1882.] which is formed over a Heated Wire in Dusty Air. 417 



a dark plane, extending downwards from the rod, clearly developed 

 itself and persisted for a long while. This result not merely shows 

 that the dark plane is not due to evaporation, but also excludes any 

 explanation depending upon an augmentation in the difference of 

 densities of fluid and foreign matter. 



The experiment was varied by using a U"* u ^ e ? through which 

 cooled water could be made to flow. When the water was not very 

 cold, the appearances were much the same as with the solid rod ; but 

 when, by means of salt and ice, the tube was cooled still further, a 

 curious complication presented itself. Along the borders of the dark 

 plane the smoke appeared considerably brighter than elsewhere. 

 Sometimes when the flow was not very regular it looked at first as if 

 the dark plane had been replaced by a bright one, but on closer ex- 

 amination the dark plane could be detected inside. There seems no 

 doubt but that the effect is caused by condensation of moisture upon 

 the smoke, due to the chilling which the damp air undergoes in pass- 

 ing close to the cold obstacle. Where the fog forms more light is 

 scattered, hence the increased brightness. That the fog should not 

 form within the smoke-free plane itself is what we might expect 

 from the interesting observations of Aitken. 



With respect to the cause of the formation of the dark plane, the 

 most natural view would seem to be that the relatively dense par- 

 ticles are thrown outwards by centrifugal force as the mixture flows 

 in curved lines round the obstacle. Even when the fluid is at rest, a 

 gradual subsidence must take place under the action of gravity ; but 

 this effect could at first only manifest itself at the top where the 

 upper boundary of the gas prevents the entrance of more dust from 

 above. It is known that air in a closed space will gradually free 

 itself from dust, but the observation of a thin dust-free stratum at 

 the top of the vessel is difficult. If we conceive a vessel full of 

 dusty air to be set into rapid rotation, the dust might be expected to 

 pass outwards in all directions from the axis, along which a dust-free 

 line would form itself. I have tried this experiment, but looking 

 along the axis through the glass top of the vessel I could see no sign 

 of a dark line, so long as the rotation was uniform. When, however, 

 the vessel was stopped, a column of comparatively smoke-free air 

 developed itself along the axis. This I attributed to the formation of 

 an inward flow along the top of the vessel, combined with a down- 

 ward flow along the axis after the manner described and explained 

 by Professor James Thomson, so that the purified air had been in 

 intimate proximity with the solid cover. It would almost seem as if 

 this kind of contact was sufficient to purify the air without the aid of 

 centrifugal force. 



The experiments made hitherto in order to elucidate this question 

 have given no decisive result. If the thin convex blade already 



