temperature, is passed
through a red-hot tube, you will not get a trace of organisms.

He then turned his attention to the mercury bath, and found on
examination that the surface of the mercury was almost always covered
with a very fine dust. He found that even the mercury itself was
positively full of organic matters; that from being constantly exposed
to the air, it had collected an immense number of these infusorial
organisms from the air. Well, under these circumstances he felt that the
case was quite clear, and that the mercury was not what it had appeared
to M. Schwann to be,--a bar to the admission of these organisms; but
that, in reality, it acted as a reservoir from which the infusion was
immediately supplied with the large quantity that had so puzzled him.

But not content with explaining the experiments of others, M. Pasteur
went to work to satisfy himself completely. He said to himself: "If
my view is right, and if, in point of fact, all these appearances 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 t