485 
existence of a limit to vaporization. 
and it may therefore not be uninteresting to urge two or 
three reasons, in part dependant upon experimental proof, 
why this should not be the case. The object, therefore, 
which I shall hold in view in the following pages, is to show 
that a limit exists to the production of vapour of any tension 
by bodies placed in vacuo, or in elastic media, beneath which 
limit they are perfectly fixed. 
Dr. Wollaston, by a beautiful train of argument and ob- 
servation, has gone far to prove that our atmosphere is of 
finite extent, its boundary being dependant upon the oppos_ 
ing powers of elasticity and gravitation.* On passing up- 
wards, from the earth's surface, the air becomes more and 
more attenuated, in consequence of the gradually diminishing 
pressure of the superincumbent part, and its tension or elas- 
ticity is proportionally diminished ; when the diminution is 
such, that the elasticity is a force, not more powerful than 
the attraction of gravity, then a limit to the atmosphere must 
occur. The particles of the atmosphere there tend to sepa- 
rate with a certain force ; but this force is not greater than 
the attraction of gravity, which tends to make them approach 
the earth and each other ; and as expansion would necessarily 
give rise to diminished tension, the force of gravity would 
then be strongest, and consequently would cause contraction, 
until the powers were balanced as before. 
Assuming this state of things as proved, the air at the limit 
of the atmosphere has a certain degree of elasticity or tension ; 
and, although it cannot there exist of smaller tension, yet, if 
portions of it were removed to a farther distance from the 
earth, or if the force of gravity over it could in any other way 
* Phil. Trans, mdcccxxii. p. 89. 
MDCCCXXVI. 3 R 
