THE MOLECULAR WEIGHT OF LIQUIDS. 271 



association or molecular complexity at any particular tem- 

 perature can be calculated. We have already seen that 

 the behaviour of non-associating or "normal" liquids may 

 be expressed by the equation — 



2*121, 



whilst for associated liquids — 



y (Mvf - y (M z/y 



— k, 



T ~ T 



where k is a number varying with the temperature, and less 

 than 2" 1 2 1. 



At each particular temperature we can multiply the 

 gaseous or normal molecular weight of the liquid by a factor 

 x, which is chosen so that the k of the last equation be- 

 comes 2*121 ; that is to say, we can select a molecular weight 

 for the liquid which gives the normal constant 2*12 1. 



The factor x at each temperature represents the molecu- 

 lar complexity of the liquid at that temperature. At pre- 

 sent, unfortunately, there is no way of verifying this method 

 of calculating the molecular complexity of liquids. It is still 

 open to adverse criticism on the score that no rigorous 

 proof has been brought forward to show that the true 

 molecular complexity is obtained in this way, but it must be 

 admitted that we can get an approximation to the molecular 

 complexity, and future investigations will show how close 

 the approximation is. 



In the case of the normal liquids the experimentally 

 found values of the molecular surface energy can be repro- 

 duced by the equation y (Mz/) f = 2*121 (t — d) up to within 

 about 30 from the critical point, and by a slight alteration 

 it may be made to fit right up to the critical point. 



By the addition of another constant /n it was found that 

 a formula of the form — 



K (r - d) 



y (MZ/) 1 



I + IAT 



agrees admirably with the experimental values of the mo- 

 lecular surface energy of methyl and ethyl alcohols, water 



