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 particles 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 the 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 be diminished, 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 particles 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 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 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 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 would just balance the attraction of gravitation and fill the receiver
