543 
ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES. 
sun’s surface they would be about 200 centims. ; out at Neptune they would be 
about 1 millim. 
We see then that the mutual action between small bodies of density that of the 
earth, will, at different distances, change sign for difierent sizes of body, ranging from 
something of the order of 4 metres diameter near the sun to the order of 2 inillims. 
diameter at the distance of Neptune. A ring of small planets, each of radius 
3-4 centims., and density 5-5, would move round the sun at the distance of the earth 
v'lthout net mutual attraction or repulsion, and each might be regarded as moving 
independently of the rest. It appears possible that if Saturn is hot enough, 
considerations of this kind may apply to his rings. 
The repulsion between small colliding bodies, even if not heated by the sun, must 
lead to some delay in their final aggregation. This is obvious when there are only 
two small bodies, and their temperature is very considerably raised by the collision. 
But there is also delay, if instead of a single pair we suppose two swarms to collide. 
Near the boundary of tlie colliding region, a body wiU experience radiation pressure 
chiefly on one side, and will tend to be driven out of the system. Of course, if the 
swarms are so dense that a member near the outside cannot see through the rest, 
this effect will be less. A body in front of another entirely screens its radiation,’ 
but the gravitation is not screened. Hence, a body near the boundary of a densely- 
packed region of collision may be repelled only by the colliding liodies just round it, 
Avhile it will be attracted by all; or, to put the same idea in another way, a body in 
a spherical swarm of uniform temperature will only be pulled ecpially in all directions 
at the centre of the swarm, but it will be equally repelled in all directions as soon as 
it is sufficiently deep to lie surrounded l)y its fellows wherever, so to sjieak, it looks. 
Inequality of Action and Reaction hetiveen Two Mutually Radiatiny Bodies. 
We have seen that two distant spheres push each other with forces TTa-hmjU B 
and ira h K /U c/", and that these, though opposite, are not equal unless li = R'. 
It would be easy to imagine cases in which the forces were not even opposite or in 
the same directions. At first sight, then, it would appear that we have two bodies 
acting upon each other witli unequal forces, but of course this statement is inexact. 
I he bodies do not act upon each other at all; each sends out a stream of momentum 
into the medium surrounding it. Some of this momentum is ultimately intercepted by 
the other, and in its passage the momentum belongs neither to one Ijody nor to the 
other. If we assume tliat the momentum is conserved, and of course everything in 
the methods of this paper depends on that assumption, the action on one of the 
bodies is equal and opposite to the reaction on tlie liglit-liearing medium contiguous 
to it. There is no failure of the law of action and reaction, but an extension of our 
idea of matter to include the medium. There should be no difficulty in this 
extension , indeed, we have made it long ago in endowing the medium with eneigy- 
