LOUIS PASTEUR. 235 



which, unlike the lactic ferment, in so far as the organism 

 by which it is brought about, — which Pasteur was en- 

 abled to separate, — exhibits the greatest activity in the 

 absence of oxygen ; indeed, when a current of air was passed 

 through the fermenting liquid, the organism — a vibrio — 

 became motionless, fell to the bottom of the fluid, and 

 the process of fermentation ceased. These two organisms 

 were taken by Pasteur to represent two classes : ( 1 ) the 

 airobies or organisms which require a supply of free oxygen 

 in order that they can live and carry on their fermentation 

 function, and (2) the anadrobies which not only can live 

 without air but flourish best in this condition. 



In both cases the organism uses up for its own nutrition 

 a comparatively small amount of the fermentable sub- 

 stance, but decomposes a much greater quantity of this 

 material, breaking it down into what are known as the pro- 

 ducts of fermentation. He found, however, that yeast could 

 bring about fermentation not only in the presence of oxygen, 

 but also when it was absent, although the amount of decom- 

 position was Very different in the two cases. Where no air 

 has access to the organism, a single part of yeast will break 

 down from 100 to 150 parts of sugar ; whilst when oxygen 

 is present it will only break down something like five or six 

 parts, and Pasteur pointed out that in the one case the 

 organism obtains a large part of the oxygen necessary for 

 its growth directly from the air, and therefore does not 

 require to break down so much sugar in order to obtain the 

 elements that it requires for its nutrition ; whilst, when air 

 is absent, the oxygen must be obtained from the sugar, as 

 a result of which a very large amount of alcohol is produced 

 during the rearrangement of the molecules rendered neces- 

 sary by the abstraction of the oxygen. 



Various forms of putrefactive processes were then 

 studied by Pasteur, who was able to show that they de- 

 pended upon organisms similar to those that he had already 

 described in the processes of fermentation, that some of these 

 were aerobic, some anaerobic, that they differed in character 

 according to (1) the nature of the medium to be broken up, 



and (2) the nature of the organism that was engaged in 



16 



