t a thousand feet. A gentle
wind blows towards us from the north-east--that is, across the crests
and snow-fields of the Oberland mountains. We are therefore bathed by
air which must have been for a good while out of practical contact
with either animal or vegetable life. I stand carefully to leeward of
the flasks, for no dust or particle from my clothes or body must be
blown towards them. An assistant ignites the spirit-lamp, into the
flame of which I plunge the pliers, thereby destroying all attached
germs or organisms. Then I snip off the sealed end of the flask.
Prior to every snipping the same process is gone through, no flask
being opened without the previous cleansing of the pliers by the
flame. In this way we charge our seven-and-twenty flasks with clean
vivifying mountain air.

We place the fifty flasks, with their necks open, over a kitchen
stove, in a temperature varying from 50 deg. to 90 deg. Fahr, and in three
days find twenty-one out of the twenty-three flasks opened on the
hayloft invaded by organisms--two only of the group remaining free
from them. After three weeks' exposure to precisely the same
conditions, not one of the twenty-seven flasks opened in free air had
given way. No germ from the kitchen air had ascended the narrow
necks, the flasks being shaped to produce this result. They are still
in the Alps, as clear, I doubt not, and as free from life as they were
when sent off from London. [Footnote: An actual experiment made at the
Bel Alp is here described.]

What is my colleague's conclusion from the experiment before us?
Twenty-seven putrescible infusions, first in vacuo, and afterwards
supplied with the most invigorating air, have shown no sign of
putrefaction or of life. And as to the others, I almost shrink from
asking him whether the hayloft has rendered them spontaneously
generative. Is not the inference here imperative that it is not the
air of the loft--which is connected through a constantly open door
with the general atmosphere--but something contained in the air, that
has produced the effects observed? What is this something? A sunbeam
entering through a chink in the roof or wall, and traversing the air
of the loft, would show it to be laden with suspended dust particles.
Indeed the dust is distinctly visible in the diffused daylight. Can
it have been the origin of the observed life? If so, are we not bound
by all antecedent experience to regard these fruitful particle