232 
PROFESSOR CLERK MAXWELL OX STRESSES IX RARIFIED 
axes of principal stress at the given point; and the part of the stress arising from 
inequalities of temperature is, in each of these principal axes, 
P 6 dh 2 ’ 
where g is the coefficient of viscosity, p the density, and 0 the absolute temperature, 
3. Now for dry air at 15° C, /x= 1 '9 X 10 -4 in centimetre-gramme-second measure, 
and 
O/jL" 
P 6 
=-0 f 315, where p is the pressure, the unit of pressure being one dyne per 
square centimetre, or nearly one millionth part of an atmosphere. 
If a sphere of 2« centimetres in diameter is T degrees centigrade hotter than the air 
at large distances from it, then, when there is a steady How of heat, the temperature 
at a distance of r centimetres from the centre will be 
0=0 O +-, and —. 
u r dr r 
Hence, at a distance of r centimetres from the centre of the sphere* the pressure in 
the direction of the radius arising from inequality of temperature wall be 0'63 
dynes per square centimetre. 
4. In Mr. Crookes’ experiments the pressure, p, was often so small that this 
stress would be capable, if it existed alone, of producing rapid motion in a radiometer. 
Indeed, if we were to consider only the normal part of the stress exerted on solid 
bodies immersed in the gas, most of the phenomena observed by Mr. Crookes could 
be. readily explained. 
5. Let us take the case of two small bodies symmetrical with respect to the axis 
joining their centres of figure. If both bodies are warmer than the air at a distance 
from them, then, in any section perpendicular to the axis joining their centres, the 
point where it cuts this line will have the highest temperature, and there will be a 
flow of heat outwards from this axis in all directions. 
Hence — will be positive for the axis, and it will be a line of maximum pressure, 
so that the bodies will repel each other. 
If both bodies are colder than the air at a distance, everything will be reversed; 
the axis will be a line of minimum pressure, and the bodies will attract each other. 
If one body is hotter and the other colder than the air at a distance, the effect will 
be smaller, and it will depend on the relative sizes of the bodies, and on their exact 
temperatures, whether the action is attractive or repulsive. 
6. If the bodies are two parallel disks very near to each other, the central parts 
will produce very little effect, because between the disks the temperature varies 
d?Q 
uniformly, and — = 0. Only near the edges will there be any stress arising from 
Ctlv 
inequality of temperature in the gas. 
