122 BACTERIA. 



substance, and as the free supply was more and more cut off it 

 became gradually more able to take what it required from the 

 sugar solution — that is, it was gradually acclimatized. In 

 reading this if instead of oxygen the word energy be used it 

 would appear that we should have a more accurate 

 physiological statement. It has already been stated that, in 

 the presence of a free supply of oxygen, relatively less alcohol 

 is formed than under conditions of anaerobiosis ; it appears as 

 though the oxidation is too complete ; there is enough oxygen 

 to supply not only the wants of the living organism but also 

 those open bonds of combination in the various molecules that 

 are unsatisfied when the yeast takes for its own use certain 

 constituent atoms from these molecules, and as a conse- 

 quence very perfect oxidation is brought about and alcohol 

 is transformed into carbonic acid gas and. water. In the 

 anaerobic condition that is brought about when the oxygen 

 in the fluid is used up, and as the carbonic acid gas accumu- 

 lates on the surface in the flask or in the vat, the vigorous 

 yeast-cells are able to separate sufficient oxygen for their own 

 use from the sugar, or sugar and water, disturbing the com- 

 positions of the fluid, and necessitating further rearrange- 

 ment of the remaining molecules ; additional oxygen can- 

 not be brought from without to satisfy the vacant bonds, and 

 other very definite products are formed. These products are 

 comparatively stable even when afterwards exposed to free 

 oxygen but not to oxygen in a nascent condition, and we have 

 as a result the alcoholic fermentation. Here, in fact, are two 

 essentially different metabolic processes at work in the proto- 

 plasm of the yeast-cells. When oxygen is freely supplied 

 the anabolic or building-up processes predominate, a fact 

 evidenced by the rapid division of the yeast-cells under these 

 conditions ; whilst, when oxygen is excluded, the catabolic 

 or breaking-down processes are in the ascendant, as shown 

 by the larger amount of the medium decomposed, accom- 

 panied, however, by a slower multiplication of the yeast-cells. 

 Summing up, Pasteur says : — 



"This being so, it is evident, we repeat, that to multiply in a fermentable 

 medium, quite out of contact with oxygen, the cells of yeasts must be 

 extremely young, full of life and health, and still under the influence of the 

 vital activity which they owe to the free oxygen which has served to form 

 them and which they have perhaps stored up for a time. When older they 

 reproduce themselves with much diiiRculty when deprived of air, and 



