Existence of a Limit to Vaporization. 345 



from the earth's surface, the air becomes more and more at- 

 tenuated, in consequence of the gradually diminishing pressure 

 of the superincumbent part, and its tension or elasticity is pro- 

 portionally 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 par- 

 ticles of the atmosphere 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 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 wei*e removed to a further distance from the 

 earth, or if the force of gravity over it could in any other way 

 be dhninished, then it would expand, and exist of a lower 

 tension ; upon the renewal of the gravitating force, either by 

 approximation to the earth's surface or otherwise, the parti- 

 cles would approach each other, until the elasticity of the whole 

 was again equal to the force of gravity. 



Inasmuch as gases and vapours undergo no change by mere 

 expansion or attenuation, which can at all disturb the analogy 

 existing between them in their permanent state under ordinary 

 circumstances, all the phaenoniena 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 in similar circumstances ; for we have no reason for 

 supposing that the particles of one vapour more than another 

 are 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 pro- 

 duced by the force of gravity upon air or vapour of the ex- 

 treme 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 would just balance the attraction of gravitation 

 and fill the receiver with vapour ; but then, if half were taken 

 out of the receiver, the remaining portion, in place of filling 

 the vessel, would submit to the force of gravity, would con- 



Vol. 68. No. 34-3. Nov. 1826. 2 X * trac 



