First of all, I draw no distinction between "dust" and "smoke." It
would be possible to draw such a distinction, but it would hardly be
in accordance with usage. Dust might be defined as smoke which had
settled, and the term smoke applied to solid particles still suspended
in the air. But at present the term "smoke" is applied to 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.
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