248 BIOGENESIS AND ABIOGENESIS vill 



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 in some- 

 thing which cannot pass through membranes ; 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 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 refine- 

 ments upon those of Redi. A lump of cotton-wool 

 is, physically speaking, a pile of many thicknesses 

 of a very fine gauze, the fineness of the meshes of 

 which depends upon the closeness of the compres- 

 sion of the wool. Now, Schroeder and Dusch 

 found, that, in the case of all the putrefiable 

 materials which they used (except milk and yolk 



