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, 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 "that "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, do not separate in the same way as lead and zinc. Moreover, homogeneous wires could not be obtained by pressing, if these alloys were mechanical mixtures; but wires of the same alloy have been proved to have the same conducting power, whether taken from the press at the beginning or end of the operation. "On the other hand, the agreement between the theoretic and actual conductivity of these alloys, as well as between the calculated and actual percentage decrement in conductivity between 0° and 100° C., indicates that, in the solid state, they are not chemical compounds. In regard to these particulars, the following law has been found to obtain for all alloys of the first and second groups, as well as for some of those belonging to the third group : "The actual percentage decrement in conductivity between 0° and 100° C. is to the calculated decrement, as the actual is to the calculated conductivity. Among the alloys of the second group, some may be regarded as mechanical mixtures. Silver and copper fused and well stirred together separate when slowly cooled, so that the mass contains different amounts of the metals at different parts. But these alloys are exceptional, and most alloys of this group may be regarded as solidified solutions of allotropic modifications of the metals in each other. "In the third group of alloys, the rapid decrement in the conductivity of those alloys of the several series, which contain but very small amounts of a metal belonging to class A, cannot be ascribed to the existence of chemical compounds of the metals. For, in the first place, the amount of one of the metals in the alloys corresponding to the turning-points of the curves representing the conductivity of the series is too small, as will be seen by the following instances:— 66 'Again, the great similarity of the curves representing the conductivity of the series of alloys belonging to this group is opposed to the existence of chemical compounds in the solid alloys. "The influence exercised upon the conductivity of metals by the presence of small quantities of other metals, does not appear to be in any way determined by the alteration of crystalline form, or tendency to crystallise, which are known to be influenced by that circumstance. "If it be assumed that the metals belonging to class B undergo a molecular change when alloyed with one another, or with metals belonging to class A, and that in each case an allotropic condition is induced by a small amount of other metals, varying with the different metals, then many of the phenomena characteristic of alloys may be explained. Thus the curve representing the conductivity of zinc-copper alloys has the same form as those of other alloys, belonging to the same group, and the percentage decrement in their conductivity between 0° and 100° C. is exactly what is indicated by the law above stated. Hence it may be inferred that solid alloys of zinc and copper are only solidified solutions of zinc, and of allotropic copper in each other. The different action of reagents upon alloys, and upon the metals constituting them, when in an isolated state, may also be referred to the existence of such allotropic modifications when they are alloyed, as well as to the existence of chemical compounds of the metals.”1 SLAGS, FLUXES, AND REFRACTORY MATERIALS § 15. Slags. The slags formed in metallurgical operations are, with few exceptions, produced by the union of silica with metallic oxides, and termed silicates. Silicates may be divided into two classes, viz. "hydrated," which contain water; and "anhydrous," which are free from water. The silicates produced by heat in metallurgical operations belong 1 Watt's Diction. of Chem. vol. iii. pp. 943, 944. E |