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 deg..  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 deg., when the melting-point of tin is taken as 228 deg..  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: 

    1.  The cohesion between the particles of A.

    2.  The cohesion between the particles of B.

    3.  The cohesion between the particles of A and the particles of B.

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.