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