i 88 i .] The Evolution of the Solar System. 267 
this integrant condition. It is, doubtless, largely or wholly 
disintegrated. As to the matter composing our atmosphere, 
we are ignorant of its condition beyond a very limited 
height. Near the surface it is composed of oxygen and ni- 
trogen, with small quantities of some chemical compounds. 
At a comparatively small distance from the surface these 
chemical compounds may cease to exist. But it is quite 
possible that here the oxygen and nitrogen are accompanied 
by a third elementary constituent, namely, hydrogen. For 
in the excessive rarity and adtive molecular motion of the 
atmosphere at this level, hydrogen may be insusceptible to 
the attraction of oxygen. Though the apparent temperature 
may be low, the absolute temperature is high in all such 
rare gases, from their greatly increased heat capacity. 
Thus, at a certain lower limit of rarity, oxygen and hydrogen 
may be so related in their vigour of molecular movement as 
to yield to their mutual attractions, and combine into water 
vapour ; while beyond this limit their absolute temperature 
may be too high to permit of such combination, and they 
will remain separate. At a still higher level it is probable 
that oxygen and nitrogen may cease to exist, as such, and 
become separated into their simpler constituents.; while hy- 
drogen, from its lighter character, may remain intaCt to a 
much greater altitude. Yet it, too, must eventually separate 
into its constituents, if it is really molecular in condition. 
Finally, the rarification may become so extreme, and the 
absolute temperature so great, that the matter concerned in 
our atmospheres may separate into its primary atoms, or 
possibly into its ethereal or completely disintegrated state. 
The density of atmospheres, therefore, depends not simply 
on the condensing effeCt of spheres upon the matter of space. 
For this condensing influence, by reducing the distance be- 
tween particles and lowering their temperature resistance, 
renders possible local aggregation into atoms and molecules, 
and thus yields a special density which greatly increases the 
susceptibility to gravity. Therefore the density of the at- 
mosphere of any planet depends only partly on its gravita- 
tive energy, and partly on the secondary condensation which 
takes place when the atmosphere becomes sufficiently dense 
to permit of chemical integration. 
Thus a small sphere might be destitute of any but a very 
rare atmosphere, from its not being able to produce the 
density necessary to set up this secondary adtion. Such 
may be the case with our moon. Its atmosphere may con- 
sist only of the primary integrations of the matter of space, 
in a state too diffuse to become apparent to us. Here there 
