xv.] SPONTANEOUS GENERATION. 353 



fluid, or solid; that it consisted, of germs remained 

 only an hypothesis of greater or less probability. 



Contemporaneously with these investigations a re- 

 markable discovery was made by Cagniard de la Tour, 

 lie found that common yeast is conrposed of a vast 

 accumulation of minute plants. The fermentation of 

 must or of wort in the fabrication of wine and of beer 

 is always accompanied by the rapid growth and multi- 

 plication of these Toruke. Thus fermentation, in so 

 far as it was accompanied by the development of mi- 

 croscopical organisms in enormous numbers, became 

 assimilated to the decomposition of an infusion of or- 

 dinary animal or vegetable matter ; and it was an ob- 

 vious suggestion that the organisms were, in some way 

 or other, the causes both of fermentation and of putre- 

 faction. 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 Plelmholtz — reduced, the mat- 

 ter to the test of experiment by a method alike ele- 

 gant and conclusive. Helmholtz separated a putrefy- 

 ing or a fermenting liquid from one which was simply 

 putrescible or fermentable by a membrane which al- 

 lowed the fluids to pass through and become inter- 

 mixed, but stopped the passage of solids. The result 

 was, that while the putrescible or the fermentable 

 liquids became impregnated with the results of the 

 putrescence or fermentation which was going on on 

 the other side of the membrane, they neither putrefied 

 (in the ordinary way) nor fermented ; nor were any of 

 the organisms which abounded in the fermenting or 

 putrefying liquid generated in them. Therefore the 

 cause of the development of these organisms must lie 



