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150 FRAGMEN TS OF SCIENCE.

the summit of Mont Blanc the blue is as uniform and co-
herent as if it formed the surface of the most close-grained
solid. A marble dome would not exhibit a stricter con-
tinuity. And Mr. Glaisher will inform you that if our hy-
pothetical shell were lifted to twice the height of Mont
.Blanc above the earth’s surface, we should 'still have the
azure overhead. Everywhere through the atmosphere those
sky-particles are strewn. They fill the Alpine valleys,
spreading like a delicate gauze in front of the slopes of
pine. They sometimes so swathe the peaks with light as
to abolish their definition. This year I have seen the
VVeisshorn thus dissolved in opalescent air. By proper
instruments the glare thrown from the sky-particles against
the retina may be quenched, and then the mountain which
it obliterated starts into sudden definition. Its extinction
in front of a dark mountain resembles exactly the with-
drawal of a veil. It is the light then taking possession of
the eye, and not the particles acting as opaque bodies, that
interferes with the definition. By day this light quenches
the stars; even by moonlight it is able to exclude from
vision all stars between the fifth and the eleventh magni-
tude. It may be likened to a noise, and the stellar radiance
to a whisper drowned by the noise.

What is the nature of the particles which shed this
light ? The celebrated De la Rive ascribes the haze of the
Alps in fine weather to floating organic germs. Now, the
possible existence of germs in such profusion has been held
up as an absurdity. It has been aflirmed that they would
darken the air, and on the assumed impossibility of their
existence in the requisite numbers, without invasion of the
solar light, a powerful argument has been based by be-
lievers in spontaneous generation. Similar arguments
have been used by the opponents of the germ theory of
epidemic disease, who have triumphantly challeged an ap-
peal. to the microscope and the chemist’s balance to decide

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