1910-11.] Study of the Constitution of Calomel Vapour. 183 
IX. — A Quantitative Study of the Constitution of Calomel Vapour. 
By Alexander Smith and Alan W. C. Menzies. 
(MS. received July 25, 1910. Read November 7, 1910.) 
The vapour density of calomel vapour corresponds to the formula 
HgCl or Hg + HgCl 2 . Odiing* showed, by amalgamation of gold-leaf, 
the presence of free mercury. Harris and V. Meyer j- separated mercury 
from the vapour by diffusion, and also showed that solid potassium hydroxide 
turned yellow (HgO) when plunged into the vapour. Diffusion might 
naturally increase the dissociation, and even a pure mercurous salt would 
certainly give mercuric oxide at 240-260°, since at that temperature 
mercurous oxide is unstable. The quantitative conclusion of these observers, 
namely, that the vapour is completely dissociated, therefore, was not 
in any way justified by their qualitative experiments, although it has 
been accepted by many chemists. Hence the proportion of unchanged 
calomel (HgCl) and its dissociation products (Hg + HgCl 2 ) still remains 
to be ascertained. 
The vapour contains, at most, three substances related by the following 
equilibria :• — 
Calomel (solid)^2HgCl$Hg + HgCl 2 . 
It was proposed to make three sets of measurements in the same tempera- 
ture-region : (1) the vapour pressures of calomel (P ca iom.) 5 (2) the vapour 
pressures of mercury (P merc .) ; (3) the vapour pressures of a mixture 
of calomel and mercury (P mixt .). In case the dissociation is negligible, 
P _ p 
+ P, 
In case the dissociation is complete, 
Pmixt. Pmerc. ~b P corros. subl. Pmerc. "b 
(iP 
2 calom 
• (l). 
.( 2 ). 
Thus, if either of these extreme cases represented the fact, the vapour 
pressures of the mixture could be obtained, both by calculation from 
the vapour pressures of mercury and of calomel, and by direct observation. 
In case undissociated calomel (HgCl) was present, its partial pressure 
(Pcaiom.) could be calculated from the observations : 
Pcaiom. Pcaiom. 2P merc , it 2 \- (Pj: 
• )P B 
* J. Chem. Soc iii. 211. 
t Berichte , xxvii., 1842, 
. ( 3 ). 
