4 BACTERIA. 



present or of sugar that was artificially introduced. By a 

 natural transition from the observation of vegetable cells to 

 a study of animal cells, we are led to consider how impor- 

 tant may be the part played by these latter in the digestive 

 tract and in the tissues of the body, especially when they are 

 called upon to act in conjunction with vegetable ferments, 

 either in normal or in abnormal positions. Fermentation, 

 then, may be looked upon as an ordinary chemical trans- 

 formation of certain substances taking place as the result of 

 the action of " living " cells, the nature of the fermentation and 

 of the substances ultimately resulting being due, firstly, to the 

 nature of the fermented body ; secondly, to the nature of 

 the organism which induces the fermentation ; and thirdly, 

 to the physical conditions under which the fermentation takes 

 place. Thus the results may be extremely complicated if a 

 mixture of ferments, say an alcoholic, a lactic, and a butyric, be 

 sowed in a single nutrient material ; but if we sow only one, 

 say the alcoholic, the sugar will split into alcohol and carbon 

 dioxide ; if we sow the butyric fermentation, butyric acid 

 will be formed as a result of the splitting up of the sugar ; 

 and so on. As will later be seen, the special name of any 

 fermentation serves to indicate merely that some special 

 product predominates. In the alcoholic fermentation, alcohol 

 is the chief product, but there are also formed as bye pro- 

 ducts, glycerine, succinic acid and a number of other 

 substances, the amount and nature of these bye products 

 varying, first, with the yeast, and second, with the condi- 

 tions under which it is allowed to grow. The character and 

 aroma of beer and wine indeed depend essentially on the 

 formation of such bye products compound aethers. It is of 

 course possible, nay, even probable, that what bacteriologically 

 may be termed impurities, may effect the same result, and 

 that special aroma and flavour may depend upon the presence 

 of small quantities of other organisms than the special yeast 

 used. Further, the activity of the process is dependent 

 in a very marked degree upon the nature of the fermenting 

 substance, and a medium which may afford ample material 

 for the carrying on of one kind of fermentation is absolutely 

 valueless as a medium for other fermentations. The only 

 real difference that exists between a pure alcoholic or butyric 

 fermentation and the complicated fermentations which take 

 place in the animal body or in putrefactive processes is, 



