act toward the body, driving the dust inward—at
any rate, there is no outward bombardment sufficient
to keep the dust away, and bodies colder than the
atmosphere surrounding them soon get dusty. Thus
if I hold this piece of glass in a magnesium flame,
or in a turpentine or camphor flame, it quickly gets
covered with smoke—white in the one case,
black in the other. I take two conical flasks
with their surfaces blackened with camphor black,
and filling one with ice, the other with boiling water,
I cork them and put a bell jar over them, under which
I burn some magnesium wire; in a quarter of an hour
or so we find that the cold one is white and hoary,
the hot one has only a few larger specks of dust on
it, these being of such size that the bombardment was
unable to sustain their weight, and they have settled
by gravitation. We thus see that when the air
in a room is warmer than the solids in it—as
will be the case when stoves, gas-burners,
etc.,
are used—things will get very dusty; whereas
when walls and objects are warmer 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