[Footnote 4: F. Guthrie, _Phil. Mag._ [5], xvii., p. 469; F.B.
Guthrie, _Journ. Chem. Soc_,. 1885, p. 94.]
Although it has been long known that on mixing certain salts the
resulting substance possessed a lower melting-point than either of the
constituent salts alone, still but few determinations of the
melting-points of mixtures of salts have been made, and even these are
often of small value, on account of the very considerable range of
temperature observed during solidification. This is due largely to the
fact that eutectic mixtures were not known, as equivalent proportions
of various salts have been employed, while eutectic mixtures are
seldom found to possess any simple arithmetical molecular relationship
between their constituents.
Eutectic salt alloys closely resemble cryohydrates in behavior. If for
simplicity we confine our attention to a fused mixture of two salts in
any proportion other than eutectic, it is found that, on cooling, the
thermometer falls steadily, until at length that salt which is in
excess of the proportion required for a eutectic mixture begins to
separate out. If this is removed, the thermometer falls until a fixed
point is reached at which the temperature remains stationary until the
whole of the mixture solidifies. On remelting, the temperature of
solidification is found to be quite fixed, and the mixture is
evidently eutectic.
It is of interest to notice that from our knowledge of the
cryohydrates it becomes possible to predict the existence,
composition, and temperature of solidification of a eutectic alloy, if
we are previously furnished with the melting-points of mixtures of the
substances in question. Or, in other cases, we may predict from the
curve of melting-points that no eutectic alloy is possible.
As an example, we may take the determinations of the melting-points of
mixtures of potassium and sodium nitrate by M. Maumené.[5] These are
graphically represented in Fig. 1, the curve being derived from the
mean of the temperatures given in the memoir. From this diagram we
should be led to expect a eutectic mixture, since the curve dips below
a horizontal line passing through the melting-point of the more
fusible of its constituents. From our curve we should expect a
eutectic mixture with about 35 per cent. KNO_{3}, and with a
temperature of solidification below 233°. Dr. Guthrie gives 32.9 per
cent. at 215°. This agreement is as good as might be expected when one
remembers that the melting-points, not being of eutectic mixtures, are
difficult to determine, and a considerable range is given; that
analyses of mixtures of potassium and sodium salts are apt to vary;
and that the two observers differ by ±7° in the temperatures given for
the melting-points of the original salts.
[Footnote 5: _Comptes Rendus_, 1883, 2, p. 45.]
[Illustration: FIG. 1.]
Dr. Tilden has drawn my attention to an interesting example of the
lowering of melting-point by the mixture of salts. The melting-point
of monohydrochloride of turpentine oil is 125°, while that of the
dihydrochloride is 50°; but on simply stirring together these
compounds in a mortar at common temperatures, they immediately
liquefy. Two molecules of the monohydrochloride and one molecule of
the dihydrochloride form a mixture which melts at about 20°.