232 CRITIQUES AND ADDRESSES. [x. 



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 attain- 

 ing 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 conclusive. 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 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 develop- 

 ment of these organisms must lie in something which 

 cannot pass through membranes ; and as Helmholtz's 

 investigations were long antecedent to Graham's re- 

 searches 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 putre- 

 faction, and at the same time gives rise to living forms 

 in a fermentable or putrescible fluid, is not a gas and is 

 not a diffusible 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 Schroeder alone, in 1859, cleared up this point by 

 experiments which are simply refinements upon those of 

 Eedi. A lump of cotton-wool is, physically speaking, a 

 pile of many thicknesses of a very fine gauze, the fineness 



