INTRODUCTION. 13 



tains lactic acid, a considerable quantity of carbonic acid, 

 which varies " according to the temperature and the duration 

 of the fermentation, but is sometimes sufficient to burst the 

 bottle or drive out the corks." The liquid contains about 

 one per cent, of alcohol. 



We have already seen that the kephir grains and the 

 sour milk, together, contain (i) a yeast fungus which is 

 capable of bringing about the fermentation of grape sugar ; 

 (2) the bacillus of lactic acid ; and (3) the rod-shaped bacteria 

 that predominate in the gelatinous mass of the tough grains. 

 Now, though this yeast fungus can set up the fermentation of 

 inverted milk sugar, it cannot affect the milk sugar itself, so 

 that we must look elsewhere for the inverting power. 



This appears to be contained in an enzyme, or ferment, 

 that is produced by a large number of bacteria, amongst 

 others by the rod-shaped organisms already referred to, 

 and by the lactic acid bacillus, and these were supposed 

 to prepare the milk sugar for the action of the yeast fungus. 

 This appeared to be a perfectly satisfactory explanation, until 

 De Bary found that, by violent agitation of the souring milk 

 to which no kephir grains have been added, alcoholic 

 fermentation may still be induced ; so that it would appear 

 that by freely oxygenating the milk during the process of 

 lactic acid fermentation, when in fact the molecules are being 

 re-arranged, oxygen is taken up into chemical combina- 

 tion, and alcohol and carbonic acid are generated ; water, in 

 all probability, being formed' as this goes on. This is 

 adduced as one of the processes of fermentation by free 

 oxidation, and is an example of a chemical fermentation 

 giving results similar to those yielded by biological fer- 

 mentation. Until, however, it can be demonstrated that 

 De Bary was working with material in which all impurities 

 were excluded, these results can scarcely be accepted as abso- 

 lutely reliable. 



In the kephir, which is a perfectly fluid mass, we have a 

 quantity of lactic acid. Now, it is a well-known fact that, 

 under ordinary circumstances, when milk turns sour, there 

 is a precipitation of the curd which forms a more or less 

 solid coagulated mass. What has become of this coagulum 

 in the kephir ? If a piece of meat be exposed to the action 

 of putrefactive germs, it will be found that after a time it 

 is reduced to a soft, pulpy, almost liquid mass. In the same 



