74 LACTIC ACID. 



stitution that it contains six atoms of each of its elements, carbon, hy- 

 drogen, and oxygen. 



The production of this acid in organic substances is very common. 

 It depends on the same principle as presented in duodenal digestion, 

 which it therefore very strikingly illustrates. As an example deserving 

 of attentive consideration, its development in milk may be offered. 



When milk is exposed to the air it eventually turns sour, the sour- 

 ness being due to the appearance of lactic acid. In its sweet state, the 

 milk may be regarded as consisting of casein, or the curdy principle, a 

 substance belonging to the protein group, insoluble in pure water, but 

 abundantly soluble if a little free or carbonated alkali be present ; of milk 

 sugar, dissolved, and of butter held in suspension in water. The ac- 

 Production of ^ on ta ^ m S P^ ace during the souring is as follows : Under 

 lactic acid the influence of atmospheric oxygen, which for this purpose 

 rom milk. mus t liswQ access, the nitrogenized principle, the casein, be- 

 gins to change, and, for reasons presently to be more particularly exam- 

 ined, impresses a change on the sugar, splitting its atom so as to give 

 rise to the production of lactic acid. As this forms, it renders the casein 

 insoluble, and the milk begins to coagulate, to prevent which a little car- 

 bonate of soda may from time to time be added. All the sugar origin- 

 ally present in the milk is soon acidified, but a much stronger solution 

 can be made by adding more milk sugar as the process of exhaustion 

 goes on, and the change can be thus kept up until the casein itself is 

 quite consumed. 



On examining this process critically, we observe that every thing de- 

 pends on the change occurring in the nitrogenized principle, the casein. 

 This, under the circumstances, takes on an incipient oxidation, and com- 

 pels the sugar atom so to divide as to give rise to the production of lac- 

 tic acid. This ceases the moment the casein ceases to change, and re- 

 commences the moment the casein is permitted to reoxidize. The de- 

 struction taking place in the casein is propagated to the sugar, the 

 physical peculiarity being that the atom of sugar is merely divided, fis- 

 sured, or split, and gives rise to the production of lactic acid, and no 

 other substance. The whole process is therefore essentially one of sub- 

 division, a conclusion which should be carefully borne in mind in apply- 

 ing these experimental principles to the physiological function of diges- 

 tion. So far as the result is concerned, the two cases are the same. 



Many other organic liquids furnish similar illustrations. Thus, in 

 Production of ^ e P era tion of making starch for commercial purposes, con- 

 lactic acid siderable quantities of that substance are turned into lactic 

 >m starch. &C Q cons tit u ting what the manufacturers term sour liquor. 

 Nor is it even requisite that so much water should be present as to give 

 the liquid condition ; for if wheat flour be made into a paste, and kept for 



