[401] 
XVIII. Influence of Change of Condition from the Liquid to the Solid State 
on Vapour-Pressure. 
By William Ramsay, Ph.D., Professor, and Sydney Young, D.Sc., Lecturer and 
Demonstrator of Chemistry in University College, Bristol. 
Communicated by Professor James Thomson, LLD., F.R.S. 
Received April 15,—Read April 24, 1884. 
[Plates 18, 19.] 
1. The object of this paper is to furnish an experimental proof of the theory advanced 
by Professor James Thomson (Brit. Assoc. Reports, 1871 and 1872, and Proc. Roy. Soc., 
vol. xxii., 27), that the pressure exerted by the vapour of a solid substance at a given 
temperature is less than that of the vapour of the substance in the liquid form at the 
same temperature. This theory was simultaneously brought forward by Kirchhoff 
>(Pogg. Ann., vol. ciii., p. 206). 
2. In confirmation of this theory, offered as a result of thermodynamic considerations 
and demonstrations, Professor Thomson made use of empirical formulas, devised by 
M. Regnault, to represent the different parts of the experimentally derived curve for 
expressing relationship between temperature and pressure of water-gas in contact with 
ice and with water ; and he showed that Regnault’s results, when rightly interpreted, 
pointed to a discontinuity in the curve, occurring at a temperature nearly coincident 
with 0°, the melting-point of ice under normal pressure. Although Professor 
Thomson’s conclusions bear remarkable testimony to the extreme accuracy of 
Regnault’s work, yet the differences of pressure in the case of water and ice are so 
minute as to require, for trustworthy determination, much greater refinement than 
Regnault’s methods admitted of. 
3. That Professor Thomson’s conclusions have not received general acceptance is 
best shown b} r the following translation from one of the most recent works on thermal 
chemistry, Naumann’s ‘ Thermochemie ’ (Brunswick, 1882), p. 178 :—“With naphtha¬ 
lene, a substance which is solid at 78°, when the temperature is raised, but when 
previously melted and cooled to 78° remains liquid, adhering in liquid drops to the 
walls of the vacuum-tube, I was able to observe that its vapour exerts the same pres- 
