532 Physical Properties of Liquids at the Boiling-Point. 



any particular liquid, the above equation would furnish a 

 method of determining n the association factor. 



There is one way of distinguishing between the effects of 

 association and constitution. If the members of an homo- 

 logous series give a gradually diminishing value of K as 

 the series is ascended, then the members of that particular 

 series show molecular association. If the value of K remains 

 approximately constant then no molecular association occurs. 

 For this reason the esters of the aliphatic dibasic acids, like 

 oxalic and succinic, have been classed along with the alcohols 

 and acids as associated liquids. 



Mention must be made here of other relations between the 

 molecular volume and the boiling-point. It has long been 

 noticed in a general way that there is a certain parallelism 

 between the molecular volume and the boiling-point, but no 

 exact relation has yet been discovered. Masson (Phil. Mag. 

 vol. xxx. p. 412, 1890) discovered empirically the relation 



Y 



-== constant, where Y and T are the molecular volume and 



temperature of boiling respectively, which he found to hold 

 closely for the alkyl halides, but apparently only for these few 

 ^classes of substances. 



On the other hand, Young (Phil. Mag. vol. xxx. p. 423, 1890) 

 showed that Masson's relation should only hold for substances 

 which have the same critical pressures, and deduced the 

 relation (from van der Waals' generalizations) 



T 

 Y = const. X p C > 



where Y is the molecular volume at the boiling-point, T e is 

 the critical temperature, and P c the critical pressure. 

 Obviously for those liquids where P e is constant, Masson's 

 relation will hold. 



Summa7y. 



An empirical relation between the latent heat of a liquid 

 and the molecular volume at the boiling-point is shown to 

 hold for most classes of liquids, with the exception of the 

 aliphatic hydrocarbons and ethers and liquids which show 

 molecular association. The relation may be written 



LM = K fr% 



where L is the latent heat at the boiling-point, M the mole- 

 cular weight, Y the molecular volume at the boiling-point, 

 and K is a constant equal to 1583. The effect of molecular 



