158 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XVII— The Explanation of an apparent Anomaly outstanding in 
the Results of Measurement of Dissociation Pressures. By 
Professor Alan W. C. Menzies. 
(MS. received May 4, 1920. Read June 21, 1920. 
Revised MS. received September 6, 1920.) 
(Abstract.) 
In 1888 Tammann* applying a form of the gas current saturation method 
to the measurement of the dissociation pressures of salt hydrates, obtained 
results which were uniformly higher by from 2 to 5 per cent, than the 
results obtained by Froweinf with the tensimeter. This anomalous 
behaviour was confirmed, in Nernst’s laboratory, by Schottky,J who re- 
ported that the initial dissociation pressures developed in tensimetric 
measurements were, in cited cases, higher than the equilibrium values. . In 
1911 Partington, § re-studying certain of the facts, added further confirma- 
tion by the use of the gas current saturation method in a somewhat 
altered form. 
Thoughtful elucidations and critical discussions of this apparent anomaly 
have been offered by Tammann, Nernst, || Partington, Brereton Baker, and 
Campbell,** those of Nernst and of Campbell being especially instructive. 
In view of notorious precedent in the case of Charles II and the Royal 
Society of London, it seemed worth while, despite the weight of triple 
authority cited above, to seek a still simpler explanation of the anomaly 
than those hitherto suggested by once again examining the experimental 
facts. The dissociation pressure of the same sample of cupric sulphate 
pentahydrate in equilibrium with trihydrate was accordingly measured, 
near 25°, by both the gas current saturation method and by the tensimetric 
method. Certain precautions were adopted so as to avoid errors that were 
suspected to have been incident to the work of the earlier experimenters. 
The only two non-preliminary measurements by the gas current satura- 
tion method gave 7*83 and 7’78 mm. mercury at 0° as the dissociation 
pressure at 25 '00° of the system CuS0 4 (5-3)H 2 0 and vapour : average, 
* Ann. Physik, xxxiii, p. 322 (1888). f Z. physik. Gliem ., i, p. 5 (1887). 
X Z. physik. Chem., lxiv, p. 415 (1908). § J. Chem. Soc., xcix, p.466 (1911). 
|| Z. physik. Chem., lxiv, p. 415 (1908). IT Ann. Rep. Progress Chem., viii, p. 34 (1912). 
** Trans. Faraday Soc., x, p. 195 (1914). 
