1910-11.] The Vapour Pressure of Dry Calomel 
519 
XXXV. — The Vapour Pressure of Dry Calomel. By Alexander 
Smith and Alan W. C. Menzies. 
(MS. received April 4, 1911. Read June 5, 1911.) 
(Abstract.) 
We have shown * that calomel vapour contains no measurable amount of 
Hg 2 Cl 2 or of HgCl, and consists wholly of the dissociation products, 
mercury and mercuric chloride. According to chemical theory, therefore, 
when, by the removal of all moisture, the dissociation is prevented, the 
vapour pressure of the dry substance should be equal only to the partial 
pressure of the undissociated molecules of the undried vapour ; that is to 
say, it should be negligibly small. Brereton Baker, however, was able to 
measure the vapour density of such dry calomel, finding it to be almost 
double the ordinary density. Since the measurement was made by the 
V. Meyer method, at 445°, it follows that this dry calomel must have 
possessed a vapour pressure approximating, if not exceeding, one atmo- 
sphere. The same anomaly occurs in the case of ammonium chloride, and 
its existence has been confirmed in that case by Abegg and Johnson, who 
found the vapour pressure of the ordinary and the dry salts to be exactly 
equal. The explanation offered in this case by Wegscheider is that 
salammoniak is dimorphous, and the form proper to low temperatures persists, 
when dry, in a temperature region in which it is unstable, and there gives, 
as the theory would predict, a high vapour pressure. The exact equality 
between the pressures is due, according to him, to a coincidence. Since 
Baker’s result seems to point to a similar dimorphism in calomel, it was 
of the highest interest to ascertain the actual value of the vapour pressure 
in the case of the dried calomel. 
The measurement was made by placing a known weight of calomel 
in a small bulb, and fusing the latter on to the elongated and down-turned 
capillary neck of a large bulb. The drying was performed by phosphorus 
pentoxide contained in another bulb (subsequently fused off) much as in 
Baker’s experiments, except that the apparatus was evacuated and was 
kept in an oven at 115°, while the pentoxide bulb hung in running cold 
water. After the period of drying was over, the apparatus was immersed 
in a bath of potassium and sodium nitrates, kept constant at 352°, and after 
* Proceedings , vol. xxxi. p. 183. 
