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carefully turned on. The air passes slowly through the cotton-wool, the
caustic potash, and the sulphuric acid in succession. Thus purified it
enters the flask F and bubbles through the liquid. Charged with vapor
it finally passes into theexperimental tube, where it is submitted to ex-
amination. The electric lamp L placed at the end of the experimental
tube furnished the necessary beam.

Wanting the cotton-wool the floating matter of the air
ran the gantlet of this system. The fact thus forced upon
my attention had a bearing too obvious to be overlooked.
It rendered at once evident to the senses Why air filtered
through cotton-wool is incompetent to generate animalcular
life. The air is rendered by this treatment Optically pure;
in other words, freed from all floating matter, germs in
cluded. But the observations also revealed the great
liability to error in experiments of this nature. They
showed that without an amount of care which was hardly
to be expected in all cases, error would be inevitable. It
was especially manifest that the chemical method of
Schultze might lead to the most erroneous consequences;
that neither acids nor alkalies had the power of rapid
destruction which they had been supposed to possess. In .
short, the employment of the luminous beam rendered
evident the cause of success in experiments rigidly con-
ducted like those of Pasteur; while it made equally evident
the certainty of failure in experiments less severely and less
skilfully carried out.

Dr. Bennett’s Emperiments.

Take, for example, the well-conceived experiments of
Dr. Hughes Bennett, described before the Royal Society
of Surgeons in Edinburgh, on January 17, 1868.1 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 so-

1British Medical Journal, 13, pt. ii. 1868.

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