286 



tion, its maximum occurring in June and July, and its minimum in 

 December and January. 



But that such a result might not rest merely on the observations 

 with one instrument, the author has collected observations made on 

 corresponding days at the observatories of Greenwich, Dublin, and 

 Paris, all which, without exception, give results very nearly agreeing 

 with those of his own observations ; and he finally concludes, that as 

 they cannot possibly arise from instrumental error, or error of obser- 

 vation, he hopes to be able ere long to prove, to the satisfaction of 

 the Society, that their cause is imperfection in the solar tables. 



On the Existence of a Limit to Vaporization. By M. Faraday, F.R.S. 

 Corresponding Member of the Royal Academy of Sciences at Paris, 

 8fC. SfC. Communicated May 26, 1826. Read June 15, 1826. 

 [Phil. Trans. 1826, p. 484.] 



The object of this paper is stated by its author to be, 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. The train of argument by which this is attempted 

 to be demonstrated may be summarily stated as follows. 



Assuming it as proved by Dr. Wollaston, that a limit exists to the 

 earth's atmosphere, where in consequence of its rarity its molecules 

 are so distant from each other, that the repulsion of any one on the 

 molecule below it is just equal to its gravity, it is clear that in this 

 case the force of gravity may be regarded as setting a limit to further 

 expansion ; and if we could exhaust a receiver to the degree of 

 tension obtaining on the surface of the atmosphere, any further sub- 

 traction of air would produce no further diminution of density, but 

 would merely produce a vacuum in the upper part of the receiver. 



But cold, as well as rarefaction, diminishes the elasticity of vapours 

 or gases, and therefore. if the temperature be greatly diminished, the 

 limit above alluded to, where gravity counteracts the elastic force, 

 will be attained with a less degree of rarefaction; and if the tempe- 

 rature were sufficiently low, it is evident that air of any given degree 

 of density would lose its disposition to expand in a direction contrary 

 to gravity. 



In the case of air, however, the cold required to render it inelastic 

 at any sensible density would of course be excessive. But if we con- 

 sider the vapours of very fixed bodies (as silver for instance), whose 

 tension even at a white heat is insensible, it is almost certain that 

 the ordinary temperature of the atmosphere is, with respect to that 

 capable of maintaining it at a sensible tension, such a degree of 

 cold as would effectually bring it under the command of gravity. 

 Supposing then silver to cool from fusion, that moment when these 

 forces became equal, would be the one in which vapour could exist 

 above the silver ; and at every lower temperature the metal would 

 be perfectly fixed. But the author regards it as probable that this 

 equilibrium at ordinary temperatures may take place with bodies 



