162 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XV. — The Densities and Degrees of Dissociation of the Saturated 
Vapours of the Ammonium Halides. By Professor Alexander 
Smith and Robert H. Lombard. 
(MS. received November 2, 1914. Read December 7, 1914.) 
The published determinations of the vapour densities of ammonium 
chloride are not consistent. Thus the percentage dissociation calculated 
from the observations at 300° to 323° ranges from 87 to 100 ; at 350°, from 
83 to 100 ; and at 445°, from 89 to 99. The results also lack quantitative 
significance because they were nearly all obtained by the Dumas or the 
V. Meyer method. When the former method was used, the densit}^ was 
that at atmospheric pressure, although the temperature corresponded to 
a much higher vapour pressure. When the latter method was used, in 
most cases, for the same reason, the vapour was highly unsaturated. In 
a few instances the vapour pressure was less than one atmosphere, and 
the vaporisation could proceed only by diffusion of the vapour into the 
air contained in the apparatus. This diffusion naturally promoted the 
dissociation, so that whether the original dissociation was great or small, 
the final density should have been none other than that corresponding to 
complete dissociation. The densities of the saturated vapours, when the 
components of the vapour are in equilibrium both with each other and with 
the solid, are much more definite and much more instructive, but, so far as the 
ammonium halides are concerned, they have not previously been determined. 
The present paper concerns the densities and the degrees of dissociation of 
the saturated vapours, and the results have confirmed our expectation 
that the latter would be found much smaller than is currently supposed. 
The solid was placed in a small bulb, connected by a capillary tube 
with a large bulb ; and the apparatus was completely evacuated before 
being sealed up. Both bulbs were then immersed in a bath of molten 
nitrates which was maintained at a constant temperature. Uniform 
distribution of the temperature was secured by vigorous stirring. Subse- 
quently the volume of the large bulb, and the weight of the salt which, 
in the form of vapour, had filled it, were measured. The pressure of the 
vapour was taken from the measurements of the dissociation pressures of 
the same salts made in a separate investigation by Smith and Calvert.* 
The temperatures were measured in both cases by platinum resistance 
thermometers standardised under identical conditions. At each temperature 
* Journal Am. Ghem. Soc., xxxvi, 1363 (1914). 
