XXII INTKODUCTION. 



the carbon necessary for its growth from the pure 

 paratartrate of ammonia. Owing to the opposition 

 of its two classes of crystals, a solution of this salt, 

 it will be remembered, does not turn the plane of polar- 

 ised light either to the right or to the left. Soon 

 after fermentation had set in, a rotation to the left 

 was noticed, proving that the equilibrium previously 

 existing between the tw r o classes of crystals had ceased. 

 The rotation reached a maximum, after which it was 

 found that all the right-handed tartrate had disap- 

 peared from the liquid. The organism thus proved 

 itself competent to select its own food. It found, as 

 it were, one of the tartrates more digestible than the 

 other, and appropriated it, to the neglect of the other. 

 No difference of chemical constitution determined its 

 choice ; for the elements, and the proportions of the 

 elements, in the two tartrates were identical. But the 

 peculiarity of structure which enabled the substance to 

 rotate the plane of polarisation to the right, also ren- 

 dered it a fit aliment for the organism. This most 

 remarkable experiment was successfully made with the 

 seeds of our common mould, Penidllium glaucum. 



Here we find Pasteur unexpectedly landed amid 

 the phenomena of fermentation. With true scientific 

 instinct he closed with the conception that ferments 

 are, in all cases, living things, and that the substances 

 formerly regarded as ferments are, in reality, the 

 food of the ferments. Touched by this wand, diffi- 



