156 
of such materials as the earth is made of and as the spectro- 
scope indicates as entering into the composition of the sun, 
and suppose this planetary body to be in comparison with 
with our globe extremely small in mass, and located at such 
a distance from the sun as to be sensibly affected by his 
rays, say for instance within Saturn’s orbit, and suppose 
further that it is retained at that distance until such changes 
as would be produced by the temperature to which it is 
there subjected are fully realised. We should then have a 
central mass of more or less solid material surrounded by 
an attenuated atmosphere of such substances as are gaseous 
at the particular temperature there prevailing and under 
the particular pressure exercised by the gravitation of the 
central mass. Now let us suppose our planetary body to 
be moved to another position considerably nearer to the 
sun, and so subjected more largely to the influence of his 
rays. An augmentation of its atmosphere would imme- 
diately be commenced. Materials non-volatilisable at its 
previous temperature would be raised into the gaseous form. 
The volume of its atmosphere would be increased whilst the 
planet’s coercive power over its elasticity would be dimi- 
nished. But let us suppose our planetary body to be once 
more replaced in its former position and subjected to the 
lesser of the two temperatures we have been considering. 
The solar heat will now no longer be able to maintain all 
that matter in the gaseous form which has been evaporated at 
the shorter of the two distances from the sun. A condensa- 
tion will accordingly be commenced through a greater or 
less extent of the cometary atmosphere, and a more or less 
dense nebulous mass will surround the central stellar 
point. This nebulosity will be again evaporated into 
