entation,
in so far as it was accompanied by the development of microscopical
organisms in enormous numbers, became assimilated to the decomposition of
an infusion of ordinary animal or vegetable matter; and it was an obvious
suggestion that the organisms were, in some way or other, the causes 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
attaining 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 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 development of these organisms must lie in something 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 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 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 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 i