328 THE CANADIAN NATURALIST. [Sept. 



Contemporaneously with these investigations a remarkable dis- 

 covery was made by Cagniard de La Tour. He found that com- 

 mon yeast is composed of a vast accumulation of minute plants. 

 The fermentation of must, or of wort, in the fabrication of wine 

 and beer, is always accompanied by the rapid growth and multi- 

 plication of these Torula3. Thus fermentation, in so far as it 

 was accompanied by the developement of microscopical organisms 

 in enormous numbers, became assimilated to the decomposition of 

 an infusion of ordinarv animal or vesretable matter ; and it was an 

 obvious suggestion that the organisms were, in some way or other, 

 the causes both of fermentation and 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 a test of experi- 

 ment by a method alike elegant and conclusive. Helmholtz sepa- 

 rated a putrefying, or 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 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 developement of these organisms must 

 lie in something which cannot pass through membrane ; and as 

 Helmholtz's investigations were long antecedent to Graham's 

 researches upon colloids, his natural conclusion was, that the 

 agent thus intercepted must be a solid material. In point of fact 

 Helmholtz's experiments narrowed the issue to this : that which 

 excites fermentation and putrefaction, and at the same time gives 

 rise to living forms in a fermentable, or putrescible fluid, is not a 

 gas and is not a difi"usible fluid ; therefore it is either a colloid, or 

 it is matter divided into very minute solid particles. 



The researches of Schroeder and Dusch in 1854, and of Schroe- 

 der alone, in 1859, cleared up this point by experiments which 

 are simply refinements upon those of Kedi. A lump of cotton- 

 wool is, physically speaking, a pile of many thicknesses of very fine 

 gauze, the fineness of the meshes of which depends upon the close- 



