184 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The first assumption gives the greatest value for the pressures of 
the mixture, the second the least. When the measurements proposed 
had been made, however, the pressure of the mixture was found to be 
even lower than the second assumption permitted. This could be explained 
provided the density of saturated mercury vapour, of saturated calomel 
vapour, or of the mixed vapours was abnormally high. Or it could be 
explained by depression in the vapour tension of the mercury in the 
mixture, due to dissolved calomel. The vapour densities were measured 
and found to be normal. The calomel vapour was then found to be 
very slowly dissolved by mercury, and the solubilities were determined. 
When the formula of the dissolved calomel is taken to be HgCl (not Hg 2 Cl 2 ), 
the final calculated results demonstrate, with surprisingly exact quantita- 
tive correspondence, that the vapour of calomel is wholly dissociated. 
The vapour pressures were all measured with the static isoteniscope 
and other apparatus used in the work on water * and on mercury, j* In the 
following table the data are given as follows : (1) temperature ; (2) solubility 
of calomel in mercury stated as mols. Hg : mols. HgCl ; (3) P merc . 5 
(4) vapour pressure of Hg as reduced by dissolved calomel (calc.) ; 
(5) Pcaiom. ; (6) P mix t. calc., assuming complete dissociation; (7) P mixt . 
observed; (8) divergence of (6) and (7). 
1. 
t. 
2. 
Solu- 
bility. 
3. 
P 
mere. 
4. 
P 
mere. 
depressed. 
5. 
p 
x calom. 
6, 
p 
A mixt.j 
calc. 
7. 
P 
mixt.) 
observed. 
8. 
A 
360° 
39*6 
802-6 
782-8 
434 
843-0 
843-6 
+ 0-6 
365 
326 
876-7 
850-6 
491 
921-5 
924-9 
+ 3-4 
370 
29-3 
956-2 
924-7 
556 
1008-3 
1013-0 
+ 4-7 
375 
273 
1041-6 
1004-8 
630 
1103-6 
1108-4 
+ 4-9 
380 
26-2 
1133-0 
1091-4 
712 
1207-5 
1211-1 
+ 3-6 
385 
25-6 
1230-9 
1184-6 
805 
1321-3 
1321-7 
+ 0-4 
390 
25*4 
1335-4 
1284-8 
906 
1444-5 
1441-4 
-3-4 
395 
25-2 
1447-0 
1391-8 
1017 
1577-6 
1570-8 
-6-8 
400 
25*1 
1566-1 
1506-1 
1135 
1719-9 
1713-4 
-6-5 
The divergences from the theory of complete dissociation vary from 
— 6 '8 to +0*4 mm.: the sum of the divergences at nine temperatures 
is — 09 mm. The average is thus only — OT mm., or about 1 part in 
12,000, which is much less than the error involved in such difficult 
measurements. 
An independent preliminary series of measurements of the vapour 
pressures, made with a mercury thermometer and a simple submerged 
* Proceedings , xxx. 523. 
t Proceedings , xxx. 521. 
