112 FLUIDITY AND PLASTICITY
acids. But while they attribute these effects to the constitution
of the molecules, it should be noted that the immediate cause of
the increase in viscosity may in each case be association, which
is the same as saying that it may be due to chemical composition
as distinguished from chemical constitution. Certainly com-
pounds containing the hydroxyl radical are often associated and
these same compounds are noted for their high viscosity, so that
in the case of ethyl acetoacetate the way seems open to explain
the greater viscosity of the enol form on the basis of an associated
molecule, quite as well as on the basis of symmetry or other
constitutive influence. At first sight it seems as though consti-
tutive influences must solely and immediately determine the
viscosity values in each of the above examples, hut Thole (1912)
seems to realize that this is not the actual case with maleic and
fumaric acids, the latter of which gives the higher viscosity in
methyl alcohol solution. He says, " The viscosities of the isomers
depend not only on the relative positions of the unsaturated
groups but also on the degree of residual affinity " which causes
molecular association. Thus the "adjacent" maleic acid may
have the lower viscosity due to slighter association. This view
is borne out by the fact noted by Thole that "barium fumarate
crystallizes with three molecules of water while barium maleate,
in which the residual affinities of the carboxyl groups are more
nearly mutually satisfied, combines with only one molecule of
water."
To what extent different constitutive influences affect the
association of compounds is an exceedingly important subject
but it is not relevant to our discussion of viscosity. Our problem
is to study the immediate effects of constitutive influences and
the cheirdeal composition of the molecule upon the viscosity and
to estimate their relative importance.
Regardless of how much uncertainty there may be in regard
to the importance of constitutive influences on viscosity, there
can be no doubt about the importance of chemical composition.
All evidence shows that this factor is of great importance. Dun-
stan and Langton (1912) have made use of this for the determine-
tion of transition points, and Thole (1913) in the detection of the
presence of racernic compounds in the liquid state, and many other
instances might be cited.