488 Mr. Faraday on the [Dec. 



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 contract into the lower half of the 

 receiver, until, by the approximation of their particles, 

 the vapour there existing should have an elasticity equal to the 

 force of gravity to which it was subject. This is a necessary 

 consequence of Dr. Wollaston's argument. 



There is yet another method of diminishing the elasticity of 

 vapour, namely, by diminution of temperature. With respect to 

 the most elastic substances, as air, and many gases, the compa- 

 ratively small range which we can command beneath common 

 temperatures does nothing more at the earth's surface than 

 diminish in a slight degree their elasticity, though two or three 

 of them, as sulphurous acid and chlorine, have been in part con- 

 densed into liquids. But with respect to innumerable bodies, 

 their tendency to form vapour is so small, that at common tem- 

 peratures the vapour produced approximates in rarity to the air 

 upon the limits of our atmosphere ; and with these, the power 

 we possess of lessening tension by diminution of temperature 

 may be quite sufficient to render it a smaller force than its 

 opponent gravity ; in which case it will be easy to comprehend 

 that the vapour would give way to the latter, and be entirely 

 condensed. The metal, silver, for instance, when violently 

 heated, as on charcoal urged by a jet of oxygen, or by the oxy- 

 hydrogen, or oxy-alcohol flame, is converted into vapour ; lower 

 the temperature, and before the metal falls beneath a white heat, 

 the tension of the vapour is so far diminished, that its existence 

 becomes inappreciable by the most delicate tests. Suppose, 

 however, that portions are formed, and that vapour of a certain 

 tension is produced at that temperature, it must be astonishingly 

 diminished by the time the metal has sunk to a mere red heat ; 

 and we can hardly conceive it possible, I think, that the silver 

 should have descended to common temperatures, before its 

 accompanying vapour will, by its gradual diminution in tension, 

 if uninfluenced by other circumstances, have had an elastic force 

 far inferior to the force of gravity ; in which case, that moment 

 at which the two forces had become equal, would be the last 

 moment in which vapour could exist around it; the metal at 

 every lower temperature being perfectly fixed. 



I have illustrated this case by silver, because, from the high 

 temperature required to make any vapour appreciable, there 

 can be little doubt that the equality of the gravitating and elastic 

 forces must take place much above common temperatures, and 

 therefore within the range which we can command. But there 

 is, I think, reason to believe, that the equality in these forces, 

 at or above ordinary temperatures, may take place with bodies 

 far more volatile than silver ; with substances indeed which boil 

 under common circumstances at 600^ or 700° F. 



