of
spontaneous generation are altogether due to the falling of minute germs
suspended in the atmosphere,--why, I ought not only to be able to show
the germs, but I ought to be able to catch and sow them, and produce
the resulting organisms." He, accordingly, constructed a very ingenious
apparatus to enable him to accomplish this trapping of this "germ dust"
in the air. He fixed in the window of his room a glass tube, in the
centre of which he had placed a ball of gun-cotton, which, as you all
know, is ordinary cotton-wool, which, from having been steeped in strong
acid, is converted into a substance of great explosive power. It is also
soluble in alcohol and ether. One end of the glass tube was, of course,
open to the external air; and at the other end of 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-cott