suspended in air, and thus for our present purpose differs in no
important respect from fog, dust, and smoke. A cloud or mist is, in
fact, fine water-dust. Rain is coarse water-dust formed by the
aggregation of smaller globules, and varying in fineness from the
Scotch mist to the tropical deluge. It has often been asked how it is
that clouds and mists are able to float about when water is so much
heavier (800 times heavier) than air. The answer to this is easy. It
depends on the resistance or viscosity of fluids, and on the smallness
of the particles concerned. Bodies falling far through fluids acquire
a "terminal velocity," at which they are in stable equilibrium--their
weight being exactly equal to the resistance--and this terminal
velocity is greater for large particles than for small; consequently
we have all sorts of rain velocity, depending on the size of the
drops; and large particles of dust settle more quickly than small.
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