346 Mr. Faraday on the 



tract into the lower half of the receiver, until, by the approxi- 

 mation 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 re- 

 spect to the most elastic substances, as air, and many gases, 

 the comparatively small range which we can command be- 

 neath 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 in- 

 numerable bodies, their tendency to form vapour is so small, 

 that at common temperatures the vapour produced approxi- 

 mates 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 de- 

 scended to common temperatures, before its accompanying 

 vapour will, by its gradual diminution in tension, if uninflu- 

 enced 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 tem- 

 peratures, and therefore within the range which we can com- 

 mand. But there is, I think, reason to believe that the equa-> 

 lity in these forces, at or above ordinary temperatures, may 

 take place with bodies far more volatile than silver; with sub- 

 stances 



