1886.] on the Electrical Deposition of Dust and Smoke. 521 



So the matter rested until 1881, when Lord Eaylcigh re-examined 

 the phenomenon, " not feeling satisfied with the explanation given by 

 the discoverer," and conclusively established the inefSciency not only 

 of all ready-made hypotheses concerning it, but also of those which 

 occurred to himself. A most interesting reversal of the experiment 

 was made by Lord Eayleigh, for, instead of holding a warm body 

 under the beam, he held a cold body above it, and witnessed the for- 

 mation of a down-streaming dark plane of great sharpness, bordered 

 by bright fringes of extra dusty air, which he attributed to the dew- 

 l^oint being passed and the consequent condensation of moisture uj)on 

 dust-nuclei. 



Three years ago I took up the subject at Liverpool, and, with 

 the active co-operation of the late Mr. J. W. Clark, then acting as 

 Demonstrator, repeated all known experiments with minute care, in 

 order to find out the cause of the singular phenomenon. 



The results of this research are numerous ; I can do no more than 

 summarise a few of them to-night, though I believe that some of them, 

 if followed up, would lead to results of considerable interest. The 

 first thing we observed was that the ascending dark plane was the 

 extension and upstreaming of a dust-free coat which invested the 

 surface of the warm body, and which was really the essential part of 

 phenomenon, the dark plane being merely an uj^streaming extension 

 of it. From gently warm bodies the upstreaming dark plane, being 

 made by the uniting of two coats, one from either side of the body, is 

 easier to see than the coats themselves, but they are usually quite 

 distinct, and on the average may be taken as about the hundredth of 

 an inch thick. 



[Appearance of coat and plane shown by diagram.] 



Round red-hot bodies the coat is obtrusively evident. We first 

 observed the coat on the concave side of a hemi-cylinder of sheet 

 copper. We tried this shape in order to disprove Lord Eayleigh's 

 suggested explanation that the dust-freeness might be due to the 

 curvature of the stream-lines and centrifugal force. This idea it 

 does disprove most effectively, for the curvature being negative on 

 the inside of the cylinder, dust ought to be thrown towards the body, 

 instead of being kept well away from it, as in fact it is. 



[Lantern slide of planes.] 



It is not easy to project the actual dark coat and plane on to a 

 screen, because the light from the dust particles is but feeble ; but it 

 is possible to send a beam along the rod, instead of across, and to 

 show that the dust-free space is more transparent than the sur- 

 rounding smoky air. In this way a projection of a thick coat and 

 plane, in light, upon the screen, is easily made. 



[Experiment. A platinum wire a few inches long is stretched 

 across a glass box, and arranged to be pulled straight at all tem- 

 peratures. A parallel beam is sent exactly along the wire, and 

 through a lens, the box is filled with brown paper smoke, and a few 

 Grove cells applied.] 



