541 
ITS EFFFX'T OX TEMPERATURF AXD ITS PRESSURE OX SMALL BODIES. 
The force on A clue to B is Tra.'b'W where Px,' = 77 N'. 
lijese are not equal unless R = Rh ?.c., unless the two bodies have the same 
teinpeiature, an illustration ot the fact that equality of action and reaction does not 
hold between the radiating and receiving bodies alone. They no longer constitute 
the whole of the momentum system. The ether, or whatever we term the light¬ 
bearing medium, is material, and takes its part in the momentum relations of the 
system. 
If the surfaces are partially or totally reflecting, the forces are easily cibtained. 
Thus if one is totally reflecting, it can be shown that the force is onlv half as o'reat as 
1 • • -I *■ ^ 
Avhen it IS fully aldsorbing. But it will he sufficient to confine ourselves to the case of 
complete absorption, followed by radiation of the absorbed heat equally in all directions 
from all parts of the surface. More general assumptions do not alter the order of the 
forces found. 
If G is the constant of gravitation = 6'67 X 10“^ and if p, p' are the densities 
of A and B, the gravitation pull is G 
Tlien on B 
Radiation push F ___ dira-l/H 
Gravitation pull P 16G\jTT^a%^pp' ’ 
or 
F __ 9 R 
P IGGXJrrahpp' 
If Cl — h ; p — p ] R = 5'32 x 10 we have 
f> ^ F' 
If we suppose the two bodies to have the temperature of the sun say, G 20 G^ A, 
and its density, say 0'25, then F = P, when 
2 4'75 X r)20(P X 10"® 
” ()-25’ ” ’ 
then a — 33,500 cent 1ms. or 335 metres. 
Of course two globes of this size would soon cool far below the temperature of the 
sun, even if for an instant they could be raised up to it. 
If we suppose 6 — 300° A—the approximate temperature of small bodies at 
the distance of the eartli from the sun—and if we take p = I, then F = P, when 
a = 19'02 centims. 
Thus two globes of water—probably nearly full absorbers at 300° A—will at that 
temperature neither attract nor repel each other if their radii are about 20 centims. 
If the density of the spheres is 11 , about that so often used for masses in the 
(Javendish experiment, F = P when 
a = 1'78 centims. 
