As an example, we may take the determinations of the melting-points of mixtures of potassium and sodium nitrate by M. Maumene.[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.  KNO_{3}, and with a temperature of solidification below 233 deg..  Dr. Guthrie gives 32.9 per cent. at 215 deg..  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 deg. in the temperatures given for the melting-points of the original salts.

   [Footnote 5:  Comptes Rendus, 1883, 2, p. 45.]

[Illustration:  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 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.]