nto contact with no air but such as had
been filtered through cotton-wool, neither putrefied, nor fermented, nor
developed living forms. It is hard to imagine what the fine sieve formed
by the cotton-wool could have stopped except minute solid particles.
Still the evidence was incomplete until it had been positively shown,
first, that ordinary air does contain such particles; and, secondly, that
filtration through cotton-wool arrests these particles and allows only
physically pure air to pass. This demonstration has been furnished within
the last year by the remarkable experiments of Professor Tyndall. It has
been a common objection of Abiogenists that, if the doctrine of Biogeny
is true, the air must be thick with germs; and they regard this as the
height of absurdity. But nature occasionally is exceedingly unreasonable,
and Professor Tyndall has proved that this particular absurdity may
nevertheless be a reality. He has demonstrated that ordinary air is no
better than a sort of stirabout of excessively minute solid particles;
that these particles are almost wholly destructible by heat; and that
they are strained off, and the air rendered optically pure, by its being
passed through cotton-wool.

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,[9]
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 cotton-wool 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