VIII 



that it consisted of germs remained only an hypo- 

 thesis of greater or less probability. 



Contemporaneously with these investigations a 

 remarkable discovery was made by Cagniard de la 

 Tour. He found that common yeast is com- 

 posed of a vast accumulation of minute plants. 

 The fermentation of must, or of wort, in the 

 fabrication of wine and of beer, is always accom- 

 panied by the rapid growth and multiplication of 

 these Torulce. Thus, fermentation, in so far as it 

 was accompanied by the development of micro- 

 scopical organisms in enormous numbers, became 

 assimilated to the decomposition of an infusion of 

 ordinary animal or vegetable matter ; and it was 

 an obvious suggestion that the organisms were, in 

 some way or other, the causes both of fermentation 

 and of putrefaction. The chemists, with Berzelius 

 and Liebig at their head, at first laughed this idea 

 to scorn ; but in 1843, a man then very young, 

 who has since performed the unexampled feat of 

 attaining to high eminence alike in Mathematics, 

 Physics, and Physiology I speak of the illustrious 

 Helmholtz reduced the matter to the test of 

 experiment by a method alike elegant and con- 

 clusive. Helmholtz separated a putrefying or a 

 fermenting liquid from one which was simply 

 putrescible or fermentable by a membrane which 

 allowed the fluids to pass through and become 

 intermixed, but stopped the passage of solids. 

 The result was, that while the putrescible or the 



