560 MESSRS. T. E. THORPE AND J. W. RODGER ON THE RELATIONS 
Anhydrides. 
Acetic aiiliydride and propionic anhydride were the only members of this series 
which were examined. The curves obtained, together with that of propionic acid, 
which is inserted for the sake of comparison, are given in fig. 16. 
Fig. 16. 
The curves for propionic anhydride and acetic anhydride are disposed in the order 
of the molecular weights. On comparing the dotted curve for propionic acid with 
those for the two anhydrides, it is evident that the absolute values of the coefficients 
for propionic anhydride are not very different from those for propionic acid at the same 
temperature, although the theoretical molecular weights of the substances differ widely. 
This is probably another instance of the marked effect of molecular complexity. 
From surface energy measurements it appears that liquid acetic anhydride contains 
simple molecules, and from the position and course of the viscosity curve for propionic 
anhydride with reference to that of acetic anhydride, it is probable that liquid 
propionic anhydride is also simply constituted. Propionic acid on the other hand 
contains molecular aggregates, and from surface-energy measurements the average 
molecular weight of the liquid is almost 130, a number wdiich is exactly the same as 
the molecular weight of gaseous and probably of liquid propionic anhydride. 
This is probably the reason for the })roximity of the curves of propionic acid and 
propionic anhydride. 
Aromatic llydrocarhons. 
Six members of this series were investigated, viz.; benzene, toluene, ethvl benzene, 
ortho-xylene, meta-xylene, and para-xylene. 
The results obtained are represented in fig. 17. 
The general character of the cuiwes for the aromatic hydrocarbons is similar to that 
of the paraffins : dyjdt is comparatively small. One of the most striking points in 
