1826.] Exist ence of a Limit to Vaporization, 437 



Dr. WoUaston, by a beautiful train of argument and observa^ 

 tiop, has gone far to prove that our atmosphere is of finite 

 extent, its boundary being dependant upon the opposing powers 

 of elasticity and gravitation.* On passing upwards from the 

 earth's surface, the air becomes more and more attenuated, in 

 corisequence of the gradually diminishing pressure of the supefr 

 incumbent part, and its tension or elasticity is proportionally 

 diminished ; v^^hen the diminution is such that the elasticity is a 

 force, not more powerful than the attraction of gravity, then 9. 

 limit to the atmosphere must occur. The particles of the atmo- 

 sphere there tend to separate 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 expan- 

 sion would necessarily give rise to diminished tension, the force 

 of gravity would then be strongest, and consequently woul4 

 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 be dimi- 

 nished, then it would expand, and exist of a lower tension ; upoi^ 

 the renewal of the gravitating force, either by approximation tp 

 the earth's surface or otherwise, the particles would approach 

 each other, until the elasticity of the whole was again equal tp 

 the force of gravity. 



Inasmuch as gases and yapours undergo no change by mer^ 

 expansion or attenuation, which can at all disturb the analogy 

 existing between them in their permanent state under ordinary 

 circumstances, all the phenomena which have been assumed as 

 occurring with the air at the limit of our atmosphere may, with 

 equal propriety, be admitted with respect to vapour in general 

 \n similar circumstances ; for we have no reason for supposing 

 that the particles of one vapour more than another axe free from 

 the influence of gravity, although the force may, and without 

 doubt does, vary, with the weight and elasticity of the particles 

 of each particular substance. 



It will be evident also that similar effects would be produced 

 by the force of gravity upon air or vapour of the extreme tenuity 

 and feeble tension referred to, whatever be the means taken to 

 bring it into that state ; and it is not necessary to imagine the 

 portion of air operated upon, as taken from the extremity of our 

 atmosphere, for a portion of that at the earth's surface, if it 

 could be expanded to the same degree by an air pump, would 

 undergo the same changes : when of a certain rarity it woulfj 

 just balance the attraction of gravitation, and fill the receiver 



^ Phil Trans. 1822, p. 89. 



