ility of air which had been filtered through
cotton-wool to generate animalcular life, had been demonstrated by
Schroeder and Pasteur: here the cause of its impotence was rendered
evident to the eye. The experiment proved that no sensible amount of
light was scattered by the molecules of the air; that the scattered
light always arose from suspended particles; and the fact that the
removal of these abolished simultaneously the power of scattering
light and of originating life, obviously detached the life-originating
power from the air, and fixed it on something suspended in the air.
Gases of all kinds passed with freedom through the plug of
cotton-wool; hence the thing whose removal by the cotton-wool rendered
the gas impotent, could not itself have been matter in the gaseous
condition. It at once occurred to me that the retina, protected as it
was, in these experiments, from all extraneous light, might be
converted into a new and powerful instrument of demonstration in
relation to the germ theory.

But the observations also revealed the danger incurred in experiments
of this nature; showing that without an amount of care far beyond that
hitherto bestowed upon them, such experiments left the door open to
errors of the gravest description. It was especially manifest that
the chemical method employed by Schultze in his experiments, and so
often resorted to since, might lead to the most erroneous
consequences; that neither acids nor alkalies had the power of rapid
destruction hitherto ascribed to them. In short, the employment of
the luminous beam rendered evident the cause of success in experiments
rigidly conducted like those of Pasteur; while it made equally evident
the certainty of failure in experiments less severely carried out.

Dr. Bennett's Experiments.

But I do not wish to leave an assertion of this kind without
illustration. Take, then', the well-conceived experiments of Dr.
Hughes Bennett, described before the Royal Society of Surgeons in
Edinburgh on January 17, 1868. [Footnote: 'British Medical Journal,'
13, pt. ii. 1868.] Into flasks containing decoctions of
liquorice-root, hay, or tea, Dr. Bennett, by an ingenious method,
forced air. The air was driven through two U-tubes, the one
containing a solution of caustic potash, the other sulphuric acid.
'All the bent tubes were filled with fragments of pumice-stone to
break up the air, so as to prevent the possibility of any germs
passing through in the