PROFESSOR CLERK MAXWELL OX STRESSES IX RARIFIED GASES. 
25 6 
The theory of thermal effusion through a small hole in a thin plate is therefore a 
very simple one. It does not involve the theory of viscosity at all. 
The finer the pores of a porous plate, and the rarer the gas which effuses through it, 
the more nearly does the passage of gas through the plate correspond to what we have 
called effusion, and the less does it depend on the viscosity of the gas. 
The coarser the pores of the plate and the denser the gas, the further does the 
phenomenon depart from simple effusion, and the more nearly does it approach to 
transpiration through a capillary tube, which depends altogether on viscosity. 
To return to the case of transpiration through a capillary tube. When the tempera¬ 
ture is uniform 
Q= 
7 rpa 1 dpi 
8 p, dz\ 
(78) 
By experiments on capillary tubes of glass, MM. Ivundt and Warburg found'" 
for the value of G for air at different pressures and at from 17° C. to 27° 0., 
G=- centimetres 
P 
(79) 
where p is the pressure in dynes per square centimetre, which is nearly the same as 
in millionths of an atmosphere. For hydrogen on glass 
centimetres 
(80) 
- When there is no flow of gas in a tube in which the temperature varies from end to 
end, the pressure is greater at the hot end than at the cold end. Putting Q.— 0 we have 
dd pda* + 4Ga 
(81) 
The quantity 6 
P 0 
is just double of that calculated in section (3) of the introduction, 
and is therefore in C.G.S measure 0'63x-y> for dry air at 15° C. Let us suppose a = 0'01 
centimetre, and the pressure 40 millimetres of mercury, then G=’00016 centimetre. 
If one end of the tube is kept at 0° C. and the other at 100° C., the pressure at the 
hot end will exceed that at the cold end by about 1 ■ 2 millionths of an atmosphere. 
The difference of pressure might be increased by using a tube of smaller bore and 
air of smaller density, but the effect is so small that though the theoretical proof of its 
existence seems satisfactory, an experimental verification of it would be difficult. 
* Pogg. Ann., July, 1876. 
