Transpiration and Radiometer Motion, 389 



make the mean radius of the passages in stucco 4*2 and 5*1 

 times those in meerschaum II. and III., those for air make it 

 4*8 and 4*8 times, and the agreement of the means 4*65 and 

 4*8 is close enough to show that our expression 



6ai? (R 2 + RiVifoOs + v i) 2 



is right enough as regards the occurrence of the mean radius 

 of the passages in it ; and moreover our equation (14) showed 

 that R entered in the form R/X, so that the discrepancy just 

 found must be due to some considerations being ignored in 

 connexion with X. 



Now it is a well known fact established by experiment 

 that gases are condensed in the passages of porous bodies. 

 The condensing action exerted by a solid surface on a gas is 

 easily expressed quantitatively, for near the end of section (8) 

 of my paper on the Laws of Molecular Force (Phil. Mag. 

 [5] xxxv.) the attraction of a cylinder of radius c, length h, 

 and density pona particle of mass m at distance z along the 

 axis from the nearest end, the law of force being 3Amw///* 4 , is 



2A7rmp[l/z-l/(z + h)-V(c 2 + z>)*+l/{c 2 +{z + h) 2 \$], 



whence the attraction on a particle at small distance z from 

 the surface of a solid may be written 2AiT7vp/z, and if the 

 particle is one of the molecules of a gas, the condition of 

 equilibrium in the gas is 



— dp/dz = 2Airpnm/z or — dp/p = 6 Awp dz/zv 2 ; 



and if p s is the pressure in the layer nearest to the surface 

 which is at distance z s from the surface, and p c is the pressure 

 at a distance z c where the effects of the solid are negligible, 

 then 



tegpJp=^logzJ Z „ . . . (20) 



IT 



a formula which makes the density of the gas in contact with 

 the solid nearly proportional to the density where the gas is 

 free, because with gases 6A.7rp/v 2 is a small fraction. This 

 formula will be investigated a little further in my next paper, 

 on " Boyle's Law at very Low Pressures/' 



A rigorous investigation for condensation in a tube would 

 be simple enough, but it suffices for our present purposes to 

 see that in most cases the density at the surface of the tube 

 will be connected with the density at the axis by the relation 



Phil. May. S. 5. Vol. 42. No. 258. Nov. 1896. 2 F 



