has come for me to approach the subject before you, I find myself
conscious of some misgivings, and the misgivings are founded upon this
ground: that the subject is not one that lends itself easily to
experimental demonstration before an audience. Many of the experiments
can only be made on a small scale, and require to be watched closely.
However, by help of diagrams and by not confining myself too closely
to our special investigation, but dealing somewhat with the wider
subject of dust in general, I may hope to render myself and my subject
intelligible if not very entertaining.

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. Nevertheless, the finest cloud
or mist that ever existed consists simply of little globules of water