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A BABEL OF THEORIES 33

dissolution, they issue from them unchanged to float in the
air, or in water, or to enter into other organised bodies."
Such was the imaginative teaching with which Bonnet
combated the doctrine of spontaneous generation. When
it came to practical experimental proof, one party pro-
fessed to demonstrate the origin of living organisms from
putrescible matter in sealed vessels; the other party denied
any such possibility if air were rigorously excluded; while a
pastry-cook, named Appert, put this latter belief to a very
practical use and started to preserve fruits and other
edibles by this method.
And here we are led to the third conundrum—What
causes matter to undergo the change known as fermenta-
tion?
It is a puzzle that must have been brought home to
many a housewife ignorant of scientific problems. Why
should the milk left in the larder at night have turned sour
by the morning? Such changes, including the putrefaction
that takes place after the death of an organism, were so
much of a mystery that the causes were considered occult
for a long time. Newton had discoursed of the effect being
due to an origin ofthe same order as catalysis—a process in
which a substance, called a catalytic agent, assists in a
chemical reaction but is itself unchanged. The myriads of
minute organisms revealed, later on, by the microscope in
fermenting and putrefying matters, were at first believed
to be mere results of the general process of putrefaction
and fermentation.
A new idea was introduced by Cagniard de Latour, who
maintained that fermentation is an effect accompanying
the growth of the ferment. That is to say, he looked upon
the ferment as something living and organised, by which
fermentation is rendered a vital act. It was the microscopic
study of beer-yeast, undertaken about the year 1836,
which brought him to the opinion that the oval cells he
observed were really alive during the production of beer,
decomposing sugar into carbonic acid and alcohol.
Turpin, the botanist, interpreted this as meaning that the
c