Although many fusible alloys have been long known, I believe no true
eutectic metallic alloy had been studied until Dr. Guthrie[6] worked
at the subject, employing the same methods as with his cryohydrates.
It is found if two metals are fused together and the mixture allowed
to cool, that the temperature falls until a point is reached at which
that metal which is present in a proportion greater than is required
to form the eutectic alloy begins to separate. If this solid be
removed as it forms, the temperature gradually falls until a fixed
point is reached, at which the eutectic alloy solidifies. Here the
thermometer remains stationary until the whole has become solid, and,
on remelting, this temperature is found to be quite fixed. In addition
to the di-eutectic alloys, we have also tri- and tetra-eutectic
alloys, and as an example of the latter we may take the
bismuth-tin-lead-cadmium eutectic alloy, melting at 71°.
[Footnote 6: _Phil. Mag._, 5th Series, xvii., p. 462.]
We have already seen with salt eutectics that, given the curve of
melting-points of a mixture in various proportions, we may predict the
existence, composition, and melting-point of the eutectic alloy. As a
matter of course, the same thing holds good for metallic eutectics. An
interesting example of this is furnished by the tin-lead alloys, the
melting-points of which have been determined by Pillichody.[7] From
these determinations we obtain the curve given in Fig. 2, and from
this curve, since it dips below a horizontal line passing through the
melting-point of the more fusible constituent, we are at once able to
predict a eutectic alloy. We should further expect this to have a
constitution between PbSn_{3} and PbSn_{4} and a melting-point
somewhat below 181°. On melting together tin and lead, and allowing
the alloy to cool, we find our expectation justified; for by pouring
off the fluid portion which remains after solidification has
commenced, and repeating this several times with the portion so
removed, we at length obtain an alloy which solidifies at the constant
temperature of 180°, when the melting-point of tin is taken as 228°.
On analysis 1.064 grm. of this alloy gave 0.885 grm. SnO_{2}, which
corresponds to Sn 65.43 per cent., or PbSn_{3.3}. This, therefore, is
the composition of the eutectic alloy, and it finds its place
naturally on the curve given in Fig. 2.
[Footnote 7: _Dingler's Polyt. Journ._, 162, p. 217;
_Jahresberichte_, 1861, p. 279.]
[Illustration: FIG. 2.]
It will be seen that the subject of eutexia embraces many points of
practical importance and of theoretical interest. Thus it has been
shown by Dr. Guthrie that the desilverizing of lead in Pattinson's
process is but a case of eutexia, the separation of lead on cooling a
bath of argentiferous lead poor in silver being analogous to the
separation of ice from a salt solution. Dr. Guthrie has also shown
that eutexia may reasonably be supposed to have played an important
part in the production and separation of many rock-forming minerals.
It is with considerable diffidence that I suggest the following as an
explanation of the multitude of facts to which previous reference has
been made.
In a mixture of two substances, A and B, we have the following forces
active, tending to produce solidification: