1886.] on the Electrical Deposition of Dust and SmoJce. 523 



8. Liquids containing solid powder in suspension (e. g. dried 

 ferric oxide in water) form a very narrow dark plane over a 

 moderately warm cylinder. But the thickness of the plane is so 

 slight that it is difficult to observe, and it gets thinner with increase 

 of temperature, instead of thicker as in the case of gases ; hence the 

 experimental rod in a liquid must be only gently warmed. 



The formation of a descending dark plane in dusty air below a 

 moderately cool body is singular, for it would not naturally have been 

 expected on the bombardment hypothesis. The dust particles must 

 be driven towards, instead of away from, a cold body ; and this fact is 

 obtrusively evident, from the fact that such a body becomes quickly 

 covered with soot in an atmosphere of MgO or other smoke, while a 

 warm body remains clear. 



It is easy to illustrate this fact by two black conical flasks, one full 

 of hot water, the other of cold, both put under a bell-jar of thick white 

 smoke. In about live minutes, if they be taken out, it will be found 

 that the hot oue is nearly free from soot, the cold one covered as 

 with hoar-frost. 



[Experiment. Either because they were left too long, or for 

 some other reason which I proceed to investigate, this experiment 

 in this particular instance completely failed.] 



Mr. Aitken, of Edinburgh, has also observed this fact, and has 

 pointed out that it explains the deposition of soot in chimneys, and 

 of lampblack on cold glass, Mr. Aitken's paper in the 'Transactions 

 of the Eoyal Society of Edinburgh ' for 1884, is a most comprehensive 

 and lucid account of all this part of the subject. His work was 

 concurrent with ours, as related in the ' Philosophical Magazine ' for 

 1884, and it is better described. 



Bodies colder than the air in contact with them have the dust in 

 it bombarded on to them ; as is well seen on a wall above hot-water 

 pipes, or on a ceiling above a gas-jet. Smoking of the gas-jet will of 

 course provide more material to be deposited, but the dust and smoke 

 naturally (or unnaturally) in the air are usually ample to effect a 

 sufficient blackening, over even a perfectly clear flame. An incan- 

 descent electric lamp hung a foot or so under a white ceiling will 

 similarly cause a small black patch. 



In rooms warmed by radiation (open fire or sunlight) objects 

 are warmer than the air and keep much dust off themselves ; though 

 the bombardment may not be sufficiently vigorous to overcome the 

 gravitation of the larger dust particles, especially over a flat hori- 

 zontal surface, where convection is sluggish. 



In stove-heated rooms things are liable to be colder than the air, 

 and thus get exceedingly dusty. 



The cause of the clearing of smoky air inside a bell-jar by the in- 

 troduction of a hot platinum wire, or other hot body, as observed by 

 Tyndall, who called it calcining of the dust, is now manifest : the 

 dust is bombarded on to the sides and floor of the vessel by the 

 warmed air. The self-formed picture of dark coat and plane formed 



