BACTERIA AND FERMENTATION 119 



each other in the proportion of these elements. It is 

 obvious, therefore, that fermentation is really only a change 

 of position, a breaking down of one compound into two 

 simpler compounds. This redistribution of the molecules 

 of the compound results in the production of some heat. 

 Thus we must add heat to the results of the work of the 

 yeasts. 



When alcohol is pure and contains no water it is termed 

 absolute alcohol. If, however, it is mixed with 16 per cent, 

 of water, it is called rectified spirit, and when mixed with 

 more than half its volume of water (56.8 per cent.) it is 

 known as proof spirit. 



We shall have to consider elsewhere a remarkable faculty 

 which some bacteria possess of producing products inimical 

 to their own growth. In some degree this is true of the 

 yeasts, for when they have set up fermentation in a saccha- 

 rine fluid there comes a time when the presence of the re- 

 sulting alcohol -is injurious to further action on their part. 

 It has become indeed a poison, and, as we have already 

 mentioned, a necessary condition for the action of a ferment 

 is the absence of poisonous substances. This limit of fer- 

 mentation is reached when the fermenting fluid contains 13 

 or 14 per cent, of alcohol. 



Having discussed shortly the " medium " and the results, 

 we may now turn to the bacteriology of the matter, and 

 enumerate some of the chief forms of the yeast plant. Pro- 

 fessor Crookshank 1 gives more than a score of different 

 members of this family of Saccharomycetes. Before dwelling 

 upon some of the chief of these, it will be desirable to con- 

 sider a number of properties common to the genus. 



The yeast cell is a round or oval body of the nature of a 

 fungus, composed of granular protoplasm surrounded by a 

 definite envelope, or capsule. It reproduces itself by bud- 

 ding, or, as it is sometimes termed, gemmation. At one end 



1 Bacteriology and Infective Diseases, Appendix. 



