FERMENTATION. 61 



the work of microscopic anaerobies, living without free 

 oxygen, the slow combustion is found very largely, if 

 not exclusively, to depend upon a class of infinitely 

 small aerobies. It is these last which have the pro- 

 perty of consuming the oxygen of the air. It is these 

 lower organisms which are the powerful agents in 

 the return to the atmosphere of all which has lived. 

 Mildew, mould, bacteria, which we have already 

 noticed, monads, two thousand of which would go to 

 make up a millimeter, all these microscopic organisms 

 are charged with the great work of re-establishing the 

 equilibrium of life by giving back to it all that it 

 has formed. 



To demonstrate the important part played every- 

 where by these microscopic organisms, Pasteur made 

 two experiments. He first introduced into vessels air 

 deprived of all dust. This process we shall have occa- 

 sion to examine in all its details, in connection with 

 the researches on spontaneous generation. In these 

 vessels, containing pure air, were placed the water of 

 yeast with sugar dissolved in it, milk, sawdust all 

 of which had been deprived by heat of the germs of 

 the lower organisms. The vessels and their contents 

 were then subjected to a temperature of twenty- 

 five to thirty-five degrees Centigrade. In a series 

 of parallel experiments, made under the same con- 

 ditions and at the same temperature, Pasteur took 

 no steps to prevent the germination of the little seeds 



