neighbourhood of strongly Illuminated Bodies. 215 



two explanations of the phenomena — one of which he consi- 

 dered to be applicable when the solid is at a red or white 

 heat; the other applicable when the body is at some more 

 moderate temperature, such as that of boiling or even warm 

 water. The first explanation is that the dust is absolutely 

 burnt and consumed by the heat; the second is that the hot 

 body warms the air in contact with it, which air therefore 

 rises, dragging the dust after it but getting a slight start in 

 advance of the dust, so that a thin stratum of the advance air 

 from either side of the body is free from dust, and the min- 

 gling of the two strata constitutes the dark plane. This goes 

 on continually as long as the convection-currents are pro- 

 duced by the body; and so the dark plane is permanent while 

 the body is hot. 



Prof. Frankland, in another paper on Dust and Disease, 

 gives a still simpler account of the matter, and considers 

 (Proc. Roy. Soc. vol. xxv. p. 542) that the observation proves 

 that " a very large proportion of the suspended particles in 

 the London atmosphere consists of water and other volatile 

 liquid or solid matter." In other words, Prof. Frankland 

 considers that the dust is simply dried up by the heat. 



These three explanations seem to have been sufficiently 

 plausible to satisfy those who may have examined or exhibited 

 the phenomena discovered by Dr. Tyndall, until in 1881 Lord 

 Rayleigh repeated and extended the original observation, 

 " not feeling satisfied with the explanation of the dark plane 

 given by the discoverer " (Roy. Soc. Dec. 21, 1882; i Nature/ 

 vol. xxviii. p. 139). He used a glass box to prevent draughts, 

 and his hot body was usually a small copper spade which 

 could be warmed from the outside of the box with a spirit- 

 lamp. He called attention to the fact that the stream-lines 

 round the obstacle follow the electrical law of flow, because 

 the warm obstacle is itself the origin of the motion. He 

 showed that smoke was not evaporated by being blown 

 through a hot glass tube into sunshine, and he conclusively 

 disproved any evaporation hypothesis by reversing the whole 

 phenomenon : cooling the rod instead of heating it, and causing 

 the dark plane to stream downwards. 



Lord Rayleigh further suggested an hypothesis of his own 

 to account for the dark plane in a simple mechanical manner, 

 viz. that the curvature of the stream-lines near the surface of 

 {he obstacle was such as to cause the heavier dust-particles to 

 be thrown outwards away from the body, and thus to leave a 

 thin layer of air free from dust. To test this hypothesis he 

 made a special centrifugal experiment with a whirling table, 

 the direct result of which was negative; but it led to the 



