With regard to this last factor, it will be seen that there are three
cases possible:
1. The cohesion of the mixture A B may be greater than the
cohesion of A + the cohesion of B.
2. The cohesion of A B may be equal to the cohesion of A + the
cohesion of B.
3. The cohesion of A B may be less than the cohesion of A + the
cohesion of B.
Now, since cohesion tends to produce solidification, we should in the
first case expect to find the melting-point of the mixture _higher_
than the mean of the melting-points of its constituents, or the curve
of melting-points would be of the form given in _a_, Fig. 3. Here no
eutectic mixture is possible.
[Illustration: FIG. 3.]
In the second case, where cohesion A B = cohesion A + B, we should
obtain melting-points for the mixture which would agree with the mean
of the melting-points of the constituents, the curve of melting-points
would be a straight line, and again no eutectic mixture would be
possible.
In the third case, however, where cohesion A B is less than cohesion A
+ B, we should find the melting-points of the mixture lower than the
mean of the melting-points of its constituents, and the curve of
melting-points would be of the form given in _e_, Fig. 3. Here, in
those cases where the difference of cohesion on mixture is
considerable, the curve of melting-points may dip below the line _e
f_. This is the _only case_ in which a eutectic mixture is possible,
and it is, of course, found at the lowest point of the curve.
If it be true, as above suggested, that the force of cohesion is at
its minimum in the eutectic alloy, we should expect to find, in
preparing a eutectic substance, either that actual expansion took
place, or that the molecular volume would gradually increase in
passing along our curve of melting-points, from either end, for each
molecule added, and that it would obtain its greatest value at the
point corresponding to the eutectic alloy.
Of this I have no direct evidence as yet, but it is a point of
considerable interest, and I may possibly return to it at some future
time.--_Chemical News._
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