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 .
If, as I have formerly shown[1], 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 60° to 80°, the gold-leaf will be found whitened by amalgamation, in consequence of the vapour which rises from the mercury beneath; but upon making the experiment in the winter of 1824–25, 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 it was so because the elastic force of any vapour which the mercury could have produced at that temperature, was less than the force of gravity upon it, and that consequently the mercury was then perfectly fixed.
Sir Humphry Davy, in his experiments on the electrical phenomena exhibited in vacuo, found, that when the temperature of the vacuum above mercury was lowered to -20° F. no further diminution, even down to -90° F., was able to effect any change, as to the power of transmitting electricity, or in the luminous appearances; and that these phenomena were then nearly of the same intensity as in the vacuum made over tin[2]. Hence, in conjunction with the preceding reasoning, I am led to conclude that they were then produced independent of any vapour of the metals, and that under the circumstances described; no vapour of mercury existed at temperatures beneath 20° F.
Concentrated sulphuric acid boils at about 600° F., but as the temperature is lowered the tension of its vapour is rapidly diminished. Signor Bellanii[3] placed a thin plate of zinc at the upper part of a closed bottle, at the bottom of which was some
