It remains yet in the order of logic, though not of history, to show
that among these solid destructible particles there really do exist
germs capable of giving rise to the development of living forms in
suitable menstrua. This piece of work was done by M. Pasteur in those
beautiful researches which will ever render his name famous; and which,
in spite of all attacks upon them, appear to me now, as they did
seven years ago, to be models of accurate experimentation and logical
reasoning. He strained air through cotton-wool, and found, as Schroeder
and Dusch had done, that it contained nothing competent to give rise to
the development of life in fluids highly fitted for that purpose. But
the important further links in the chain of evidence added by Pasteur
are three. In the first place he subjected to microscopic examination
the cottonwool which had served as strainer, and found that sundry
bodies clearly recognisable as germs were among the solid particles
strained off. Secondly, he proved that these germs were competent to
give rise to living forms by simply sowing them in a solution fitted for
their development. And, thirdly, he showed that the incapacity of air
strained through cotton-wool to give rise to life was not due to any
occult change effected in the constituents of the air by the wool, by
proving that the cotton-wool might be dispensed with altogether, and
perfectly free access left between the exterior air and that in the
experimental flask. If the neck of the flask is drawn out into a tube
and bent downwards; and if, after the contained fluid has been carefully
boiled, the tube is heated sufficiently to destroy any germs which may
be present in the air which enters as the fluid cools, the apparatus may
be left to itself for any time and no life will appear in the fluid.
The reason is plain. Although there is free communication between the
atmosphere laden with germs and the germless air in the flask, contact
between the two takes place only in the tube; and as the germs cannot
fall upwards, and there are no currents, they never reach the interior
of the flask. But if the tube be broken short off where it proceeds from
the flask, and free access be thus given to germs falling vertically out
of the air, the fluid, which has remained clear and desert for months,
becomes, in a few days, turbid and full of life.