|| 172 FLUIDITY AND PLASTICITY
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;| HI POSITIVE CTJRVATION ANTD CHEMICAL COMBINATION
4 Before considering the meaning of positive curvature in detail,
it is necessary to emphasize the fact that a minimum in the fluid-
JJ ity-volume concentration curve is not necessary to indicate that
chemical combination is taking place and when a minimum does
occur, its location, according to numerous investigators, notably
Kndlay (1909) and Denison (1913), does not correspond to the
exact composition of the compound formed. This is proved, if
proof be needed, by the fact that the minimum usually changes
with the temperature and may disappear altogether. The ques-
tion then is, assuming that a chemical combination is formed by
the mixing of the two components of a binary mixture, how can the
|* data be used to show what this compound is? To answer this,
I; we will present three cases of increasing complexity, in all of
*; which there is the same amount of chemical combination, it being
I' assumed that in the feeble combination with which we are dealing
I the two components A and B are always in equilibrium with a
f small amount of the compound C so that
}' A + B & C
L, Case I.—The fluidity-temperature curves of two closely related
• jfl substances are represented by the curves A and B in Fig. 63a.
4! If there were no combination between the components on mixing,
w the curve for the 50 per cent mixture would lie half-way between
ft, the curves A and B (dotted). Let it be assumed that this mix-
jl ture does show the maximum amount of combination and that
the curve is thereby lowered to 0.5S. Using the data plotted in
Kg. 63a it becomes possible to plot the fluidity-volume concen-
tration curves for the various temperatures £1, £2, *2> etc., as shown
in Fig. 63&. In this case there is a well-defined minimum in the
fluidity-volume concentration curve in the 50 per cent mixture
and the deviation of the curves from the normal (dotted) curves
is constant in amount.
Case II.—.Let us now assume that we are dealing with two
substances whose fluidities are widely different, although they
still run parallel to each other. With the same amount of combi-
nation as before, the curve 0.5B falls between the curves A and B,
Fig. 64&. As a result the fluidity-volume concentration curves,
Fig. 646, no longer exhibit a minimum although, by assumption,