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ALLOYS

tlie union so effected the term cementation is applied. A very interesting observation was made in 1820 by Faraday and Stodart, who in the course of an investigation on the alloys of iron with other metals, note their failure to produce certain alloys by cementation, but consider it “remarkable ” that platinum will unite with steel at a temperature at which the steel is still solid. This early recognition of the fact that alloys can be produced by cementation is very curious, and shows that Faraday and Stodart had observed a creeping molecular action to occur at a temperature below the fusing-point of either platinum or carburized iron. So long ago as 1878 W. Spring showed that under a pressure varying from 13 to 47 tons per square inch, metallic filings will unite into solid masses which under pressure behave like fluids and truly flow through the apertures of the receptacle in which they are compressed. This fact lends support to the view that cohesion is a form of chemical affinity. By compressing, in a finely divided state, 15 parts of bismuth, 8 parts of lead, 4 parts of tin, and 3 parts of cadmium, an alloy is produced which fuses at 100° C. Hallock found that metals might be united without pressure, if they were heated to the meltingpoint of the alloy to be formed, that is, to a point which as a rule is much below the melting-point of the least fusible constituent. It has long been known that brass may be formed by electrodeposition from a solution containing copper and zinc. In this connexion J. B. Senderens lias made a very interesting scries of experiments on the precipitation of one metal from solution by another metal. He points out that K-ichter in his researches, published between the years 1796-99, was led to the promulgation of the law which is now expressed as follows: “Metals are precipitated from their saline solution atom for atom of the same, valency.” Senderens shows that this is not rigorously accurate, since the amount of the precipitating metal is always in excess. Thus, in the well-known reaction employed in refineries for the precipitation by copper of silver from sulphate solutions, in accordance with the equation—AgSCq + Cu = Ag + CuS04, if the solution of sulphate of silver contains 4-5 grammes of the salt in a litre of water, a plate of copper immersed in it for seven days will be found to have lost 0-012 gramme more than theory demands. Senderens shows that this fact is of singular interest in relation to the allotropy of iron, but his work is quoted here on account of its bearing on the formation of alloys in the “wet way.” Several chemists, among whom Blanche and Brugnatelli may be specially mentioned, have thought that alloys might be so prepared, while Gay-Lussac at first favoured that opinion, but abandoned it later. So long ago as 1857, in a remarkable paper on “The Reciprocal l recipitation of Metals, Odling showed that a piece of coppercoated with cadmium, and a coil of copper and cadmium foils rolled up together, behave very differently when treated with hydrochloric acid, the metallic deposit of cadmium partaking of the character of an alloy. Such precipitation of one metaf on another he attributes to the affinity of one metal for the other. He in fact formed alloys in the “wet way.” More recently Mylius and Fromm have shown that alloys may be precipitated from dilute solutions by zinc, cadmium, tin, lead, and copper. Thus a strip of zinc plunged in a solution of sulphate of silver containing not more than 0-03 gramme of silver in the litre, becomes covered with a flocculent precipitate which is a true alloy of silver and zinc, and in the same way, when copper is precipitated from its sulphate by zinc, the alloy formed is brass. They have also formed certain alloys of definite composition such as AuCdCu,Cd, and, more interesting still, Cu,Sn. If volatile metals in the form of vapour are brought in contact with non-volatile metals umon n ill in many cases ensue. It has been shown that alloys of platinum and palladium with cadmium, zinc, and magnesium'mayJ be so produced. In the classical monograph on alloys published by Matthiessen in 1860 (Phil. Tram. R. g.), it was clearly Alloys as stated that alloys must be considered to be solidisoiid _ fied solutions. Fewgeneralizations have been more solutwns. fruitful in results, and much modern work has been devoted to the development of the view. The constitution of solutions has been very closely studied in recent years, and from the point of view of their freezingpoints F, Guthrie has shown that solutions of metals m

each other behave like ordinary aqueous solutions of salts. His actual results for solutions of common salt in water are given in Fig. 1. c If, for instance, a g*40’ thermometer be 2 placed in a 10 per | + 20" cent, solution of *2 salt in water which ? is being slowly cooled by means 0 of an external freezing mixture, the mercury will halt in its fall at about 8° C., owing to the separation of pure salt-free ice. This gives the point d on the branch AB. The mercury then continues to fall until the temperature of 22° C. is reached, and the cryo-hydrate or eutectic of ice and salt solidifies. This eutectic, as has been abundantly shown, consists merely of a very intimate mixture of ice and salt in juxtaposition. As the degree of concentration of salt in the original solution increases, the initial freezing-point on the branch AB will become lower and lower, while the second freezing point always remains constant at - 22° C.; and when the solution contains 23-5 per cent, of salt, both freezing-points coincide in the point B at - 22° C. The salt branch of the diagram is a very steep one, because the melting-point of pure salt is above 700° C. Take on this branch a point e representing water containing more salt than 23‘5 per cent., say 25 per cent. In this case the first solid to separate on cooling is pure salt, and it does so at - 12° C, which, for this degree of concentration is the first halting stage of the thermometer ; the second is, as before, the solidification of the eutectic of salt and ice, which always has the same composition, and freezes at the same temperature, namely 22° C. The diagram therefore has two branches joining at B a horizontal line. This curve has been described at some length, because curves representing the freezing-points of any series of alloys may be derived in the same way. Guthrie considered that alloys in cooling behave like a cooling mass of clear molten granite. This would throw off in cooling “ atomically definite ” bodies, leaving behind a fluid mass not definite in composition, since the quartz and felspar undergo solidification before the mica. In alloys much the same thing happens, for when a molten mass of lead and bismuth, or bismuth and tin, cools, a certain alloy of the metal falls out, just as the quartz and felspar did, and ultimately the most fusible alloy of the series is left. This is called by Guthrie the eutectic alloy, but the proportions between the constituent metals are not atomic, and Guthrie pointed out 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 he adds “ that certain metals may and do unite with one another in the small multiple of their combining weights, may be conceded; the constitution of eutectic alloys is not in the ratio of any simple multiple of their chemical equivalents, but their composition is not on that account less fixed, nor are their properties the less definite.” The constitution of eutectic alloys as revealed by the microscope will be dealt -with in the article on Metallography. Mendeleeff regards solutions as strictly definite atomic chemical combinations at temperatures higher than their dissociation temperatures. Definite chemical substances may be either formed or decomposed at temperatures which are higher than those at which dissociation begins; the same phenomenon occurs in solutions. In order to show how close the relation is between freezing solutions of salt in water and in an alloy, no simpler case could well be taken