48 SCIENCE PROGRESS 



Table III. Data of Change of State 



The density is that in the vapour state, and is half the 

 molecular weight. Tc, T B , T F are the absolute values of the 

 critical, boiling and freezing point temperatures, and pc is 

 the critical pressure in atmospheres. Tc/pc = v c /\ and is seen 

 to increase with increasing density more than with temperature 

 or molecular complexity. Indeed, with more complex molecules 

 of the type of fluorbenzene, which is given as an example, as it 

 is well studied and normal, this ratio appears to be little more 

 than half the lowest value otherwise found in the table, and thus 

 among simple substances. No doubt the meaning is a variation 

 in the critical volume which cannot be satisfactorily investigated 

 for want of sufficient reliable data. The last column is the 

 reduced normal boiling point, and the mean of the values 

 given is 0*62 or very nearly 2/3, which is a rough and useful 

 approximation to this ratio. It may be noted that there is much 

 less regularity in the relation of the freezing point to the 

 others, as would be anticipated from the complex molecular 

 conditions which appear at and near the solid state. 



By making use of the principle that equally reduced vapour 



