An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

OR SCIENTIFIC METHOD. 283
of temperature were then further studied. A temperature
of from 20 C. to 24 C. was most favourable to it ; while the
process was stopped at freezing-point (o C.) and again at
60 C. ; and boiling destroyed it. Afterwards the connection
with atmospheric air was examined. Helmholtz showed that
the oxygen produced by electrolysis in a sealed-up tube con
taining a boiled fermentible fluid did not cause fermenta
tion. This was simply oxygen that was absolutely unmixed
with any organic or other foreign matter, and differed in
this respect from atmospheric oxygen, since air always has
extremely minute living organisms in it. Hoffmann showed
that air filtered through cotton-wool was incapable of causing
fermentation. All these negative instances went to confirm
the theory that the yeast-cells were the immediate cause of
the process, especially the fact that the cause of the process
was destroyed by heat. But the most ingenious "negative"
experiment was made by Helmholtz.
" He placed a sealed
bladder full of grape-juice in a vat of fermenting juice, and
found that the fluid in the bladder did not ferment. Thus
the cause of the fermentation could not pass through the
wall of the bladder. If the fermentation were excited, as
Liebig held, by a separate substance formed by the yeast-
cells, and presumably soluble, one would have expected it to
pass through the wall of the bladder ; but if the process
were caused by the small yeast-cells, then one can see why
fermentation was not excited, as the yeast-cells could not
pass through the membrane."
The theory of causation by yeast-cells was not proved by
these applications of the Double Method of Difference; but
this Method proved a number of facts about the cause, which
lent support to that theory, and laid the foundation for the
splendid researches of Pasteur.
8. In the most
"
"
exact sciences, where the causes
and effects which we examine are susceptible of degrees
of intensity, or at any rate of being
may not only observe and compare
" more or we
less,"
events but measure
them. Jevons s statement is fully justified :
question in science is first a matter of fact only,
"
Every
then a
matter of quantity, and by degrees becomes more and