1886.] on the Electrical Deposition of Dust and Smolce. 625 



instant the jar is clear of smoke : it has all been condensed on the 

 sides and floor of the vessel. 



[Experiment.] * 



The kind of smoke used is quite immaterial, it is all acted ujion 

 in the same way, but to make the effect visible to an audience, it is 

 better to use something which does not dirty or render opaque the 

 glass. Burning magnesium ribbon makes a very good and clean smoke. 

 For exi)eriments on a large scale a cheap smoke is obtained by burning 

 sulphur in the neighbourhood of a pan of ammonia. Whether 

 positive or negative electricity be used seems to make no difference. 



Instead of a single point, a double set of points may be used, each 

 connected with one pole of the machine. A round knob will act 

 instead of a point, but not so quickly. Brush discharge, or anything 

 that electrifies the air itself, is the most effectual. When a knob 

 or pair of knobs is used, the lines of force are interestingly mapped 

 out by the dust-flakes. 



The cause of the phenomenon is manifest enough. The electri- 

 fied or polarised particles attract each other, and are attracted by the 

 opposite poles, just as iron filings are influenced near a magnet. In 

 thinking over what manifestations of this aggregating power of elec- 

 tricity were already known, the beautiful observations of Lord 

 Rayleigh on water-jets occurred to me ; though the cause in this case 

 is not so clear. The experiment is not so well known as it should be, 

 and, being an extremely simple one, I venture now to show it. A 

 vertical water-jet two feet high, from an opening ^V i^^h in diameter, 

 scatters into drops and falls as a shower like rain ; but hold a piece of 

 rubbed sealing wax a yard or so distant from the place where the 

 jet breaks into drops, and they at once cease to scatter : they fall in 

 large blobs as a thunder-shower. 



[Experiment. The rain may be allowed to patter on to paper, 

 when the difference in sound is very distinct. The air should be free 

 from electricity beforehand, as the jet is extremely sensitive. | 



Clouds can probably be caused to rain by discharging electricity 

 into them ; at any rate, a cloud of steam in a bell-jar rapidly turns 

 into Scotch mist or fine rain, and so disappears. 



[Experiment. Insulation in this case is a slight difficulty, but it 

 is easily managed.] 



To make a thick mist or fog it is sufficient to introduce a scrap of 

 burning sulj)hur under the bell-jar ; instantly the country mist becomes 

 more lilvc a town fog, but this also is rapidly dispersed by electricity. 



[Experiment.] 



I have ventured to think it possible that the coagulation or com- 

 bination of oppositely charged dust particles is a gross imitation of 



* Note added July 1886, My attention has just been drawn to a paragraph 

 in 'The Mechanics' Magazine' for November 1850, wherein it appears that this 

 phenomenon was at that time observed by a Mr. C, F. Guitard ; a fact of which 

 I had been quite ignorant. 



