due to any occult change effected in
the constituents of the air by the wool, by proving that the cotton-wool
might be dispensed with altogether, and perfectly free access left
between the exterior air and that in the experimental flask. If the neck
of the flask is drawn out into a tube and bent downwards; and if, after
the contained fluid has been carefully boiled, the tube is heated
sufficiently to destroy any germs which may be present in the air which
enters as the fluid cools, the apparatus may be left to itself for any
time and no life will appear in the fluid. The reason is plain. Although
there is free communication between the atmosphere laden with germs and
the germless air in the flask, contact between the two takes place only
in the tube; and as the germs cannot fall upwards, and there are no
currents, they never reach the interior of the flask. But if the tube be
broken short off where it proceeds from the flask, and free access be
thus given to germs falling vertically out of the air, the fluid, which
has remained clear and desert for months, becomes, in a few days, turbid
and full of life.

[Footnote 9: _Lectures to Working Men on the Causes of the Phenomena of
Organic Nature_, 1863. (See Vol. II. of these Essays.)]

These experiments have been repeated over and over again by independent
observers with entire success; and there is one very simple mode of
seeing the facts for one's self, which I may as well describe.

Prepare a solution (much used by M. Pasteur, and often called "Pasteur's
solution") composed of water with tartrate of ammonia, sugar, and yeast-
ash dissolved therein.[10] Divide it into three portions in as many
flasks; boil all three for a quarter of an hour; and, while the steam is
passing out, stop the neck of one with a large plug of cotton-wool, so
that this also may be thoroughly steamed. Now set the flasks aside to
cool, and, when their contents are cold, add to one of the open ones a
drop of filtered infusion of hay which has stood for twenty-four hours,
and is consequently hill of the active and excessively minute organisms
known as _Bacteria_. In a couple of days of ordinary warm weather the
contents of this flask will be milky from the enormous multiplication of
_Bacteria_. The other flask, open and exposed to the air, will, sooner or
later, become milky with _Bacteria_, and patches of mould may appear in
it; while the liquid in the flask, the neck of which is plugged with
cott