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filled with unfiltered air, and still no trace of the beam is
visible. Why? By pure accident I stumbled on this flask
in our apparatus-room, where it had remained quiet for
some time. Here are three other flasks which' have also
been kept quiet for a couple of days ; they are all optically
empty. The still air of the flasks has deposited its dust,
germs and all, and is itself practically free from suspended
matter. Hence, manifestly, the result of Pasteur.

I have had a chamber erected, the lower half of which
is of wood, its upper half being enclosed by four glazed
window-frames. The chamber tapers to a truncated cone
at the top. It measures in plan 3 ft. by 2 ft. 6 in., and its
height is 5 ft. 10 in. On the 6th of February this chamber
was closed, every crevice that could admit dust, or cause
displacement of the air, being carefully pasted over with
paper. The electric beam at first revealed the dust within
the chamber as it did in the air of the laboratory. The
chamber was examined almost daily ; a perceptible diminu-
tion of the floating matter being noticed as time advanced.
At the end of a week the chamber was optically empty,
exhibiting no trace of matter competent to scatter the light.
But where the beam entered, and where it quitted the
chamber, the white circles stamped upon the interior sur-
faces of the glass showed what had become of the dust. It
clung to those surfaces, and from them instead of from the
air, the light was scattered. If the electric beam were sent
through the air of the Paris caves, the cause of its impotence
as generator of life would, I venture to predict, be revealed.

These experiments illustrate the application of a lumi-
nous beam to researches of this kind. They prove that the
germs which produce infusorial and fungoid life share the
fate of the ordinary visible dust with which they are inter-
mixed; that such germs attach themselves to the sides of
vessels, and fall gradually to the bottom of spaces filled
with perfectly still air. But I will now turn to a far more


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