XXIV INTRODUCTION. 



Under this head are also rightly ranked the phe- 

 nomena of putrefaction. As M. Radot well expresses 

 it, the fermentation of sugar may be described as the 

 putrefaction of sugar. In this particular field M. 

 Pasteur, 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 

 found it to be the work of a minute fungus, the my- 

 coderma aceti, which, requiring free oxygen for its 

 nutrition, overspreads the surface of the ferme&ting 

 liquid. By the alcoholic ferment the sugar of the 

 grape-juice is transformed into carbonic acid ,u r as and 



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

 foreign member. 



