Cloud-spherules are falling therefore, but falling very slowly.
To recognize the presence of dust in air there are two principal
tests; the first is, the obvious one of looking at it with plenty of
light, the way one is accustomed to look for anything else; the other
is a method of Mr. John Aitken's, viz., to observe the condensation of
water vapor.
Take these in order. When a sunbeam enters a darkened room through a
chink, it is commonly said to be rendered visible by the motes or dust
particles dancing in it; but of course really it is not the motes
which make the sunbeam visible, but the sunbeam the motes. A dust
particle is illuminated like any other solid screen, and is able to
send a sufficient fraction of light to our eyes to render itself
visible. If there are no such particles in the beam--nothing but
clear, invisible air--then of course nothing is seen, and the beam
plunges on its way quite invisible to us unless we place our eyes in
its course. In other words, to be visible, light must enter the eye.
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