BETWEEN THE VISCOSITY OF LIQUIDS AND TUEIR CHEMICAL NATURE. 559 
boiling point, the curve for formic acid is to the right of all the others ; at the same 
temperatures it has the greatest viscosity. On passing to acetic acid the viscosity 
falls, the curve for this acid lying uniformly to the left of the formic acid curve. The 
curve for propionic acid, in a similar way, comes to the left of that for acetic acid. 
The order of the curves for the three acids is exactly the opposite to what invariably 
obtains in the case of strictly homologous substances. The curve for butyric acid, 
however, takes up a normal position to the right of that for propionic acid ; isobutyric 
acid also conforms to the general rule, the curve being uniformly to the left of that 
of butyric acid, and to the right of that of propionic acid. 
This exceptional behaviour of the fatty acids is in all probability to be traced to 
differences in molecular complexity. Eotvos first suggested that the fatty acids 
contain complex molecules, and according to Ramsay and Shields all the acids we 
have examined contain molecular aggregates, and at all temperatures the complexity 
of formic and acetic acids is somewhat the same, and more than twice as great as that 
of any of the other acids, so that at all temperatures the weights of the liquid 
molecules of these two acids are greater than those of the others. It is thus possible 
to give a definite reason for the apparently anomalous position of the curves of the 
lowest acids. In the case of the normal paraffins and their monohalogen derivatives 
we are dealing with liquids which in all likelihood contain simple molecules, the 
molecular wmights of the gas and the liquid are here the same, and here the curves are 
disposed in accordance with the ordinary gaseous molecular weights. 
In the case of the acids, however, the effect of the complexity of the liquid molecule 
is supei-added, the molecular weights of gas and liquid are no longer the same, and 
the curves no longer follow one another in the order of the theoretical molecular 
weights, but their disposition evidently depends upon the weights of the liquid 
molecules. Although this reasoning indicates why the viscosity curve of formic 
acid should lie to the right of the others, it does not explain why the curve 
for formic should lie to the right of that for acetic acid, because from the measure¬ 
ments of Ramsay and Shields, the molecular weight of liquid acetic acid is at 
all temperatures greater than that of formic acid, and its viscosity at any temperature 
would thus be expected to be greater instead of being less than that of formic acid. 
It is noticeable from the numbers given by Ramsay and Shields that with the 
exception of acetic acid the complexity of the first four acids diminishes with rise in 
molecular weight. If the anomalous position of the viscosity curves is due solely to 
the effect of complexity, it is indicated that acetic acid is really no exception to this 
rule. If, however, the results obtained by the surface-energy method of estimating 
complexity are valid, it must be admitted that the anomalous position of the curve 
for formic acid, with relation to that of acetic acid, is due to some peculiarity in the 
constitution of formic acid, which may be associated with the fact that it is the initial 
member of the homologous series, and does not contain a CH^ or a CHg group. 
