increases its strength, the maximum being attained with about 20 per cent. of tin. The entire series of copper-tin alloys is very interesting. Two only appear to be homogeneous, and these correspond respectively to the formula SnCu, and SnCu,. Notwithstanding the comparatively small difference in their composition, TENSILE STRENGTH TONS PER SQ. INCH ALUMINIUM BRONZE 8 45 35 30 25 10 the appearance of the fractured surfaces of these alloys is quite different, the latter being yellowish grey in colour with a mirror-like fracture, whilst the former is blue with a rough fracture. Having a higher specific gravity than the mean of its constituents, the alloy SnCu, stands out from the rest of the series. An interesting example of the influence of foreign elements on the strength of metals is afforded by aluminium bronze, the strongest of all the copper alloys. An alloy of 10 per cent. of aluminium with 90 per cent. of copper, rolled into plates, has a tensile strength of from 100,000 to 120,000 lbs. per square inch, and in castings has a tensile strength of 70,000 to 80,000 lbs. per square inch. This alloy, which was discovered by the late Dr. Percy, closely resembles gold in appearance. The effect of smaller proportions of aluminium on copper is shown in the accompanying diagram (Fig. 17). FIG. 17. In discussing the influence of foreign elements on iron, the terms "cast iron," "wrought iron," and "steel" are used, as this classification is still in general use among engineers. In 1878, however, an international commission at Philadelphia decided to adopt a classification based on the amount of carbon contained in the metal: I. Pig iron with 2.3 per cent. and more of carbon; melts at a comparatively low temperature (1075° to 1275° C.) and cannot be forged. a. White pig iron; all the carbon is combined with the iron, the compound is very hard, brittle, white, and is made solely for the purpose of being converted into malleable iron. b. Grey pig iron; in which more or less of the carbon is present in the form of graphite. The metal is soft, tough, grey to black, and is used for conversion into malleable iron or for the production of castings. II. Steel with 1.6 to 0.4 per cent. of carben, melts at 1400° to 1500° C. By sudden cooling of a red-hot mass the hardness is considerably increased. III. Weld iron with less than 0.2 per cent. of carbon, melts at 1600° C. and above. It cannot be appreciably hardened. The Philadelphia Commission decided that 1. Every malleable compound of iron, containing the ordinary elements of that metal, which is obtained either by the union of pasty masses of iron or by any process not involving fusion, and which cannot be hardened by the ordinary method, shall be called weld iron. This is what has formerly been known as wrought iron. 2. Any analogous compound, which by any cause hardens, shall be called weld steel. This has hitherto been termed puddled steel. 3. Every malleable compound of iron, containing the ordinary constituents of that metal, which is obtained and poured in the fused state, but which does not harden by the ordinary methods, shall be known as ingot iron. 4. Every compound similar to the last, but capable of hardening from any cause whatever, shall be called ingot steel. Colour. The colour of metals is influenced by their purity. Thus, iron becomes white by admixture with carbon, silicon, sulphur, phosphorus, and other elements. By the alteration in the colour of a metal it is possible to detect the presence of a very small quantity of impurity, especially in the case of gold. The lustre of metals is due to their great power of reflecting light. It varies with the purity and the degree of polish of the metal. It is therefore of great use in detecting the presence of impurities, a notable example of this being afforded by copper. Fusibility. All metals are fusible. On account of the difficulty experienced in determining high temperatures, only the melting points of the metals that fuse at temperatures below 1000° have been ascertained with absolute accuracy. The melting points of the more important metals are given in the table on pp. 58, 59. Arsenic sublimes at 180°, but it may be fused under the pressure of its own vapour. When strongly heated, metals pass from a brownish-red to a clear red colour, which gradually increases in luminosity and transparency to a dazzling white. The temperatures corresponding to the different colours have been estimated by Pouillet to be On solidifying from a molten state, metals frequently exhibit excrescences due to the expulsion of absorbed gases. This expulsion often occurs shortly before the solidification, and causes a sudden outburst of metal through the surface. In this way silver, when molten, absorbs oxygen, and expels it on solidification. In the case of steel, the evolution of gas continues long after the metal has solidified on the surface. When a metal passes from the liquid to the solid state, it either does so suddenly, or it passes through an intermediate pasty stage. This fact is occasionally of great metallurgical importance. Thus, white pig iron is more suitable for dry puddling than is grey pig iron, as the former becomes very pasty, whilst the latter does not. On solidification after melting, metals usually crystallise. Crystallisation also occurs when metals are condensed from a state of vapour or are deposited by the electrolytic decomposition of metallic solutions. Metals most frequently crystallise in the cubic system. This is the case with platinum, gold, silver, copper, lead, and iron, and probably with tin and zinc. Tin also crystallises in the tetragonal system, and the iron-manganese carbide spiegeleisen crystallises in the rhombic system. Antimony, arsenic, and zinc crystallise in the hexagonal system, whilst bismuth crystallises in rhombohedra resembling cubes. Tin, zinc, and lead are thus dimorphous-that is, they may be developed according to two systems of crystallisation. The crystallisation of metals is of great importance, as the formation of crystals, due to continued vibration, intense cold, sudden alterations of temperature, or the presence of impurities may render a metal absolutely useless. Crystallisation may serve to indicate the quality of the metal, as in the case of foundry pig iron; to indicate the presence of impurities, as in the case of the film of antimony produced on lead that is refined by steam; and, lastly, to separate metals on a large scale, as is the case in Pattinson's process of desilverising lead. Welding is the property possessed by metals, which on cooling from the molten state pass through a plastic stage before becoming perfectly solid, of being joined together by the cohesion of the molecules that is induced by the application of an extraneous |