germs in the mercury used to isolate his
air. He was never sure that they did not cling to the instruments he
employed, or to his own person. Thus when he opened his hermetically
sealed flasks upon the Mer de Glace, he had his eye upon the file used
to detach the drawn-out necks of his bottles; and he was careful to
stand to leeward when each flask was opened. Using these precautions,
he found the glacier air incompetent, in nineteen cases out of twenty,
to generate life; while similar flasks, opened amid the vegetation of
the lowlands, were soon crowded with living things. M. Pouchet
repeated Pasteur's experiments in the Pyrenees, adopting the
precaution of holding his flasks above his head, and obtaining a
different result. Now great care would be needed to render this
procedure a real precaution. The luminous beam at once shows us its
possible effect. Let smoking brown paper be placed at the open mouth
of a glass shade, so that the smoke shall ascend and fill the shade. A
beam sent through the shade forms a bright track through the smoke.
When the closed fist is placed underneath the shade, a vertical wind
of surprising violence, considering the small elevation of
temperature, rises from the band, displacing by comparatively dark air
the illuminated smoke. Unless special care were taken such a wind
would rise from M. Pouchet's body as he held his flasks above his
head, and thus the precaution of Pasteur, of not coming between the
wind and the flask, would be annulled.

Let me now direct attention to another result of Pasteur, the cause
and significance of which are at once revealed by the luminous beam.
He prepared twenty one flasks, each containing a decoction of yeast,
filtered and clear. He boiled the decoction so as to destroy whatever
germs it might contain, and, while the space above the liquid was
filled with pure steam, he sealed his flasks with a blow-pipe. He
opened ten of them in the deep, damp caves of the Paris Observatory,
and eleven of them in the courtyard of the establishment. Of the
former, one only showed signs of life subsequently. In nine out of
the ten flasks no organisms of any kind were developed. In all the
others organisms speedily appeared.

Now here is an experiment conducted in Paris, on which we can throw
obvious light in London. Causing our luminous beam to pass through a
large flask filled with the air of this room, and charged with its
germs and its dust, the beam is