542 PROFESSOR J. H. POYNTINO ON RADIATION IN THE SOLAR SYSTEM: 
This does not throw any doubt on the results of Cavendish experiments, for it 
only holds when the radiators are in an enclosure of very low absolute temperatuie. 
In all Cavendish experiments tlie greatest care is taken to make the attracted body 
and its enclosure of one uniform temperature. 
The really interesting case is that of two small meteorites, in interplanetary space. 
To judge from the specimens which succeed in penetrating the earths atmospheie 
they are very dense. Let us suppose them to have density 5-5 that of the earth 
and temperature 300° A, that which they will have at the earth’s distance. Then 
F = P when 
a = 3‘4 centims. 
If the radii of the bodies are less than the values found for equality of F and P in 
the different cases, the net effect is repulsion. 
The ratio of F to P is inversely as the square of the radius, so that, as the radii are 
decreased from the values giving F = P, the radiation repulsion soon becomes 
enormously greater than the gravitation pull, and the latter may be neglected in 
comparison. Thus for two drops of water at 300° A in a zero enclosure, with radii 
0-001 centim., the pressure is nearly 400,000,000 times the pull. 
It is not, however, that the radiation force is great, or even its acceleration. 
The 
force becomes exceedingly minute, but the gravitation much moie minute. 
Thus consider two drops of water at 300° placed in a zero enclosure at a distance 
d = 10a apart. Our assumption of parallel radiation from one to the other is now 
only a rough approximation, but the result will be of the right ordei. ^ ^ 
The radiation push is 7 ra^R/Ud 5 and the acceleration is 3rtR/4U(P = X — 
approximately. 
This only becomes considerable when the drops approach molecular dimensions, 
and long before this they cease to absorb fully the stream of momentum falling on 
them. Still, even molecules are selective absorbers, and absorb especially each othei s 
radiations. And we may expect that if two gas molecules collide and set each other 
radiating much more violently tlian before, they will be practically in an enclosure 
of much lower temperature than their own, and their mutual radiation may result in 
verv rapid repulsion—repulsion of the order of the fourth power of the temperature 
reached. 
Radiation Pressure between Small Bodies at Different Distances from the Sun. 
We have seen above, tliat if two small spheres of density 5-5 are at the distance of 
the earth from the sun, their gravitation will be balanced liy tlieir radiation pressure 
when the radius of each is 3-4 centims. Now the balancing radius is proportional to 
the square of the temperature, that is, inversely proportional to the distance, since the 
temperature (Part I.) is inversely as the square root of the distance. ihus, at tlie 
distance of Mercury, the radii would be about 9 centims. ; a million miles from the 
