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 deg., while that of the dihydrochloride is 50 deg.; 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 deg.. 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 deg.. [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 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.]