6l2 



NA TURE 



YApril 2i, 1884 



on the surface of the Vjody. Let me describe the appearance and 

 mode of seeing it by help of a diagram. [For full description 

 see Philosophical Magnzine for March 1884.] 



Surrounding all bodies warmer than the air is a thin region 

 free from dust which shows itself as a dark space when examined 

 by looking along a cylinder illuminated transversely, and with a 

 dark background. At high temperatures the coat is thick ; at 

 very low temperatures it is absent, and dust then rapidly collects 

 on the rod. On a warm surface only the heavy particles are able 

 to settle — there is evidently some action tending to drive small 

 bodies away. An excess of temperature of a degree or two is 

 sufficient to establish this dust-free coat, and it is easy to see the 

 dust-free plane rising from it. The appearances may also be 

 examined by looking along a cylinder lo-varJs the source of 

 light, when the dust-free spaces will appear brighter than the 

 rest. A rod of electric-light carbon warmed and fixed horizon- 

 tally across a bell-jar full of dense smoke is very suitable for this 

 experiment, and by means of a lens the dust-free regions may be 

 thus projected on to a screen. Diminished pressure makes the 

 coat thicker. Increased pressure makes it thinner. In hydrogen 

 it is thicker, and in carbonic acid thinner, than in air. We have 

 also succeeded in observing it in liquids — for instance, in water 

 holding fine rouge in suspension, the solid body being a metal 

 steam tube. Quantitative determinations are now in progress. 



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Fig. 2 



Fig. I shows the appearance when looking along a copper oy 

 carbon rod laterally illuminated ; the paths of the dust particles 

 are roughly indicated. Fig. 2 shows the coat on a semi-cylinder 

 of sheet copper with the concave side turned towards the light. 



It is difficult to give the full explanation of the dust-free spaces 

 in a few words, but we may say roughly that there is a molecular 

 bombardment from all warm surfaces by means of which small 

 suspended bodies get driven outwards and kept away from the 

 surface. It is a sort of differential bombardment of the gas 

 molecules on the two faces of a dust particle somewhat analogous 

 to the action on Mr. Crookes' radiometer vanes. Near cold 

 surfaces the bombardment is very feeble, and if they are cold 

 enough it appears to act towards the body, driving the dust in- 

 ward — at any rate there is no outward bombardment sufficient to 

 keep the dust away, and bodies colder than the atmosphere sur- 

 rounding 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 black- 

 ened with camphor black, and filling one with ice, the other with 



boiling water, I cork them and put a bell-jar over them, undei 

 which I burn some magnesium wire ; in a quarter of an hour 01 

 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 tlie case 

 when stoves, gas-burners, &c. , 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 sur- 

 face 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 elec- 

 tricity into it. It is easily possible to electrify air by means of a 

 point or flame, and an electrified body has this curious jiruperty, 

 that the dust near it at once aggregates together into larger 

 particles. It is not difficult to understand why this liajtpens ; 

 each of the particles becomes polarised by induction, and they 

 then cling together end to end, just like iron filings near a mag 

 net. 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 coagu- 

 lated, and a space can thus be cleared of it. I will illustrate this 

 action by making some artificial fogs in a bell-jar furnishel with 

 a metal point. First burn some magnesium wire, electrify it by 

 a few turns of this small Voss machine, and the smoke has be- 

 come snow ; the particles are elongated, and by pointing to the 

 charged rod indicate the lines of electrostatic force very beauti- 

 fully : 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 w ith soot, but the air is clear. Turpen- 

 tine 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 one can go 

 in and find the walls thickly covered with stringy blacks, notably 

 on the gas-pipes and everything most easily charged by 

 induction. Next fill a liell-jar full of steam, and electrify, 

 paying attention to insulation of the supply point in this 

 case. In a few seconds the air looks clear, and turning on 

 a beam of light we see the globules of water dancing about, no 

 longer fine and impalpable, but separately visible and rapidly 

 falling. Finally make a London fog by burning turpentine and 

 sulphur, adding a little sulphuric acid, either directly as vapour 

 or indirectly by a trace of nitric oxide, and then blowing in steam. 

 Electrify and it soon becomes clear, although it takes a little longer 

 than before ; and on removing the bell-jar we find that even the 

 smell of SOohas disappeared, and only a little vapour of turpentine 

 remains. Similarly we can make a Widnes fog by sulphuretted 

 hydrogen, chlorine, sulphuric acid, and a little steam. Probably 

 the steam assists the clearing when gases have to be dealt with. 

 It may be possible to clear the air of tunnels by simply dis- 

 charging electricity into the air — the electricity being supplied by 

 Holtz machines, driven say by small turbines — a very handy form 

 of power, difficult to get out of order. Or possibly some hydio- 

 electric arrangement might be devised for the locomotive steam 

 to do the work. I even hope to make some impression on a 

 London fog, discharging from lightning-conductors or captive 

 balloons carrying flames, but it is premature to say anything about 

 this matter yet. 1 have, however, cleared a room of smoke vci-y 

 quickly with a small hand machine. 



It will naturally strike you how closely allied these phenomena 

 must be to the fact of popular science that " thunder clears the 

 air." Ozone is undoubtedly generated by the flashes, and may 

 have a beneficial effect, but the dust-coagulating and -expellin:; 

 power of the electricity has a much more rapid effect, though ii 

 ■ See Nati-ke, July 26, i88j (p. 297). 



