1826.] On the existence of a Limit to Vaporization. 201 



with vapour ; but then, if half were taken out of the receiver, 

 the remaining portion, in place of filling the vessel, would sub- 

 mit to the force of gravity, would contract into the lower half 

 of the receiver, until, by the approximation of the 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 com- 

 paratively 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 

 condensed into liquids. But with respect to innumerable 

 bodies, their tendency to form vapour is so small, that at com- 

 mon temperatures 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 com- 

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

 entirely condensed. The metal, silver for instance, when vio- 

 lently 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 dimi- 

 nished, 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 tem- 

 perature ; 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. 



