r than the
air--as will be the case in sunshine, or when open fireplaces are
used, things will tend to keep themselves more free from dust. Mr.
Aitken points out that soot in a chimney is an illustration of this
kind of deposition of dust; and as another illustration it strikes me
as just possible that the dirtiness of snow during a thaw may be
partly due to the bombardment on to the cold surface of dust out of
the warmer air above. Mr. Aitken has indeed suggested a sort of
practical dust or smoke filter on this principle, passing air between
two surfaces--one hot and one cold--so as to vigorously bombard the
particles on to the cold surface and leave the air free.

But we have found another and apparently much more effectual mode of
clearing air than this. We do it by discharging electricity into it.
It is easily possible to electrify air by means of a point or flame,
and an electrified body has this curious property, that the dust near
it at once aggregates together into larger particles. It is not
difficult to understand why this happens; each of the particles
becomes polarized by induction, and they then cling together end to
end, just like iron filings near a magnet. A feeble charge is often
sufficient to start this coagulating action. And when the particles
have grown into big ones, they easily and quickly fall. A stronger
charge forcibly drives them on to all electrified surfaces, where they
cling. A fine water fog in a bell jar, electrified, turns first into a
coarse fog or Scotch mist, and then into rain. Smoke also has its
particles coagulated, and a space can thus be cleared of it. I will
illustrate this action by making some artificial fogs in a bell-jar
furnished with a metal point. First burn some magnesium wire,
electrify it by a few turns of this small Voss machine, and the smoke
has become snow; the particles are elongated, and by pointing to the
charged rod indicate the lines of electrostatic force very
beautifully; electrify further, and the air is perfectly clear. Next
burn turpentine, and electrify gently; the dense black smoke
coagulates into black masses over an inch long; electrify further, and
the glass is covered with soot, but the air is clear. Turpentine smoke
acts very well, and can be tried on a larger scale; a room filled with
turpentine smoke, so dense that a gas-light is invisible inside it,
begins to clear in a minute or two after the machine begins to turn,
and in a quarter of an hour