FIRST DISCOVERIES. 37 



ance of a small organism which played the part of 

 ferment. Pasteur applied this mode of fermentation 

 to the paratartrate of ammonia. He saw that this 

 salt also fermented, depositing the same organism. 

 All appeared as if the course of things was the same 

 as in the case of the right-handed tartrate. But 

 Pasteur, having had the idea of following the course 

 of the operation with the aid of the polariscope, soon 

 detected a profound difference between the two fer- 

 mentations. In the case of the paratartrate, the 

 liquid, at first inert, gradually assumed a sensible 

 power of deviation to the left, which augmented by 

 degrees and attained a maximum. The fermentation 

 was then suspended ; there was no longer any of the 

 right-handed acid in the liquid, which, when evaporated 

 and mixed with its own volume of alcohol, immediately 

 furnished a beautiful crystallisation of left-handed 

 tartrate of ammonia. 



From that moment a great new fact was esta- 

 blished namely, that the molecular dissymmetry 

 proper to organic matters intervened in a phenomenon 

 of the physiological order, and did so as a modifier 

 of chemical affinity. The kind of dissymmetry proper 

 to the molecular arrangement of the left-handed 

 tartaric acid was, no doubt, the sole cause of the dif- 

 ference between this acid and the right-handed acid, 

 in regard to the fermentation produced by a micro- 

 scopic fungus. We shall see later on that organised fer- 



