594 
MESSRS. T. E. THORPE AND J. W. RODGER ON THE RELATIONS 
is only one-tenth of what it was in the case of the formic acid comparison. Propionic 
and isobutyric acid give values which are uniformly less by some 30 units than those 
of the corresponding iodides, so that sufficiently high up in the acid series the 
members conform to the rule that the compound of lower molecular weight has the 
lower viscosity. 
This behaviour of the acids with reference to the iodides is very probably to be 
attributed to the high molecular complexity of the lowest acids. If the viscosity 
coefficient is greater the higher the molecular weight of the liquid molecule, on using 
Ramsay and Shields’ observations to estimate the molecular complexity, it follows 
that acetic acid should have a slightly larger viscosity than ethyl iodide, and higher 
acids should have smaller viscosities than the corresponding iodides just as is the 
case in the above table. Formic acid, on the other hand, although its complexity 
appears from surface-energy observations to be about the same as that of acetic acid, 
has, at its boiling-point, a smaller liquid molecular weight than methyl iodide, and 
would be expected to have a correspondingly smaller viscosity. This is, however, 
not the case, and points either to the fact, as already stated, that the surface-energy 
observations employed do not give a correct measure of the complexity, or that 
formic acid, as initial member of the homologous series, gives a peculiar v^alue of the 
viscosity coefficient. 
On comparing the alcohols with the iodides, methyl alcohol is seen to have a much 
smaller coefficient than methyl iodide, but for the remaining alcohols given in the 
table the coefficients are decidedly greater than those of the iodides ; for the 
saturated alcohols the difference is about 100 units. 
These results cannot be easily explained. According to surface-energy measure¬ 
ments the weights of the molecules of the liquid alcohols are invariably lower than the 
ordinary molecular weights of corresponding iodides. Methyl alcohol, however, is 
the only alcohol which has a lower viscosity than the corresponding iodide. 
In explanation of these anomalies the following points must be borne in mind :— 
1. The degree of molecular complexity may not be accurately indicated by surface- 
energy measurements. 
2. The molecular complexity of the alcohols may increase with rise in molecular 
weight. 
3. The boiling-point may not be a suitable condition of comparison. 
That the boiling-point is a suitable condition of comj^arison in the case of the 
chlorides, bromides, and iodides is shown b}^ the fairly definite relations exhibited. 
Hence, it is probable that the molecular complexity of the alcohols does increase with 
rise in molecular weight, contrary to the conclusion arrived at from surface-energy 
measurements. 
