it he placed an aspirator, a contrivance for causing a current of the
external air to pass through the tube. He kept this apparatus going
for four-and-twenty hours, and then removed the 'dusted' gun-cotton,
and dissolved it in alcohol and ether. He then allowed this to stand
for a few hours, and the result was, that a very fine dust was
gradually deposited at the bottom of it. That dust, on being
transferred to the stage of a microscope, was found to contain an
enormous number of starch grains. You know that the materials of our
food and the greater portion of plants are composed of starch, and we
are constantly making use of it in a variety of ways, so that there is
always a quantity of it suspended in the air. It is these starch
grains which form many of those bright specks that we see dancing in a
ray of light sometimes. But besides these, M. Pasteur found also an
immense number of other organic substances such as spores of fungi,
which had been floating about in the air and had got caged in this way.

He went farther, and said to himself, "If these really are the things
that give rise to the appearance of spontaneous generation, I ought to
be able to take a ball of this 'dusted' gun-cotton and put it into one
of my vessels, containing that boiled infusion which has been kept away
from the air, and in which no infusoria are at present developed, and
then, if I am right, the introduction of this gun-cotton will give rise
to organisms."

Accordingly, he took one of these vessels of infusion, which had been
kept eighteen months, without the least appearance of life, and by a
most ingenious contrivance, he managed to break it open and introduce
such a ball of gun-cotton, without allowing the infusion or the cotton
ball to come into contact with any air but that which had been subjected
to a red heat, and in twenty-four hours he had the satisfaction of