matter in the wrong place, and whatever injurious or disagreeable
effects it produces are largely due to our own dealings with nature. So
soon as we dispense with horsepower and adopt purely mechanical means of
traction and conveyance, we can almost wholly abolish disease-bearing
dust from our streets, and ultimately from all our highways; while
another kind of dust, that caused by the imperfect combustion of coal,
may be got rid of with equal facility so soon as we consider pure air,
sunlight, and natural beauty to be of more importance to the population
as a whole than are the prejudices or the vested interests of those who
produce the smoke.

But though we can thus minimize the dangers and the inconveniences
arising from the grosser forms of dust, we cannot wholly abolish it; and
it is, indeed, fortunate 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.