184 LOUIS PASTEUR, 



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

 tact with air. The butyric ferment 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 aerobies and 

 anaerobies, the former requiring free oxygen to main- 

 tain life, the latter capable of living without free 

 oxygen, but able to wrest this element from its com- 

 binations with other elements. This destruction of 

 pre-existing compounds and formation of new ones, 

 through the increase and multiplication of the orga^ 

 nism, 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 bs described as the 

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

 teur, whose contributions to the subject are of the 

 highest value, was preceded by Schwann, a man of 

 great merit, of whom the world has heard too little. 1 

 Schwann placed decoctions of meat in flasks, sterilised 

 the decoctions 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 putrefaction 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 expense of the organic 

 substance, and cause its putrefaction. 



The grasp of Pasteur on this class of subjects 

 was embracing. He studied acetic fermentation, and 



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

 foreign member. 



