it was found that if you connected the mouth of the vessel with a
red-hot tube in such a way that the air would have to pass through the
tube before reaching the infusion, that then you would get no
animalcules. Yet another thing was noticed: if you took two flasks
containing the same kind of infusion, and left one entirely exposed to
the air, and in the mouth of the other placed a ball of cotton wool, so
that the air would have to filter itself through it before reaching the
infusion, that then, although you might have plenty of animalcules in
the first flask, you would certainly obtain none from the second.

These experiments, you see, all tended towards one conclusion--that the
infusoria were developed from little minute spores or eggs which were
constantly floating in the atmosphere, which lose their power of
germination if subjected to heat. But one observer now made another
experiment which seemed to go entirely the other way, and puzzled him
altogether. He took some of this boiled infusion that I have been
speaking of, and by the use of a mercurial bath--a kind of trough used
in laboratories--he deftly inverted a vessel containing the infusion
into the mercury, so that the latter reached a little beyond the level
of the mouth of the 'inverted' vessel. You see that he thus had a
quantity of the infusion shut off from any possible communication with
the outer air by being inverted upon a bed of mercury.

He then prepared some pure oxygen and nitrogen gases, and passed them by
means of a tube going from the outside of the vessel, up through the
mercury into the infusion; so that he thus had it exposed to a
perfectly pure atmosphere of the same constituents as the external air.
Of course, he expected he would get no infusorial animalcules at all in
that infusion; but, to his great dismay and discomfiture, he found he
almost always did get them.