FLUIDITY AND TEMPERATURE

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Unfortunately, fluidity-temperature curves are not generally
linear throughout. Even the aliphatic hydrocarbons, which are
supposedly unassociated, give curves which depart considerably
from linearity. For this peculiarity two possible explanations
suggest themselves. The effect of expansion is not linear except
in the case of mercury and there may be changes in the molecular
weight, either association or dissociation. Later each of these
causes will be considered in detail. Suffice it for the present
to note that the alcohols, Fig. 46, give fluidity-temperature
curves which are strongly curved at low fluidities, but the curves
tend to become linear at high fluidities as is true of the other

classes of compounds. In a_________________________

given homologous series the evi-
dence at hand shows a tendency
for the fluidity-temperature
curves near the ordinary boil-
ing-points to become parallel
to each other with equal dis-
tances between them. But
they must all meet at absolute
zero of temperature, which
requires that in the higher
members of the series at least

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7

7

7

7

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Temperature absolute
FIG. 49.—Diagram illustrating the
there Should be a region of fluidity-temperature curves of a- homol-
. & . ogous series of liquids.
rapid curvature. This is the
region of "softening" which has been observed in many viscous
substances. According to the views here presented the phe-
nomenon of softening may not occur in any substance con-
sisting of monatomic molecules but will certainly become
manifest as we increase the complexity of the molecule. These
ideas are represented diagrammatically in Fig. 49. If they
represent accurately the actual behavior of substances, the
fluidity-temperature curves have the following properties: (1)
They all approach linearity as the fluidity increases, i.e., each
tends to become asymptotic to a line which forms an acute
angle with the temperature-axis. (2) As the fluidity becomes
very small, each curve tends to become asymptotic to the tem-
perature-axis itself. Thus the general form of the fluidity-
temperature curve must be a hyperbola, since this is the