HIS LIFF AND LABOURS. 183 



ing between the two 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. 

 Xo 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 

 turn 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, Penicillium 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, difficulties 

 fell rapidly before him. He proved the ferment of 

 lactic acid to be an organism of a certain kind. The 

 ferment of butyric acid he proved to be an organism of 

 a different kind. He was soon led to the fundamental 

 conclusion that the capacity of an organism to act as a 

 ferment depended on its power to live without air. 

 The fermentation of beer was sufficient to suggest this 

 idea. The yeast-plant, like many others, can live 

 either with or without free air. It flourishes best in 

 contact with free air, for it is then spared the labour 

 of wresting from the malt the oxygen required for its 

 sustenance. Supplied with free air, however, it prac- 

 tically ceases to be a ferment ; while in the brewing- vat, 

 where the work of fermentation is active, the budding 



