VAKIATION OF MOLECULAR SURFACE-ENERGY WITH TEMPERATURE. 653 
that the volume of one gramme of the vapour of acetic acid at 50°, the lowest tempera¬ 
ture at which measurements Avere possible, and at a pressure nearly that of the 
saturated vapour (51‘35 millims., instead of 56'56)is 3567 cub. centims., corresponding 
to the molecular weight, 109'6, instead of 60, the molecular weight of CoH^O^, or 120, 
that of 
The alcohols show no such deviation in their vapour densities at any observed 
temperatures and pressures; but that does not preclude the occurrence of association 
between simple molecules to- form more complex molecular groups in the liquid 
substances. It is known that if the molecules in the gaseous state be caused to 
recede, dissociation takes place; for instance, at the same temperature, 50°, but 
at a pressure of 13'4 millims., one gramme of acetic acid gas occupied no less than , 
15,100 cub. centims., implying a molecular weight of 99’2, considerably greater than 
that of 0311.^03 (60), but not so great as that of C^HgO^ ( 120 ). And many other 
similar instances could be given. 
Now, since association is promoted by the approach of molecules, it is clear that 
as molecules in the liquid state are much nearer each other than in the gaseous state, 
it is not unreasonable to suppose that some liquids may consist in whole or part of 
associated molecules, even although their gases show no signs of abnormal density. 
These observations are illustrated by the following tables, which show the variation 
with ten)perature of the molecular surface-energy of acetic acid, and of methyl and 
ethyl alcohols, assuming that the molecular weight in each case is tliat of the simple 
molecule, CoH^Og, CH^O, or CgHgO. By choosing suitable molecular weights, 
implying a mixture of simple and complex molecules, the relation can, of course, be 
made a hnear one. 
