52 THE STORY OF GERM LIFE. 



gradual conversion of the alcohol into acetic acid. 

 At the close of the process practically all of the 

 alcohol has disappeared. Ordinarily, however, 

 not all of it has been converted into acetic acid, 

 for the oxidation does not all sf^p at this step. 

 As tffe oxidation goes on, some of the acid is 

 oxidized into carbonic dioxide, which is, of course, 

 dissipated at once into the airj and if the process 

 is allowed to continue unchecked for a long 

 enough period much of the acetic acid will be lost 

 in this way. 



The oxidation of the alcohol in all commer- 

 cial production of vinegar is brought about by 

 the growth of bacteria fflvthe liquid. When the 

 vinegar production is going on properly, there is 

 formed on the top of the liquid a dense felted mass 

 known as the "mother of vinegar." This mass 

 proves to be made of bacteria which have the 

 power of absorbing oxygen from the air, or, at all 

 events, of causing the alcohol to unite with oxy- 

 gen. It was at first thought that a single species 

 of bacterium was thus the cause of the oxidation 

 of alcohol, and this was named Mycoderma aceti. 

 But further study has shown that several have 

 the power, and that even in the commercial man- 

 ufacture of vinegar several species play a part 

 (Fig. 18), although the different species are not yet 

 very thoroughly studied. Each appears to act 

 best under different conditions. Some of them 

 act slowly, and others rapidly, the slow-growing 

 species appearing to produce the larger amount 

 of acid in the end. After the amount of acetic 

 acid reaches a certain percentage, the bacteria are 

 unable to produce more, even though there be al- 

 cohol still left unoxidized. A percentage as high 

 as fourteen per cent, commonly destroys all their 



