90 
Dr. Wollaston on the 
our atmosphere. For, if the density be throughout as the 
compressing force, then must a stratum of given thickness at 
every height be compressed by a superincumbent atmo- 
sphere, bearing a constant ratio to its own weight, whatever 
be its distance from the earth. But if air consist of any ulti- 
mate particles no longer divisible, then must expansion of 
the medium composed of them cease at that distance, where 
the force of gravity downwards upon a single particle is 
equal to the resistance arising from the repulsive force of the 
medium. 
On the latter supposition of limited divisibility, the atmo- 
sphere which surrounds us will be conceived to be a medium 
of finite extent, and may be peculiar to our planet, since its 
properties would afford no ground to presume that similar 
matter exists in any other planet. But if we adopt the hypo- 
thesis of unlimited expansion, we must conceive the same 
kind of matter to pervade all space, where it would not be 
in equilibrio, unless the sun, the moon, and all the planets 
possess their respective shares of it condensed around them, 
in degrees dependent on the force of their respective attrac- 
tions, excepting in those instances where the tendency to 
accumulate may be counteracted by the interference of other 
kinds of matter, or of other powers of which we have no ex- 
perience, and concerning which we cannot expect to reason 
correctly. 
Now, though we have not the means of ascertaining the 
extent of our own atmosphere, those of other planetary bo- 
dies are nevertheless objects for astronomical investigation ; 
and it may be deserving of consideration, whether, in any in- 
stance, a deficiency of such matter can be proved, and whe- 
