526 Mr. D. Tyrer on Relations between the Physical 



It will be observed from the Table that there is a close 

 agreement between the calculated and observed values of 

 the latent heat. The only classes of substances which do 

 not seem to conform to the relationship are the aliphatic 

 hydrocarbons and ethers and of course associated liquids 

 as the hydroxy! compounds. The validity of the relation is 

 in general affected by the following factors : — 



(1) Experimental errors in the values of the latent heats. 

 These are rather considerable as a comparison of the results 

 of different observers shows. The difference between the 

 calculated and observed values of the latent heat does, as a 

 general rule, come easily within the range of experimental 

 errors except in the fews cases mentioned above. 



(2) Association of the molecules and their dissociation on 

 vaporization. 



The Effect of Molecular Association on the Relation. 



The last portion of the above Table deals with liquids 

 which show molecular association, and it will be observed 

 that in these cases there are very wide divergences between 

 the calculated and observed values of the latent heats. The 

 question may be divided into two cases, viz. : — 



(a) Liquids whose molecules are associated but which do 



not dissociate on vaporization. 



(b) Liquids whose molecules dissociate on vaporization. 



Case (a). — In the first case the relation should obviously 

 be written 



JMn = K%/Vn, 



where n is the association factor. 

 The true latent heat then becomes 



Mn* 



Whereas the latent heat Li calculated according to the 

 equation without considering association is 



t _kVv 



For this class of liquids, therefore, the calculated values of 

 the latent heats will be greater than the observed values. 



Case (b). — The second case includes liquids like water, 

 whose molecules are associated in the liquid state, but are 

 normal in the vapour state, and also liquids like amyl iodide, 



