MOLECULAR ASSOCIATION 27 



very nearly the case for the liquid, but not at all for the 

 vapour. On the other hand there is evidence that the 

 molecular magnitude, which is nearly normal at the critical 

 point, as appears from p. 23, is increased in the liquid on 

 fall of temperature by formation of double molecules ; the 

 osmotic methods, in fact, give for liquid alcohol dissolved 

 in hydroxyl-free solvents, abnormally high values (Part II, 

 p. 60), which indicates the same conclusion as to liquid 

 alcohol itself at ordinary temperature. But alcohol 

 vapour at ordinary temperature possesses the normal 

 density. 



The striking fact that the formation of double molecules 

 which takes places in solution and not in vapour should 

 give rise to abnormal relations in the vapour only may be 

 explained by the following considerations : imagine a liquid, 

 say ethyl alcohol, in equilibrium with its saturated vapour, 

 and assume that each has the normal molecular weight 

 corresponding to C 2 H 6 O, and that the density of liquid 

 and vapour obey the rule of corresponding states. Now 

 let some of the simple molecules in the liquid be converted 

 into double ones ; the kinetic equilibrium due to the equal 

 evaporation and condensation of molecules C 2 H 6 is now 

 upset, since fewer evaporate. Accordingly fewer must 

 condense, so that a reduction in the amount of the vapour 

 must take place, causing a decrease in its density and 

 pressure, but an increase in the volume of a given weight, 

 as the above numbers indicate. 



If, further, we accept the law of correspondence as a 

 further condition for the characteristic equation, it is to be 

 remarked that any equation that has as many character- 

 istic variables (P, F, T) as constants (a, 6, K), i. e. of the 

 form 



/ (P, F, T, a, 6, E) = o, 



leads to the same conclusion if it includes the critical 

 state l . 



1 Meslin, Compt. Rend. 116. 135. 



