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.
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