Few
of the fairy tales of science are more marvelous than these recent
discoveries as to the varied effects and important uses of dust in the
economy of nature.
The question why the sky and the deep ocean are both blue did not much
concern the earlier physicists. It was thought to be the natural color
of pure air and water, so pale as not to be visible when small
quantities were seen, and only exhibiting its true tint when we looked
through great depth of atmosphere or of organic water. But this theory
did not explain the familiar facts of the gorgeous tints seen at sunset
and sunrise, not only in the atmosphere and on the clouds near the
horizon, but also in equally resplendent hues when the invisible sun
shines upon Alpine peaks and snowfields. A true theory should explain
all these colors, which comprise almost every tint of the rainbow.
The explanation was found through experiments on the visibility or
non-visibility of air, which were made by the late Professor Tyndall
about the year 1868. Everyone: has seen the floating dust in a sunbeam
when sunshine enters a partially darkened room; but it is not generally
known that if there was absolutely no dust in the air the path of the
sunbeam would be totally black and invisible, while if only very little
dust was present in very minute particles the air would be as blue as
the summer sky.
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