Page:Experimental researches in chemistry and.djvu/214

1826.] On the existence of a Limit to Vaporisation.

[Read June 15, 1826.]

is well known that within the limits recognized by experiment, the constitution of vapour in contact with the body from which it rises, is such, that its tension increases with increased temperature, and diminishes with diminished temperature; and though in the latter case we Eau, with many substances, so far attenuate the vapour as soon to make its presence inappreciable to our tests, yet an opinion is very prevalent, and I believe general, that still small portions are produced; the tension being correspondent to the comparatively low temperature of the substance. Upon this view it has been supposed that every substance in vacuo or surrounded by vapour or gas, having no chemical action upon it, has an atmosphere of its own around it; and that our atmosphere must contain, diffused through it, minute portions of the vapours of all those substances with which it is in contact, even down to the earths and metals. I believe that a theory of meteorites has been formed upon this opinion.

Perhaps the point has never been distinctly considered, and it may therefore not be uninteresting to urge two or three reasons, in part dependent 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 observation, has gone far to prove that our atmosphere is of finite extent, its boundary being dependent upon the opposing powers of elasticity and gravitation. On passing upwards, 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 elasticity is