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. KNO3, 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.
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°.
III. EUTECTIC METALLIC ALLOYS.
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°.
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 PbSn3 and PbSn4 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. SnO2, which corresponds to Sn 65.43 per cent., or PbSn3.3. This, therefore, is the composition of the eutectic alloy, and it finds its place naturally on the curve given in Fig. 2.
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.