1881.] The Evolution of the Solar System. 265 
But if the effect of condensation has been to add any new 
matter to that originally possessed by the nebulous earth, 
then the influence upon outer substance must have increased. 
And there is no reason to doubt that such new matter has 
been added. For as the nebulous mass condensed into its 
solid state it would, of course, denude a vast extent of space 
of its material contents, thus tending to produce an abnormal 
vacuum unless exterior matter pressed, both through its 
power of diffusion and as a result of terrestrial attraction, 
into this denuded space. It is very probable, then, that the 
condensation of the earth has added to the quantity of sub- 
stance within its range of influence, and that any solidifi- 
cation of its existing atmosphere would tend to produce a 
new and denser atmosphere, through the influence of in- 
creased gravitative energy. 
But now we may again ask, what portion of this gaseous 
envelope of the earth really belongs to it as atmosphere ? 
There are two forces involved in considering this question — 
that of gravitation, and that of centrifugal energy of rota- 
tion. Gravitation in itself does not cause the matter upon 
which it adts to rotate. Rotation is a secondary effedt, whose 
cause we need not consider here. But we can safely say 
that this cause weakens as we reach a considerable distance 
outward from the centre, and that much of the rotation 
which exists comes from a tertiary cause, that of fridtion. 
But this, again, is an influence which weakens outwardly, 
through the growing rarity of matter. The particles com- 
posing the solid crust of the earth are forced to rotate in 
unison, through their mutual fridtion. It is the same with 
the particles composing the lower atmosphere. But as we 
ascend into the region of the rarer atmosphere this fridtionai 
effedt rapidly decreases. It decreases, in fadt, just where it 
should increase, if the necessary result is to be produced. 
For as we leave the centre the rapidity of rotation must 
increase, through the increased length of orbit of the par- 
ticles. Necessarily, then, as both the forces that produce 
rotation decrease in this rare outer atmosphere, the rotation 
itself must lag. The spheres of outer atmosphere must 
fall behind those of the inner atmosphere through the rapid 
diminution of the fridtionai influence. Instead, then, of the 
earth possessing an atmosphere which revolves in unison 
with it to a certain point outwards, and then suddenly 
ceases to revolve, it is far more probable that its atmosphere 
begins to lose its rotating energy as soon as its denser regions 
are passed, and that it rotates less and less rapidly with the 
continual decrease in fridtionai influence, until all rotation 
vol. hi. (third series). T 
