e as
we possess all tends in the contrary direction, and is in favour of the
same slow and gradual changes occurring then as now.

But this conclusion in nowise conflicts with the deductions of the
physicist from his no less clear and certain data. It may be certain that
this globe has cooled down from a condition in which life could not have
existed; it may be certain that, in so cooling, its contracting crust
must have undergone sudden convulsions, which were to our earthquakes as
an earthquake is to the vibration caused by the periodical eruption of a
Geyser; but in that case, the earth must, like other respectable parents,
have sowed her wild oats, and got through her turbulent youth, before we,
her children, have any knowledge of her.

So far as the evidence afforded by the superficial crust of the earth
goes, the modern geologist can, _ex animo_, repeat the saying of Hutton,
"We find no vestige of a beginning--no prospect of an end." However, he
will add, with Hutton, "But in thus tracing back the natural operations
which have succeeded each other, and mark to us the course of time past,
we come to a period in which we cannot see any further." And if he seek
to peer into the darkness of this period, he will welcome the light
proffered by physics and mathematics.

IV

YEAST

[1871]

It has been known, from time immemorial, that the sweet liquids which may
be obtained by expressing the juices of the fruits and stems of various
plants, or by steeping malted barley in hot water, or by mixing honey
with water--are liable to undergo a series of very singular changes, if
freely exposed to the air and left to themselves, in warm weather.
However clear and pellucid the liquid may have been when first prepared,
however carefully it may have been freed, by straining and filtration,
from even the finest visible impurities, it will not remain clear. After
a time it will become cloudy and turbid; little bubbles will be seen
rising to the surface, and their abundance will increase until the liquid
hisses as if it were simmering on the fire. By degrees, some of the solid
particles which produce the turbidity of the liquid collect at its
surface into a scum, which is blown up by the emerging air-bubbles into a
thick, foamy froth. Another moiety sinks to the bottom, and accumulates
as a muddy sediment, or "lees."

When this action has continued, with more or less violence, for a certain
time, it gradually moderates.