Fermentation and Kindred Phenomena. 95 



phenomena which have long been noticed and even employed 

 practically, although their cause was not understood. A few of 

 these changes deserve our attention. 



The Lactic Ferment. — Everyone knows that when milk is 

 kept it becomes sour and curdles. 



As early as 1780 the Swedish chemist Scheele extracted from 

 sour milk a peculiar acid, which he named from its occurrence 

 lactic acid. It is obvious that the souring of milk is due to the 

 development of this acid ; but the question arises — From what 

 special substance in the milk is it formed, and what is the cause 

 of its development ? 



Careful experiments have completely answered these ques- 

 tions. One of the chief constituents of milk is sugar, not 

 exactly the same as occurs in the sugar cane, but one which is 

 very analogous. This " milk sugar," as it is called, is extracted 

 from the whey of milk (chiefly in Switzerland), and is a com- 

 mercial product. Now it has been found that in proportion as 

 milk becomes sour the quantity of this sugar diminishes, and 

 under suitable conditions it disappears altogether. Further, if 

 a solution of sugar is mixed with a few drops of sour milk, the 

 sugar solution becomes sour and the acidity increases rapidly. It 

 is, therefore, pretty clear that the development of lactic acid in 

 milk is due to some transformation which the sugar suffers, and 

 we have only to compare the formulae of the two to see that a 

 simple chemical relationship exists between them. In fact, 

 every particle of sugar contains the necessary atoms to form 

 two particles of lactic acid, and we may represent the conversion 

 of the former into the latter by the following chemical equation : 



C 9 H 12 6 = 2 C 3 H 6 3 



Sugar. Lactic Acid. 



But what is the cause of this transformation ? Pasteur, guided 

 by his previous researches in alcoholic fermentation, sought for 

 and found the lactic ferment which consists of minute rods or 

 bacilli, which are often jointed or beaded. They can readily be 

 seen in a droplet of sour milk with a £ inch power. By removing 

 some of them from sour milk, and sowing them in a suitable 



