238 



On Phenomena exhibited by Dusty Air. 



Hot 



Surface. 



4>< 



T> 



, Cold 

 Sm-face. 



4>< 



T< 



Now, the conductivity of a gas for heat, and the viscosity 

 which resists its own motion, are each of the nature of a 

 diffusion or interchange of molecules ; and, as shown in 

 Maxwell's 'Heat,' the three things — the diffusivity , the conduc- 

 tivity, and the effective (or " kinematic") viscosity, are all 

 proportional to one another, and vary directly with the square 

 of the absolute temperature, and inversely as the pressure. 



Returning to the warm solid, with its up-streaming air- 

 currents and maximum-velocity layer towards which dust is 

 bombarded from the surface, it is evident that a rise in the tem- 

 perature of the solid would throw this layer further out from 

 its surface (and so thicken the dust-free coat), both because 

 the conductivity of the gas improves, and because its viscosity 

 near the surface increases with temperature. A fall of tem- 

 perature, on the other hand, would throw the maximum- 

 velocity layer inwards, until, when the solid was colder than 

 the air, the maximum down-streaming layer might be very 

 close to the surface, because its viscosity would be least there, 

 and because the conductivity of the gas would then be so poor. 

 (N.B. We never assume that the dusty air is warmed by 

 radiation from the solid, because the far more powerful elec- 

 tric-light radiation produces no particular effect on the dust 

 except in the neighbourhood of bodies.) The effect of an 

 increased pressure would be to diminish both the conductivity 

 and the viscosity in the same proportion ; and so for both 

 reasons it would throw the maximum-velocity layer inwards, 

 in fact it would act like cold. Rarefaction of the air would 

 act like an increase of temperature. 



Hydrogen has a higher conductivity and viscosity than air, 

 consequently its maximum-velocity layer should be further 

 from the surface of the warm solid exciting the currents. Car- 



