solid
particles produced by combustion only, and "dust" to particles owing
their floating existence to some other cause. This is evidently an
unessential distinction, and for the present I shall use either term
without distinction, meaning by dust or smoke, solid particles
floating in the air. Then "fog"; this differs from smoke only in the
fact that the particles are liquid instead of solid. And the three
terms dust, smoke, and fog, come to much the same thing, only that the
latter term is applied when the suspended particles are liquid. I do
not think, however, that we usually apply the term "fog" when the
liquid particles are pure water; we call it then mostly either mist or
cloud. The name "fog," at any rate in towns, carries with it the idea
of a hideous, greasy compound, consisting of smoke and mist and
sulphur and filth, as unlike the mists on a Highland mountain as a
country meadow is unlike a city slum. Nevertheless, the finest cloud
or mist that ever existed consists simply of little globules of water
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