we cannot do so, since it has now been
discovered that it is to the presence of dust we owe much of the beauty,
and perhaps even the very habitability of the earth we live upon. 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.

This was proved by passing a ray of electric light lengthways through a
long glass cylinder filled with air of varying degrees of purity as
regards dust. In the air of an ordinary room, however clean and well
ventilated, the interior of the, cylinder appears brilliantly
illuminated. But if the cylinder is exhausted and then filled with air
which is passed slowly through a fine gauze of intensely heated platinum
wire, so as to burn up all the floating dust particles, which are mainly
organic, the light will pass through the cylinder without illuminating
the interior, which, viewed laterally, will appear as if filled with a
dense black cloud. If, now, more air is passed into the cylinder
through the heated gauze, but so rapidly that the dust particles are not
wholly consumed, a slight blue haze will begin to appear, which will
gradually become a pure blue, equal to that of a summer sky. If more and
more dust