174

FLUIDITY AND PLASTICITY

Case III.—In the usual case in practice, the fluidity-tempera-
ture curves are not parallel, so that the fluidities may be identical
at one temperature but very different at another. We then
obtain a series of curves as shown in Fig. 65a and 65&. At low
temperatures there is a good minimum in the fluidity-volume
concentration curves, but it gradually shifts to the right as the
temperature is raised, until at the highest temperatures it dis-
appears altogether. It is manifestly erroneous to assume that
the composition of the hydrate changes on this account. On the
other hand, the deviation from the expected linear curves as

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a

FIG. 65.—Diagram illustrating how the minimum in the fluidity-concentra-
tion curve may shift with the temperature. The maximum deviation from the
linear curve is the significant quantity. This quantity floes not vary with the
temperature and it indicates the composition of the solvate.

measured vertically is everywhere the same as in the simpler
cases. In practice, the hydration is generally less at the higher
temperatures so that the deviation should grow less as the tem-
perature is raised, but the cases already given are sufficient to
show that the deviation of the observed fluidity-volume concentra-
tion curve from the linear curve, which would be expected were
there no combination between the components of the solution,
can alone furnish trustworthy information.
Were the components of the mixture non-associated, it seems
possible to calculate not only the composition of the solvate
formed but also the percentage of it existing in the solution.
But substances which form feeble combinations on mixing are
usually themselves associated, and it is quite likely that this