28O AGRICULTURAL BACTERIOLOGY. 



that the mother of vinegar acted thus chemically rather than 

 physiologically. But the error of this conclusion was later 

 shown by two facts, (i) The ordinary acetic acid fermenta- 

 tion is stopped by the accumulation of the acid, and it will not 

 occur at all if the amount of acetic acid in the solution be 

 more than 14 per cent. The formation of acid by platinum 

 black is entirely uninfluenced by such an accumulation. (2) 

 The formation of the acid by the natural process is most 

 abundant at about 95 R, diminishing rapidly if the tempera- 

 ture is raised above this. The oxidation by platinum black 

 goes on much more rapidly at higher temperatures and may 

 in itself produce so much heat as actually to ignite the alcohol. 

 The two processes must, evidently, be veiy different in nature. 



ACETIC ACID PRODUCTION A TRUE FERMENTATION. 



That the "mother of vinegar" is a necessary agent in the 

 acetic fermentation has been recognized from the earliest times. 

 This mother of vinegar is a soft, felted mass, semi-solid, and 

 commonly forming a scum on the surface of the fermenting 

 alcohol. Its causal connection with the phenomenon is indi- 

 cated by the name given it, motlier of vinegar. The fermenta- 

 tion does not occur if this material be wanting, and a very 

 small bit placed on the surface of an alcoholic solution soon 

 extends itself and covers the whole surface, inducing an active 

 acetic acid formation. This mother of vinegar proves to be ,1 

 mass of microorganisms. It was first named Mycodcrma by 

 Persoon, who studied it in 1822 without having any suspicion 

 that the skin was the cause of the acetic acid. Its organic 

 nature was proved a little later by Kiitzing, who showed that 

 this skin was made of numerous minute, living organisms, and 

 who positively asserted that they were the cause of the acetic 

 fermentation. But in regard to this fermentation, just as in re- 

 gard to others, the chemist Licbig checked the advance of 

 discovery by his own theories of fermentation, which regarded 



