to sense as dust
or powder placed on the palm of the hand. In the still air the dust
gradually sinks to the floor or sticks to the walls and ceiling, until
finally, by this self-cleansing process, the air is entirely freed
from mechanically suspended matter.

Thus, far, I think, we have made our footing sure. Let us proceed.
Chop up a beefsteak and allow it to remain for two or three hours just
covered with warm water; you thus extract the juice of the beef in a
concentrated form. By properly boiling the liquid and filtering it,
you can obtain from it a perfectly transparent beef-tea. Expose a
number of vessels containing this tea to the moteless air of your
chamber; and expose a number of vessels containing precisely the same
liquid to the dust-laden air. In three days every one of the latter
stinks, and examined with the microscope every one of them is found
swarming with the bacteria of putrefaction. After three months, or
three years, the beef-tea within the chamber is found in every case as
sweet and clear, and as free from bacteria, as it was at the moment
when it was first put in. There is absolutely no difference between
the air within and that without save that the one is dustless and the
other dust-laden.

Clinch the experiment thus: Open the door of your chamber and allow
the dust to enter it. In three days afterwards you have every vessel
within the chamber swarming with bacteria, and in a state of active
putrefaction. Here, also, the inference is quite as certain as in the
case of the powder sown in your garden. Multiply your proofs by
building fifty chambers instead of one, and by employing every
imaginable infusion of wild animals and tame; of flesh, fish, fowl,
and viscera; of vegetables of the most various kinds. If in all these
cases you find the dust infallibly producing its crop of bacteria,
while neither the dustless air nor the nutritive infusion, nor both
together, are ever able to produce this crop, your conclusion is
simply irresistible that the dust of the air contains the germs of the
crop which has appeared in your infusions. I repeat there is no
inference of experimental science more certain than this one. In the
presence of such facts, to use the words of a paper lately published
in the 'Philosophical Transactions,' it would be simply monstrous to
affirm that these swarming crops of bacteria are spontaneously
generated.

Is there then no experimental proof of spontaneous g