neighbourhood of strongly Illuminated Bodies. 231 



There remains only the H distribution " hypothesis of 

 Tyndall (originally given Proc. Roy. Inst. 1870, vol. vi. p. 3), 

 viz. that the dust lags a little behind the air as it starts off in 

 a convection-current. It is not so easy to negative this, and 

 in fact in some form or other it is certainly true that the dust 

 does get filtered out of the air, being all made to keep out- 

 side the dark coat, while some of the air passes in. The only 

 question is, Why does the dust get driven out of the air in 

 this way ? what is there near the surface of a warm body 

 which keeps back the dust ? A mere lagging behind of the 

 dust particles by reason of their extra inertia seems to us a 

 quite insufficient statement. The first formation of a coat on 

 the surface of a solid before the convection-currents have 

 started, and the case of the interior of a glass tube, may be 

 adduced as negativing any purely convective explanation : 

 the dust-free coat is seen not only on the outside of a tube 

 where convection-currents are in full swdng, but it is well 

 marked also as an internal lining of the tube where the air 

 and dust are both stagnant. The internal lining, though 

 thick, is of irregular thickness, and has not the sharp boundary 

 of the outside coat. The universal effect of convection- 

 currents is to sharpen the boundary of the coat, but to thin 

 it down. This is, indeed, a very important fact, and leads 

 straight to the conclusion that the formation of the dark coat 

 is an operation which requires time, though only a very short 

 time, and that it is possible to carry off a great part of the 

 coat more quickly than it can be renewed. 



Suggested Explanations. 



The provisional explanations which have occurred to us as 

 possible during the course of the investigation have been 

 very numerous, and have been mostly one after another dis- 

 carded, and only one or two are at all worthy of being here 

 mentioned. 



One notion which occurred pretty early in the experiment, 

 if it had held its ground, would have reduced the whole thing 

 to the merest mechanical phenomenon, just as Dr. TyndalPs 

 original ingenious explanation (to which, by the w T ay, we were 

 more than once tempted to revert) or Lord Rayleigh's centri- 

 fugal-force notion would, if true, have deprived the dark 

 spaces of most of their physical interest. It was in the belief 

 that something deeper than common mechanical principle 

 was at the root of the appearances that we were originally 

 tempted to examine them so closely ; and we are not sorry 

 that the barbarous simplicity of the notion now to be men- 

 tioned has failed to explain them, though at the same time 



