3 86 



SCIENCE PROGRESS. 



A study of the general physical behaviour of pure hydroxy- 

 liquids, particularly such properties as surface tension, 

 viscosity, vapour - pressure, boiling - point, and heat of 

 vaporisation (see ''Science Progress," vol. iii.,pp. 226-230), 

 points to the conclusion that their molecules, as distin- 

 guished from those of liquids in general, are really 

 aggregates of gaseous molecules. Hence it is not surprising 

 that in solution there is evidence of these molecular 

 aggregates, and that curves are obtained for their molecular 

 weights at different concentrations resembling those of 

 dissociating gases. Indeed, so definite seems to be the 

 rule that such is the constitution of a hydroxy-liquid, and 

 so uniformly does it give rise to anomalies in certain 

 solvents, that attempts have been made to use observations 

 on the freezing-point of solutions to ascertain in the case 

 of tautomeric compounds of doubtful chemical structure 

 whether or not a hydroxyl-group occurs in the molecule 

 (Auwers, 1894). Why hydroxy-compounds behave in 

 this way is difficult to say. The only hypothesis yet put 

 forward is due to Briihl (1895), wno concludes that in the 

 simple molecules of these substances oxygen is tetravalent 

 and that when they exist as simple molecules they are in 

 reality unsaturated compounds. The only other substances 

 which resemble them in giving associated solutions are com- 

 pounds whose molecules contain a labile hydrogen atom. 



Effect of tcmpei'attire. — Very little work has yet been 

 done to ascertain how the molecular weight of a substance 

 in an associated solution alters with the temperature. The 

 following are Beckmann's values of the molecular weight of 

 benzoic acid determined from the freezing-point and boiling- 

 point of solutions in benzene. The theoretical value is 122.. 



