Generalizations of Van der Waals. 155 



employed, and this has been fully described in the Trans. 

 Chem. Soc. 1891, p. 37. This method is applicable to sub- 

 stances that attack mercury, and from the data it affords the 

 specific volumes both of liquid and saturated vapour may be 

 calculated ; it is also available at any temperature up to the 

 critical point. A modification of the method is described in 

 the Trans. Chem. Soc. lix. p. 929. 



The object of this paper is to show how far the generaliza- 

 tions of Van der Waals have been verified by the experimental 

 results. 



If all the relations were strictly true, it would obviously 

 make no difference whether the specific (or molecular) volumes 

 were compared at corresponding temperatures or correspond- 

 ing pressures ; but it may be stated at once that it is only in 

 a limited number of cases that, when the absolute temperatures 

 are proportional to the absolute critical temperatures, the 

 vapour-pressures are also proportional to the critical pressures. 

 It is therefore necessary to compare the various substances, 

 not only at corresponding temperatures but also at corre- 

 sponding pressures. The best mode of procedure would pro- 

 bably be to give the temperatures, pressures, and volumes of 

 each substance in terms of the critical constants of that body ; 

 but the critical volumes of only a few of the compounds have 

 been directly determined, and it has therefore been necessary 

 to compare the various substances with one of them taken as 

 a standard. The very simple relations observed between the 

 four monohalogen derivatives of benzene (Trans. Chem. Soc. 

 1889, p. £86), and the fact that the constants of fluorbenzene 

 have been determined up to the critical point, render that 

 substance the most suitable for the purpose. 



The experimental data and the ratios calculated from them 

 are given in the following tables. 



I. Critical constants — temperature, pressure, volume. 

 II. Corresponding pressures. 



III. Corresponding temperatures. 



IV. Boiling-points on absolute scale at corresponding 



pressures. 

 V. Vapour-pressures at corresponding temperatures. 

 VI. Molecular volumes of liquids at corresponding pres- 

 sures. 

 VII. Molecular volumes of liquids at corresponding tem- 

 peratures. 

 VIII. Molecular volumes of saturated vapour at corre- 

 sponding pressures. 

 IX. Molecular volumes of saturated vapour at correspond- 

 ing temperatures. 

 X. Ratios of absolute temperatures to those of fluorben- 

 zene at corresponding pressures. 

 M2 



