FERMENTATION. 121 



is going on very rapidly; there is a growth and multi- 

 plication of the yeast-cells, a transformation of the sugar into 

 alcohol and carbonic acid gas — all without the presence of free 

 oxygen. Pasteur set himself to reconcile these two apparently 

 contradictory facts, and as a result of his observations he 

 made one of the most important of his discoveries, important 

 in its relation to the life history of both fermentative and 

 pathogenic organisms ; and specially important because of 

 the parts that the aerobic and anaerobic conditions under 

 which organisms exist play in determining the growth of these 

 organisms and the spread of certain specific infective diseases. 

 He found that old cells with thick membranes, even con- 

 taining spores, were not able to multiply in media other- 

 wise suitable unless oxygen was present ; and that the vigour 

 of the rejuvenating process depended to a very great extent 

 on the amount of free oxygen that was contained within the 

 fermenting fluid. He found, in fact, that with these cells, as 

 in the case of fungi and of other animal and vegetable cells, 

 a sudden and complete cutting oiF of the supply of free 

 oxygen immediately proved fatal to them, so that the cells 

 which had been exposed to the air for some time — that is, which 

 had acquired an aerobic habit — were rendered inactive by the 

 cutting off of the supply of oxygen. Along with this he 

 found that the yeast organisms could multiply in liquids 

 containing nutrient materials for a considerable length of 

 time before there was any appearance of alcohol in the fluid. 

 After a time, however, the free oxygen being used up, it 

 would naturally be expected that the growth of the yeast-cells 

 would also come to an end, but such was not found to be the 

 case. The yeast-cells continued to multiply, the oxygen in the 

 fluid was used up and replaced by carbonic acid gas, and car- 

 bonic-acid gas was found on the surface, cutting off any further 

 oxygen supply from without. How was it that these cells 

 went on multiplying ? Pasteur answered the question as 

 follows : During the gradual elimination of oxygen the yeast- 

 cells, having once become vigorous from the presence of a 

 good supply of oxygen, and of other nutrient requisites, 

 had acquired an activity that they did not before possess ; 

 they had at the same time become gradually acclimatized, 

 and their protoplasm had become so far altered that, as the 

 free oxygen was partially cut off', it was able to wrest from the 

 sugar what oxygen it required for the building up of its own 



