264 The Evolution of the Solar System. [May, 
utmost quantity of matter, except as they may be added to 
by meteoric accretion. One hypothesis argues that the 
Earth, and presumably all planets, will eventually absorb all 
its seas and atmosphere, and afterwards roll on through 
space, a dead world. Yet it is possible that these conclu- 
sions have been too hastily reached. 
It was formerly argued that the atmosphere, from its rate 
of decrease of density, must cease to exist at a height of 
some fifty miles from the surface. Yet we now know that 
it is sufficiently dense to produce incandescence, through 
fridtion with meteoric bodies, at a height of several hundred 
miles, and may extend to a much greater height in a more 
diffused state. Thus we have some reason to believe that it 
has no real limit, but shades off imperceptibly into the 
matter of space, this matter being condensed around every 
orb, and gradually rarefied outwardly. 
It might be supposed, however, that we could imagine a 
true limit to the atmosphere at that point where it ceases to 
be sufficiently controlled by the earth’s atmosphere to be 
obliged to rotate with it. But we hope to show that there 
is no such limiting point, and that the atmosphere in this 
respedt also shades off into the inter-spheral conditions of 
substance. The earth is a globe of matter limited in size 
by two diverse forces — the centripetal force of gravity, and 
the centrifugal force of rotation. Its gravitative vigour de- 
pends upon its material contents and their degree of con- 
densation. To this vigour the atmosphere adds very little. 
The density of the material possessed by the earth is not a 
diredt consequent of gravitative force. It arises partly from 
a secondary effedt, that of local aggregation from the 
gaseous into the liquid and solid forms. Through this local 
adtion the condensation of matter became much greater, and 
thus its immediate gravitative effedt was greatly enhanced. 
This has, of course, much increased the density of the at- 
mosphere, and every such increase in the density and decrease 
of size in the solid mass must add to the atmospheric density, 
by bringing the gaseous substance nearer to the centre of 
gravity, and increasing the gravitative pressure upon it. 
But if we pass outward into space, to the point to which the 
material now composing the earth and its atmosphere ex- 
tended in its nebulous condition, we will find that any sub- 
stance beyond this point is entirely unaffedted by the 
earth’s condensation and solidification. The effedt upon 
this outer sphere of matter depends only on the quantity of 
material within it, and it matters not whether that material 
be in a state of gaseous diffusion or of solid condensation. 
