seen crossing the flask from side to
side. But here is another similar flask, which cuts a clear gap out
of the beam. It is 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.
Acting upon this obvious suggestion I set aside three other flasks,
filled, in the first instance, with mote-laden air. They are now
optically empty. Our former experiments proved that the
life-producing particles attach themselves to the fibres of
cotton-wool. In the present experiment the motes have been brought by
gentle air-currents, established by slight differences of temperature
within our closed vessels, into contact with the interior surface, to
which they adhere. The air of these flasks has deposited its dust,
germs and all, and is practically free from suspended matter.

I had a chamber erected, the lower half of which is of wood, its upper
half being enclosed by four glazed window-frames. It 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 February 6 it 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 diminution 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. Such must have been the case in the
stagnant caves of the Paris Observatory. Were our electric beam sent
through the air of these caves its track would be invisible; thus
showing the indissoluble association of the scattering of light by air
and its power to generate life.

I will now turn to what seems to me a more interesting application of
the luminous beam than any hitherto described. My reference to
Professor Lister's interpretation of the fact, that air which has
passed through the lungs cannot produce putrefaction, is fresh in your
memories. 'Why air,' said he, 'introduced into the pleural cavity,
through a wounded lung, should have such wholly different effects from
that entering through a permanently open wound, penetrating from
without, was to me a complete mystery, till I heard of the germ
theory of putref