BACTERIA. 6.1 



liquids, ethyl-alcohol is converted into acetic acid, and by 

 farther oxidation the latter is converted into carbonic acid 

 and water. This has been recently confirmed by A . J. Brmvn. 

 to whom we are indebted for the most complete researches on 

 the chemical action of acetic acid bacteria. 



2. LACTIC ACID BACTERIA. 



When milk is exposed at a temperature of 35 to 42 C. 

 it will soon become sour, and a considerable portion of the 

 acid produced is lactic acid, which is formed by the agency 

 of various species of bacteria. When a certain quantity of 

 lactic acid has been formed, the fermentation ceases. It will 

 recommence, however, if the liquid be neutralised with car- 

 bonate of lime, or on the addition of a small quantity of 

 pepsine or pancreatine, which causes the casei'ne of the milk 

 to be dissolved. 



A method commonly employed for inducing a spontaneous 

 lactic acid fermentation is the following : To a liter of water 

 are added 100 grams of sugar, 10 grams of casei'ne or old 

 cheese, and an abundant quantity of powdered carbonate 

 of lime. This mixture is exposed in an open vessel to a 

 temperature of 35 C to 40 C. The liquid is occasionally 

 agitated, or a current of air is passed through it. After 

 completion of the fermentation the liquid is evaporated, 

 when calcium lactate crystallises out, and from this the 

 lactic acid is liberated by treatment with sulphuric acid. 



In addition to milk-sugar, lactic acid bacteria are also 

 capable of fermenting cane-sugar, glucose, maltose, and 

 various other substances. According to Bourquelofs investi- 

 gations, a species of lactic acid bacterium, which makes its 

 appearance in the spontaneous acid fermentation of milk, 

 is capable of fermenting cane-sugar without previously 

 inverting it. 



In milk-sugar solutions which were free from casein e, 

 Fokker could only obtain feeble lactic acid fermentations, 



