metals separating on cooling; the lead retaining 1.6 per cent of zinc, and the zinc retaining 1.2 per cent of lead. In all cases of mixed metals there is not that total and complete alteration in properties which is the distinctive feature of chemical action between a metal and a non-metal. Alloys still possess the metallic character, but there is frequently a considerable evolution of heat evinced; the melting points, as in fusible metal, are considerably lowered; the mean density is increased, and the colour and other physical properties are considerably modified. Most often, however, as already indicated, alloys seem to be mixtures of definite compounds with an excess of one or other metal, and the separation of their components from each other is generally easily effected by simple means. Thus an alloy of lead and copper may be largely separated by exposing the mixture to a temperature above that of the fusing point of lead, but below that of copper, when the lead liquates out, leaving behind a porous mass of copper, containing a little lead. Also in remelting brass, a considerable quantity of zinc is lost by volatilisation. When silver is amalgamated with mercury the amalgam is dissolved in an excess of mercury, which excess is removed by simple pressure; and the remaining portion of mercury is completely separated by the agency of heat. Many metals combine together when melted, and only remain in union within certain ranges of temperature, as shown by the wide differences between their melting and solidifying points. Matthiessen regards it as probable that the condition of an alloy of two metals in the liquid state may be either that of--(1) a solution of one metal in another; (2) chemical combination; (3) mechanical mixture; or (4) a solution or mixture of two or all of the above; and that similar differences may obtain as to its condition in the solid state. He also classifies the solid alloys composed of two metals according to their chemical nature. 1. Solidified solutions of one metal in another lead-tin, cadmium-tin, zinc-tin, zinc-cadmium, and lead-cadmium alloys. 2. Solidified solutions of one metal in the allotropic modification of another: lead-bismuth, tin-bismuth, tin-copper, zinccopper, lead-silver, and tin-silver alloys. 3. Solidified solutions of allotropic modifications of the metals in each other: bismuth-gold, bismuth-silver, palladiumsilver, platinum-silver, gold-copper, and gold-silver alloys. 4. Chemical compounds of the alloys corresponding to Sn, Au, Sn,Au, and Au,Sn. 5. Solidified solutions of chemical compounds in each other: the alloys intermediate between those corresponding to the above formulæ. 6. Mechanical mixtures of solidified solutions of one metal in another: alloys of lead and zinc containing more than 1.2 per cent of lead, or 1.6 per cent of zinc. 7. Mechanical mixtures of solidified solutions of one metal in the allotropic modification of another: alloys of zinc and bismuth containing more than 14 per cent zinc, or more than 24 per cent bismuth. 8. Mechanical mixtures of solidified solutions of allotropic modifications of the two metals in each other: most of the silver-copper alloys.1 As before stated, many mixtures, the components of which may be united in the liquid condition, separate to some extent on cooling. In fact, a cooling mass of mixed metals often behaves like water containing suspended matter does in freezing, when the ice first formed rejects the foreign matter; and so the portion of the alloy which first solidifies rejects certain other portions of the constituent metals. Thus a mixture of lead, antimony, and copper poured into a cylindrical mould, allowed to cool, and then broken, will show that while the copper and antimony have united the lead has been rejected, and driven to the centre of the mass. A mixture of lead and zinc separates in a similar way. Silver and copper alloys behave in a similar manner, but in any mixture of fused silver and copper, one particular alloy of silver and copper is formed, this is driven inwards or outwards according 1 Matthiessen, Brit. Assoc. 1863. to whether copper or silver is in excess in the bath. In all cases the separation is never complete, a small quantity at least of the other metal being found in each portion of the separated constituents. The solid mass in the above three cases is a mixture of solidified solutions of the metals in each other.1 Dr. Guthrie gave considerable attention to this subject some years ago, and came to the conclusion that certain alloys in cooling behave as a cooling mass of granite would; clear molten granite in cooling would throw off "atomically definite" bodies, leaving behind a fluid mass, which is not definite in composition, as the quartz and felspar solidify before the mica. The same thing takes place in cooling fused mixed metals; for when a mixture of lead and bismuth or bismuth and tin cools, a certain alloy of the metals falls out, and the most fusible alloy of the series is left, which Guthrie calls the eutectic alloy. It is the most fusible alloy of the series, but the proportions between the constituent metals are not atomic proportions; and Guthrie says, that the preconceived notion that the alloy of minimum temperature of fusion must have its constituents in simple atomic proportions, and that it must be a chemical compound, seems to have misled previous investigators; but that certain metals may, and do unite with one another in small multiples of their combining weights may be conceded; the constitution of eutectic alloys is not in the ratio of a simple multiple of the chemical equivalents of the constituents, but their composition is not on that account less fixed, nor are their properties the less definite." 2 66 "Matthiessen considers that the conductivity for heat and electricity are among the characters best calculated to indicate the chemical nature of alloys. With respect to electric conductivity, he divides metals into two classes : "A. Metals which, when alloyed with each other, conduct 1 Roberts-Austen, Journal of Society of Arts, 1888, p. 1115. electricity in the ratios of their relative volumes-lead, tin, zinc, and cadmium. "B. Metals which, when alloyed with each other, or with a metal of class A, do not conduct electricity in the ratios of their relative volumes, but always in a lower degree than that calculated from the mean of their volumes-bismuth, antimony, platinum, palladium, iron, aluminium, gold, copper, silver, etc. "The curves representing the conductivity of different series of alloys have the relation shown in the accompanying diagrams. "Group I. Those belonging to the alloys of metals in class A are almost straight lines. That of lead-tin alloys is given as a type, Fig. 1. “Group II. The curves of alloys of metals in class B show a rapid decrement on both sides of the curve, the turning points being connected together by nearly straight lines. That of gold-silver alloys is given as the type, Fig. 2. "Group III. The curves of alloys of metals in class A with those in class B show a rapid decrement on the side beginning with the metal belonging to class B, then turning and going in a straight line to the other side, beginning with metal belonging to class A. That of tin-copper alloys is given as the type, Fig. 3. "In regard to alloys of the first group, if they were mechanical mixtures, the metals composing them, unless their specific gravities were the same, would separate into two layers when melted and slowly cooled, as in the case of lead-zinc alloys. But the alloys of lead and tin, for example, |