360 LAY SERMONS, ADDRESSES, AND REVIEWS. [xv. 



in something which cannot pass through membranes ; 

 and, as Helmholtz's investigations were long antece* 

 dent 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 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 Recli. A lump of cotton- wool is, physically speak- 

 ing, a pile of many thicknesses of a very fine gauze, 

 the fineness of the meshes of which depends upon the 

 closeness of the compression of the wool. Now, 

 Schroeder and Dusch found that, in the case of all the 

 putrefiable materials which they used (except milk 

 and yolk of egg), an infusion boiled, and then allowed 

 to come into contact with no air but such as had been 

 filtered through cotton-wool, neither putrefied nor fer- 

 mented, nor developed living forms. It is hard to 

 imagine what the fine sieve formed by the cotton- wool 

 could have stopped except minute solid particles. 

 Still the evidence was incomplete until it had been 

 positively shown, first, that ordinary air does contain 

 such particles; and, secondly, that filtration through 

 cotton-wool arrests these particles and allows only 

 physically pure air to pass. This demonstration has 

 been furnished within the last year by the remarkable 

 experiments of Professor Tyndall. It has been a 

 common objection of Abiogenists that, if the doctrine 

 of Biogeny is true, the air must be thick with germs ; 



