Although many fusible alloys have been long known, I believe no true eutectic metallic alloy had been studied until Dr. Guthrie6 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°.
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 and PbSn 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, which corresponds to Sn 65.43 per cent., or PbSn. This, therefore, is the composition of the eutectic alloy, and it finds its place naturally on the curve given in 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.
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.
In the second case, where cohesionA 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 cohesionA 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.
Read before the Birmingham Philosophical Society, January 22, 1885.
Guthrie, Phil. Mag. , xvii., p. 462.
Guthrie, Phil. Mag., 4th Series, xlix., pp. 1, 206, 266; 5th Series, i., pp. 49, 354, 446, vi., p. 35.
F. Guthrie, Phil. Mag. , xvii., p. 469; F.B. Guthrie, Journ. Chem. Soc,. 1885, p. 94.
Comptes Rendus, 1883, 2, p. 45.
Phil. Mag., 5th Series, xvii., p. 462.
Dingler's Polyt. Jour., 162, p. 217; Jahresberichte, 1861, p. 279.