§ IX.] Viscous fermentation. 89 



It does not strictly belong to our subject, but it is perhaps 

 not superfluous to remark that every white membrane which 

 makes its appearance spontaneously on the surface of a fluid 

 suitable for forming vinegar is not necessarily mother of vinegar. 

 The white and ultimately wrinkled film which usually forms on 

 beer or wine, is a well-known object, and to the naked eye looks 

 so like the membrane of vinegar as to be often mistaken for it. 

 But under the microscope it is distinguished from it by being 

 formed from a comparatively large Sprouting Fungus, Saccharo- 

 myces Mycoderma, which has no direct coimection with the 

 formation of vinegar. On the contrary, it converts alcohol and 

 other bodies in ■ solution by oxidation into carbonic acid and 

 water. Indirectly it may, indeed, in this way promote the for- 

 mation of vinegar by destroying any excessive amount of alcohol 

 and acid which would impede the development of the Micro- 

 coccus aceti, and so providing it with a substratum favourable 

 to its vegetation. 



4. We now come to a series of examples of phenomena of 

 fermentation and decomposition produced by Bacteria in the 

 sugars and in the allied carbohydrates. When in the following 

 remarks we speak simply of saccharine solutions, it is always to 

 be understood that they contain also the constituents required 

 for nutrient solutions. 



We must first of all say a word or two respecting the so-called 

 viscous fermentations (25, p. 572, 43, 44). The juices of plants 

 which contain sugar, such as onion and beet, when extracted 

 by crushing often assume a sticky viscous character, and produce 

 carbonic acid and in many cases also mannite. Organisms 

 also, to be described presently, make their appearance as a 

 sediment in the viscous mass. If a small portion of the sub- 

 stance is introduced into a suitable solution of cane-sugar which 

 was before free from germs, the same viscidity is caused in it 

 as the organisms develope. These must therefore be regarded 

 as the causes of the change. The organisms in question are, 

 according to Pasteur, of two kinds. The first is a Micrococcus 



