existence of a limit to vaporization. 489 
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 tempe- 
rature of the vacuum above mercury was lowered to 20° F. 
no further diminution, even down to — 20° 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.* Hence, in conjunction with the preceding reason- 
ing, I am led to conclude, that they were then produced 
independant 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 6oo° F. but as 
the temperature is lowered the tension of its vapour is rapidly 
diminished. Signor BELLANi-f placed a thin plate of zinc at 
the upper part of a closed bottle, at the bottom of which was 
some concentrated sulphuric acid. No action had taken place 
at the end of two years, the zinc then remaining as bright as 
at first ; and this fact is very properly adduced in illustration 
of the fixedness of sulphuric acid at common temperatures. 
Here I should again presume, that the elastic force which 
tended to form vapour, was surpassed by the force of gravity. 
Whether it be admitted or not, that in these experiments 
the limit of volatilization, according to the principle of the ba- 
lance of forces before stated had been obtained, I think, we 
can hardly doubt that such is the case at common tempera- 
tures, with respect to the silver, and with all bodies which 
* Phil. Trans, mdcccxxii. p. 71. f Gionale di Fisica, v. 197. 
