Temperatures, Pressures, and Volumes. 7 



vapour at low temperatures. The densities of the saturated 

 vapours of the alcohols, on the other hand, are normal at low 

 temperatures, and complex molecules cannot, therefore, he 

 present; but there is considerable evidence of their existence 

 in the liquid state. 



Ramsay and Shields (Phil. Trans. 184 A. p. 647 ; Trans. 

 Chem. Soc. lxiii. p. 1089) have recently studied the surface- 

 energy of a large number of compounds, and have described 

 a method by which the molecular complexity of liquids may 

 be ascertained. They show that most of the liquids investi- 

 gated have the same molecular weight in the liquid as in the 

 gaseous state, but that there is greater molecular complexity 

 in the liquid state in the case of the fatty acids and 

 alcohols. As regards homologous compounds — both acids and 

 alcohols — the complexity diminishes with rise of molecular 

 weight, and as regards individual compounds it diminishes 

 with rise of temperature; in order of complexity methyl 

 alcohol comes next to acetic acid. 



These conclusions, based on totally different considerations, 

 agree perfectly with those suggested in this paper and in the 

 previous one on the same subject. 



An explanation of the relatively high molecular volumes of 

 the saturated vapour of the alcohols at low temperatures 

 would be afforded by the assumption that complex molecules 

 exist to some extent at the critical points, an assumption 

 which is supported by the high critical densities. 



It has been shown (Phil. Mag. Nov. 1890, p. 423) that if 

 the generalizations of Van der Waals were strictly true, the 

 following relation should be true for all substances : — 



v_V_T / 

 w'"V / "T /X p' 



where v and v 1 are the molecular volumes of saturated vapour, 

 Y and V those of liquid, and T and T' the boiling-points on 

 the absolute scale of temperature of any two substances at 

 corresponding pressures p and p 1 . This relation should hold 

 good at the critical point, or 



py 



-m-= constant, 



where P, V, and T are the critical pressure, the critical 

 molecular volume, and the absolute critical temperature of any 

 substance. As the critical volumes are now known, it is possible 

 to test this relation at the critical points of the various com- 



py 



pounds, and the values of — pp- are given in the table below. 



