ust, and transported to the Alps. Thither,
to an elevation of about 7,000 feet above the sea, I invite my
co-enquirer to accompany me. It is the month of July, and the weather
is favourable to putrefaction. We open our box at the Bel-Alp, and
count out fifty-four flasks, with their liquids as clear as filtered
drinking water. In six flasks, however, the infusion is found muddy.
We closely examine these, and discover that every one of them has had
its fragile end broken off in the transit from London. Air has entered
the flasks, and the observed muddiness is the result. My colleague
knows as well as I do what this means. Examined with a pocket-lens,
or even with a microscope of insufficient power, nothing is seen in
the muddy liquid; but regarded with a magnifying power of a thousand
diameters or so, what an astonishing appearance does it present!
Leeuwenhoek estimated the population of a single drop of stagnant
water at 500,000,000: probably the population of a drop of our turbid
infusion would be this many times multiplied. The field of the
microscope is crowded with organisms, some wabbling slowly, others
shooting rapidly across the microscopic field. They dart hither and
thither like a rain of minute projectiles; they pirouette and spin so
quickly round, that the retention of the retinal impression transforms
the little living rod into a twirling wheel. And yet the most
celebrated naturalists tell us they are vegetables. From the rod-like
shape which they so frequently assume, these organisms are called
'bacteria'--a term, be it here remarked, which covers organisms of
very diverse kinds.

Has this multitudinous life been spontaneously generated in these six
flasks, or is it the progeny of living germinal matter carried into
the flasks by the entering air? If the infusions have a
self-generative power, how are the sterility and consequent clearness
of the fifty-four uninjured flasks to be accounted for? My colleague
may urge--and fairly urge--that the assumption of germinal matter is
by no means necessary; that the air itself may be the one thing needed
to wake up the dormant infusions. We will examine this point
immediately. But meanwhile I would remind him that I am working on
the exact lines laid down by our most conspicuous heterogenist. He
distinctly affirms that the withdrawal of the atmospheric pressure
above the infusion favours the production of organisms; and he
accounts for their absence in ti