between the viscosity of liquids and their chemical nature. 705 
magnitude of the temperature. At equal slope the viscosity of benzene, say, although 
its viscosity curve is peculiar, accords with those of higher homologues, and, indeed, 
of most homologous substances. Benzene may therefore be said to be comparable 
with other substances at equal slope, but it has still to be explained why the 
temperature of benzene is higher than that of its higher homologue at equal slope, 
for this temperature relation is the reverse of what holds for almost all simply- 
constituted liquids. 
It follows, therefore, from the above general discussion, (1) that a comprehensive 
view of the physico-chemical relationships of a series of substances can only be 
obtained by studying the variation of the physical property over as wide a range of 
temperature as possible; (2) that the graphical or algebraical representation of the 
results so obtained wiU indicate whether particular members of a series are exceptional 
in behaviour as compared with their congeners ; and (3) if such exceptional behaviour 
occurs, it may be detected either in the viscosity-magnitude or the temperature, no 
matter whether we use the boiling-point, a corresponding temperature, or a tempera¬ 
ture of equal slope as the condition of comparison. 
Other Methods of Obtaining and Comparing Viscosity-magnitudes. 
It might at first sight be supposed that the most suitable method of obtaining 
physico-chemical relationships would have been to deal with the curves expressing the 
relations between temperature and the molecular viscosity {rjdr) or the molecular 
viscosity work (rjcP), instead of concerning ourselves, as we have done, with the 
curves for rj, the viscosity coefficient. 
From the fact, however, that molecular aggregation afiects the values of cP and cP 
to an extent which cannot at present be satisfactorily estimated, we concluded that 
the question should, in the first instance, be approached by deducing slopes from the 
curves for the viscosity coefficients, and not from curves involving the quantities cP 
and #. 
We have, however, made a series of comparisons using curves for molecular 
viscosity, theoretical values of cP being used in obtaining them. The result of this 
method is, that the constants in Slotte’s formula, and the coefficients /3 and y in the 
modified formula, the values of the temperatures of equal slope, and the values 
of the molecular viscosity read off at these temperatures, although difiering in 
magnitude from those already given, exhibit amongst themselves practically the same 
general relationships as have already been described. 
The same conclusions apply to the method in which curves for molecular viscosity 
work are employed. 
4 X 
mdcccxciv.—A. 
