LUfht Absorption and I luorescence. 21) 



not the least interesting of these is the problem of the three 

 states of mutter and the difference in their energy contents. 

 Following on the theory of allot ropy, it is an obvious con- 

 clusion to draw that the three states are equilibrium 

 mixtures of different molecular phases. Whilst it cannot 

 be assumed that any change of state is due to a change of 

 phase of every molecule present, yet it is not impossible 

 that, ut any rate, the latent heat of evaporation should 

 approximate to the amount of energy associated with a 

 change of phase — that is to say, one or more quanta at the 

 infra-red fundamental for every molecule. The values of 

 the infra-red fundamentals are at present known tor very 

 few substances, but it is possible to make use of the most 

 important molecular frequency, in the infra-red, which, as I 

 have shown, is the infra-red fundamental in the cases of 

 water and sulphur dioxide. Tn Table I. are given the 

 relations between the latent heat of evaponttion per molecule 

 and the quantum at the infra-red frequency. The agree- 

 ment is so close that I have calculated the latent heat on the 

 basis of it being equal to the number of infra-red quanta 

 shown in the seventh column. The calculated values ex- 

 pressed as calories per gram-molecule are given in the last 

 column. 



In the cases of carbon tetrachloride and sulphur it may be 

 noted that the infra-red measurements are not so trustworthy 

 as those of the other compounds *. At the same time it 

 must be noted that there are some compounds for which 

 this relation does not appear to hold. Now, it is well known 

 that water is associated in the liquid condition, and therefore 

 the suggestion may be made that when the latent heat is 

 equal to two or more infra-red quanta, this is due to the 

 molecule of the liquid being more complex than the molecule 

 of the vapour. The cases in which the relation does not 

 hold would then be explicable on the ground that the 

 molecular association is indefinite — that is to say, the liquid 

 contains an equilibrium mixture of two molecular complexes. 

 This application of the molecular phase hypothesis cannot, 

 however, be further discussed here. 



In conclusion, reference may be made to the deduction 

 from the molecular phase hypothesis that, since a specific 

 molecular phase is essential for a molecule to take part in a 

 given reaction, a definite amount of energy must, in general, 

 be supplied to the molecule before the reaction can take 

 place. Whilst this is not new, the conception of the critical 



* Coblentz, Pub. Carnegie Inst. Washington, No. 35 (190o). 



