184 LOUIS PASTEUR, 



torula is completely cut off by the sides of the vessel, 

 and by a deep layer of carbonic acid gas, from all con- 

 tact with air. The butyric ferment not only lives with- 

 out air, but Pasteur showed that air is fatal to it. He 

 finally divided microscopic organisms into two great 

 classes, which he named respectively aerobies and an- 

 aerobies, the former requiring 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 formation of new ones, through the in- 

 crease and multiplication of the organism, constitute 

 the process of fermentation. 



Under this head are also rightly ranked the phe- 

 nomena of putrefaction. As M. Eadot well expresses 

 it, the fermentation of sugar may be described as the 

 putrefaction of sugar. In this particular field M. Pas- 

 teur, whose contributions to the subject are of the high- 

 est value, was preceded by Schwann, a man of great 

 merit, of whom the world has heard too little.* Schwann 

 placed decoctions of meat in flasks, sterilised the decoc- 

 tions by boiling, and then supplied them with calcined 

 air, the power of which to support life he showed to 

 be unimpaired. Under these circumstances putrefaction 

 never set in. Hence the conclusion of Schwann, that pu- 

 trefaction was not due to the contact of air, as affirmed 

 by Gay-Lussac, but to something suspended in the air 

 which heat was able to destroy. This something consists 

 of living organisms, which nourish themselves at the ex- 

 pense of the organic substance, and cause its putrefaction. 



The grasp of Pasteur on this class of subjects 

 was embracing. He studied acetic fermentation, and 



* It was late in the day when the Royal Society made him a 

 foreign member. 



