by which fermented liquids could be surely manufactured. No doubt,
therefore, it was soon discovered that the most certain, as well as
the most expeditious, way of making a sweet juice ferment was to add
to it a little of the scum, or lees, of another fermenting juice.
And it can hardly be questioned that this singular excitation of
fermentation in one fluid, by a sort of infection, or inoculation,
of a little ferment taken from some other fluid, together with the
strange swelling, foaming, and hissing of the fermented substance,
must have always attracted attention from the more thoughtful.
Nevertheless, the commencement of the scientific analysis of the
phenomena dates from a period not earlier than the first half of the
seventeenth century.

At this time, Van Helmont made a first step, by pointing out that the
peculiar hissing and bubbling of a fermented liquid is due, not to the
evolution of common air (which he, as the inventor of the term "gas,"
calls "gas ventosum"), but to that of a peculiar kind of air such
as is occasionally met with in caves, mines, and wells, and which he
calls "gas sylvestre."

But a century elapsed before the nature of this "gas sylvestre," or,
as it was afterwards called, "fixed air," was clearly determined, and
it was found to be identical with that deadly "choke-damp" by which
the lives of those who descend into old wells, or mines, or brewers'
vats, are sometimes suddenly ended; and with the poisonous aëriform
fluid which is produced by the combustion of charcoal, and now goes by
the name of carbonic acid gas.

During the same time it gradually became clear that the presence of
sugar was essential to the production of alcohol and the evolution of