On the existence of a Limit to Vaporisation. Q1826. 



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 elas- 

 tic 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*, 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 con- 

 sequence of the vapour which rises from the mercury beneath ; 

 but upon making the experiment in the winter of 1824-25, 1 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 phe- 

 nomena exhibited in vacuo, found, that when the temperature 

 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 tint- Hence, 

 in conjunction with the preceding reasoning, I am led to con- 

 clude 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. 3 but as 

 the temperature is lowered the tension of its vapour is rapidly 

 diminished. Signer BellaniJ placed a thin plate of zinc at the 

 upper part of a closed bottle, at the bottom of which was some 



* Quarterly Journal of Science, x. 354. See page 57. 



f Phil. Trans. 1822, p. 71. J Giornale di Fisica, v. 197. 



