Mr. Faraday on the 
488 
that the silver should have descended to common tempera- 
tures, 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 be- 
come 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 tempe- 
ratures, 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 6oo° or 700° F. 
If, as I have formerly shown,* some clean mercury be put 
at the bottom of a clean dry bottle, a piece of gold leaf 
attached to the under part of the stopper by which it is closed, 
and the whole left for some months at a temperature of from 
6 o° to 8o°, the gold leaf will be found whitened by amalga- 
mation, in consequence of the vapour which rises from the 
mercury beneath ; but upon making the experiment in the 
winter of 1824-5, I was unable to obtain the effect, however 
near the gold leaf was brought to the surface of the mercury ; 
and I am now inclined to believe, because the elastic force of 
any vapour which the mercury could have produced at that 
* Quarterly Journal of Science, X. 354. 
