' it is necessary to assume 

 that it consists of an equal mixture of right and left ethidene lactic acid. On 



1 An asymmetric carbon atom is one in which the four atoms, or groups of atoms, united to 



CH a 

 I 

 it are all different. In lactic acid we find the following grouping, II — C — Oil. The central 



COOK 



carbon represents the asymmetric atom. Such an arrangement is always optically active. < me 

 is able to conceive the arrangement of the atoms in space, according to the above grouping, >>r 

 CI I, 



as follows: HO— C— H. This latter represents a different configuration. The two arrange- 



1 



coon 



ments are optically antagonistic. A mixture of the two is optically inactive. The reader is 

 referred to a text-book on general chemistry for the suggestive illustrations of the tetrahedral 

 space pictures. 



Vol. I.— 35 



