THE YEAST FUNGI. 151 



exciting fermentation ; it is found especially on wine, 

 then on beer, juice of fruits, sauerkraut, &c. 



Formerly it was supposed that this organism caused acetic 

 fermentation in fermented fluids ; but according to Nageli 

 the relation of the fungus to acetic fermentation is not a 

 causal one. It is found especially on the surface of markedly 

 acid fluids, e.g., wine poor in alcohol ; in that case it occasions 

 no formation of acetic acid, the latter is due rather to 

 specific fission fungi, which, however, cannot vegetate in 

 markedly acid fluids. This fungus acts in the first place like 

 a covering of mould, it leads to the combustion of the acid, 

 and diminution of the amount of acid in the fluid. Thus it 

 prepares the soilfor the reception and multiplication of the 

 fission fungus which produces acetic acid; the occurrence 

 of this scum is therefore a necessary preparation for the 

 acetic fermentation. Nageli distinguishes the following 

 scums on fermented fluids : 1. Acetic mother (Essigmutter), 

 a very thick, tough, glue-like scum, with a smooth surface ; 

 oxidises the alcohol to acetic acid, consists of bacteria= 

 Mycoderma aceti. 2. Acetic skin (Essigliautclieri), thin, 

 slimy, smooth, or finely wrinkled, oxidises the alcohol to 

 acetic acid, consists of bacteria=Mycoderma cerevisiae. 3. 

 Mesentcric skin (KaJimliaut, Gekrosehaut) , comparatively Composition 

 strong and firm, folded like the mesentery, consists of yeast 

 fungi (Saccharomyces mesentericus), which destroy the fruit 

 acids. At a later period the acetic fungus (bacterium) 

 appears in the material, and oxidises the alcohol to acetic 

 acid = Mycoderma vini. 4. Smooth skin (Falsclie Kahmhaut, 

 Glatthaut), comparatively strong, but without folds, of a 

 loosely granular composition, consists of yeast fungi, does 

 not destroy the fruit acids to any marked extent, and does 

 not permit the acetic fungus to settle in the fluid. 5. Acetic 

 ether skin (Essiyatlierlidutclieri), thin, not folded. Composed 

 of yeast fungi (Saccharomyces sphaericus) and of bacteria 

 (acetic fungus), the combined action of which converts a part 

 of the sugar into acetic ether. The acetic mother and acetic 

 skin are formed on alcoholic fluids, which contain few fruit 

 acids, but on the other hand may contain a good deal of acetic 

 acid; for example, on beer, on vinegar, to which wine or 

 beer is added, seldom on slightly acid wine. The other 

 skins, on the other hand, appear constantly on fluids which 

 contain a certain quantity of fruit acids, the mesenteric 

 skin on fermented wine juice and other fruit juices, the 

 smooth skin at times on similar fluids which are altered by 

 the addition of sugar and other materials. More recent 

 investigations by Hansen 011 the membranes occurring on 



