PROPERTIES OF MATTER IN THE GASEOUS STATE. 
831 
From equation (127) we have 
Px—y , 3 1 H s ry+Oj 
Pi 11 a/ 7t pet r/r? 
(135) 
/3H 
where, as before, is the quantity of heat carried across a unit of surface. 
At points near to the surface. 
113. In equations (131), (132), and (135) no account has been taken of the dis¬ 
continuity in the immediate neighbourhood of the surface; hence the results obtained 
from these equations may not hold good within the layer of gas of thickness s, which 
is adjacent to the surface. 
In order to take this discontinuity into account, the equations of steady conditions 
should be modified in the manner described in Art. 84, but for this particular case the 
same thing may be accomplished in a somewhat simpler manner. 
Suppose the solid surface to be either spherical or cylindrical at the point considered, 
Q 
and put c l for the radius. Then it is obvious that when — is very large the pressure 
on the surface will be but slightly affected by the layer immediately adjacent to the 
surface, i.e., putting p Ci for the pressure at the surface, and p Cx+s for the pressure at a 
distance s from the surface, 1 — ——— is small when - is large. 
Pc x +s Pi s 
When, however, the gas surrounding the surface is limited by another surface, 
of pressure or 
Maxwell is 
“stress” arising from the inequality of temperature. 
P*—Pi= 
3u 2 dfO_ 
fid dx~ 
The result given by Professor 
where y is the coefficient of viscosity, 6 the absolute temperature, and x any one of the three directions 
x, y, z. This result, when transformed to the present notation, becomes 
And if we put, as in equation (80), 
we have 
Px-Pi 
pj dx* 
M 
Px—P i_6 ?8 1 d ?t 
jq 7 t t dx^ 
It is thus seen that the two results are identical in form, but that Professor Maxwell makes the 
pressure just three times as great as that given by equation (133). 
In the abstract published in ‘ Nature,’ Maxwell has not given the details of the method by which he 
arrived at his result. 
MDCCCLXXIX, 
.') O 
