INTRODUCTION. XX111 



culties fell rapidly before him. He proved the ferment 

 of lactic acid to be an organism of a certain kind. 

 The ferment of butyric acid he proved to be an 

 organism of a different kind. He was soon led to the\ 

 fundamental conclusion that the capacity of an organ- 

 ism to act as a ferment depended on its power to live 

 without air. The fermentation of beer was sufficient 

 to suggest this idea. The yeast-plant, like many others, 

 can live either with or without free air. It flourishes 

 best in contact with free air, for it is then spared 

 the labour of wresting from the malt the oxygen 

 required for its sustenance. Supplied with free air, 

 however, it practically ceases to be a ferment ; while 

 in the brewing vat, where the work of fermentation 

 is active, the budding torula is completely cut off by 

 the sides of the vessel, and by a deep layer of carbonic 

 acid gas, from all contact with air. The butyric fer- 

 ment not only lives without air, but Pasteur showed 

 that air is fatal to it. He finally divided microscopic 

 organisms into two great classes, which he named 

 respectively arables and ancerobics, the former requir- 

 ing free oxygen to maintain life, the latter capable of 

 living without free oxygen, but able to wrest this 

 element from its combinations with other elements. 

 This destruction of pre-existing compounds and forma- 

 tion of new ones, caused by the increase and multipli- 

 cation of the organism, constitute the process of 

 fermentation. 



