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

282 THE THEORY OF INDUCTION
"
if A, then a the "), investigator is justified in assuming
that our knowledge of the negative instances (the pro
perties of the other elements) warrants the statement
that no other element will produce that particular reaction
(i.e.,
the limited number
"
if not A, then not a But ").
of the elements places Chemistry in an exceptional posi
tion. In other branches of science, the great difficulty
lies in the examination of the negative instances. Dr
Hill has forcibly expressed this truth: "Paradoxical as
it sounds, the ingenuity of the man of science is taxed
not in making observations and devising experiments,
but in planning how to unmake them the real ;
difficulty
is experienced ... in devising an experiment in which
the supposed predisposing condition is absent, while
other conditions remain the same."
In illustration of the Double Method of Difference, we
shall analyse the investigation occasioned by the suggestions
made through the Method of Single Agreement, mentioned
in 3, Example (//).
A suggestion had been made (and controverted) that the
process of fermentation was directly connected with the
presence of living yeast- cells. Accordingly a series of
searching experimental investigations into the negative in
stances (of no fermentation) was undertaken : these afford
a beautiful example of the successful treatment of the nega
"
tive instance. Gay-Lussac showed that clean grapes or
boiled grape juice, passed into the Torricellian vacuum of a
barometer- tube, remained free from fermentation for any
length of time, but that if a single bubble of air were ad
mitted, fermentation soon appeared. Schwann repeated
Gay-Lussac s experiment, and showed that if the air were
admitted to the vacuum through a red-hot tube then fer
mentation did not occur" (M Kendrick, loc. tit.} Thus, by
two applications of the Double Method of Difference, the
following results were established : it was something in
atmospheric air that caused fermentation ; and the cause
(whatever it might be) was destroyed by heat. The effects