who handles constructional forms has to deal is the maintenance
of a due relation between absolute strength and a useful degree of
elasticity. Only after many failures has the fact been grasped that
a very high degree of strength is inconsistent with a trustworthy
degree of elasticity. The reasons were not understood until the
researches of Wöhler demonstrated the difference between the effects
of merely dead loads and of live loads, and between repetitions of
stress of one kind only, and the vastly more destructive effects of
both kinds alternating.
The texture of metals and alloys is related to the character of the operations which can be done upon them. Broadly the malleable and ductile metals and alloys show a fibrous character when ruptured, the fusible ones a crystalline fracture. The difference is seen both in the workshop and in the specimens ruptured in testing-machines. A piece of wrought iron, or mild steel or copper, if torn asunder shows long lustrous fibres, resembling a bundle of threads in appearance. A piece of cast iron, or steel or bronze, shows on rupture a granular, crystalline surface destitute of any fibre. The ductile metals and alloys also extend from 10 to 30% with reduction of area before they fracture, the crystalline ones snap shortly without warning. In some instances, however, the method of application of stress exercises an influence. Wrought iron and mild steel may be made to show a short and crystalline fracture by a sudden application of stress, while if drawn asunder slowly they develop the silky, fibrous appearance. The men who design and work in metals have to take account of these vital differences and characteristics, and must be careful not to apply treatment suitable to one kind to another of a dissimilar character. Tools, appliances and methods have little in common. Between the work of the smith, the sheet-metal worker and the founder, there is a great gulf. An artistic taste will recognize the essential differences, and not endeavour, apart from questions of strength, to graft a design suitable for one on another. It is bad taste to imitate the tracery of the ductile wrought iron in cast designs, the foliations of ancient wrought-iron grilles and screens in heavy cast iron. Severe simplicity is also most in harmony with constructional designs in plated work, where stresses occur in straight lines. From this point of view' the lattice-girder bridge is an ideal design in steel.
One of the most valuable characteristics of the iron alloys is their capacity for hardening, which they owe in the main to the presence of certain small percentages of carbon relatively to minute quantities of other elements: as manganese, tungsten, 'nickel and others of less importance. The capacity for hardening is an invaluable Property not only in regard to cutting-tools, but also in prolonging the life of parts subjected to severe friction. Great advances have been made in the utilization of this property as a result of the growth of the precision grinding-machines, which are able to correct the inaccuracies of hardened work as effectually as those of soft materials. It is utilized in the spindles of machine-tools, in the balls and rollers for high-speed bearings, slides, pivots and such like;
Methods of Union.—The methods of union of works in metal are extremely varied. An advantage in casting is that the most complicated shapes are made in one piece. But all other complicated forms have to be united by other means—as welding, soldering, riveting or bolting. The two first-named are trustworthy, but are evidently unsuitable for the greater portion of engineers’ work, for which riveting and bolting are the methods adopted. Even the simple elements of rivets and bolts have produced immense developments since the days when bolts were made by hand, holes cored or hand-drilled, and rivets formed and closed by hand labour. Nut- and bolt-making machinery; both for forging and screw cutting, operates automatically, and drilling machinery is highly specialized. Hand-riveting on large contracts has been wholly displaced by power-riveting machines. The methods of union adopted are not allowed to impair the strength of structures, which is calculated on the weakest sections through the rivet or bolt holes. Hence much ingenuity is exercised in order to obtain the strongest joint which is consistent with security of union. This is the explanation of all the varied forms of riveted joints, which to casual observers often appear to be of a fanciful character.
Protection of Surfaces:—The protection and coloration of metals and alloys includes a large number of industries. The engineer uses paints for his iron and steel. A small amount of work is treated by the Bower-Barff and allied processes, by which a coating of magnetic oxide is left on the metal. Hot tar—Angus Smith’s process—is used for water-pipes. Boiled linseed-oil is employed as a non-corrosive coating preceding the application of the lead and iron oxide paints. In steam boilers artificial galvanic couples are often set up by the suspension of zinc plates in the boiler, so that the corrosion of the zinc may preserve the steel boiler plates from' waste. Various artificial protective coatings are applied to the plates of steel ships. Bright surfaces are protected with oil or with lacquer. The ornamental bronzes and brasses are generally lacquered, though in engineers’ machinery they are as a rule not protected with any coating. For ornamental work lacquering divides favour with colouring—sometimes done with coloured lacquers, but often with chemical colourings, of which the copper and iron salts are the chief basis. (J. G. H.)
Literature.—Prehistoric: Worsaae, Nordiske Oldsager i Kjobenhavn (1854); Perrin, Etude préhistorique—Age du bronze (1870). Classical: Layard, Nineveh and Babylon (1853); Pliny, Natural History, bk. xxxiv.; Bröndsted, Den Fikoroniske Cista (1847); Gerhard, various monographs (1843–1867); Müller, Etrusker, &c., and other works; Ciampi, Dell’ Antica toreutica, (1815); Von Bibra, Die Bronzen und Kupfer-Legirungen der alten und ältesten Völker (1869); C. Bischoff, Das Kupfer in der vorchristlichen Zeit (1865) Medieval, &c.: Digby Wyatt, Metal-Work of the Middle Ages (1849); Shaw, Ornamental Metal-Work (1836); Drury Fortnum, S.K.M. Handbook of Bronzes (1877); King, Orfèvrerie et ouvrages en métal du moyen âge (1852–1854); Hefner-Alteneck, Serrurerie du moyen âge (1869); Viollet-le-duc, Dict. du mobilier, “Serrurerie” and “Orfèvrerie,” (1858 &c.); Lacroix, Trésor de S. Denis, and L’Art du moyen âge (various dates); Karch, Die Rätselbilder an der Broncethüre zu Augsburg (1869); Krug, Entwürf für Gold-, Silber-, und Bronze-Arbeiter; Linas, Orfèvrerie merovingienne (1864), and Orfèvrerie du XIIIᵐᵉ siècle (1856); Bordeaux, Serrurerie du moyen âge (1858); Didron, Manuel des œuvres de bronze et d’orfèvrerie du moyen âge (1859); Du Sommerard, Arts au moyen âge (1838–1846). and Musée de Cluny (1852); Rico y Sinobas, Trabajos de metales (1871); Bock, Die Goldschmiedekunst des Mittelalters (1855), and Kleinodien des heil.-römischen Reiches; Jouy, Les gemmes et les joyaux (1865); Texier, Dictionnaire d’orfèvrerie (1857); Virgil Solis, Designs for Gold- and Silversmiths (1512), (facsimile reproduction, 1862); Molinier, Les Bronzes de la Renaissance (1886); Servant, Les bronzes d’art (1880); Wilhelm Bode, Italian Bronze Statuettes of the Renaissance (Eng. trans. by W. Grétor, first 2 vols., 1909). Practical Treatises: Theophilus, Diversarum artium schedula (12th cent.), (see Quellenschriften für Kunstgeschichte, VII., Vienna 1877); Cellini, Trattati dell’ oreficeria e della scultura (ed. Milanesi, Florence, 1856); Vasari, Tre arti del disegno, pt. ii. (Milanesi’s ed., 1882); Garnier, Manuel du ciseleur (1859); Haas, Der Metallarbeiter (1902).
METAMERISM (Gr. μετά, after, μέρος, a part), a technical term used in natural science: In chemistry it denotes the existence of different substances containing the same elements in the same proportions and having the same molecular weight; it is a form of isomerism.
In zoology, metamerism is the repetition of parts in an organized body, a phenomenon which is, as E. Haeckel, W. Bateson and others have recognized, only a special case of a tendency to repetition of structural units or parts which finds one expression in bilateral symmetry. It occurs in almost every group of the animal kingdom, but is most conspicuous in segmented worms, arthropods and vertebrates. In certain worms (the Cestoidea and some Planarians) metameric segmentation is accompanied by the separation of the completed metameres one by one from the older (anterior) extremity of the chain (strobilation), but it by no means follows that metameric segmentation has a necessary origin in such completion and separation of the “meres.” On the contrary, metamerism seems to arise from a property of organisms which is sometimes more (eumerogenesis) and sometimes less (dysmerogenesis) fully exhibited, and in some groups not exhibited at all. The most complete and, at the same time, simplest instances of metameric segmentation are to be seen in the larger Chaetopods, where some hundreds of segments succeed one another—each practically indistinguishable in structure from the segment in front or from that behind; muscles, right and left appendage or parapodium, colour-pattern of the skin, gut, blood-vessels, coelom, nephridia, nerve-ganglion and nerves are precisely alike in neighbouring segments. The segment which is least like the others is the first, for that carries the mouth and a lobe projecting beyond it—the prostomium. If (as sometimes happens) any of the hinder segments completes itself by developing a prostomium, the chain breaks at that point and the segment which has developed a prostomium becomes the first or head-bearing segment of a new individual. Compare such an instance of metameric segmentation with that presented by one of the higher Arthropods—e.g. the crayfish. Here the somites are not so clearly marked in the tegumentary structures; nevertheless, by examining the indications given by the paired parapodia, we find that there are twenty-one somites present—a limited definite number which is also the precise number found in all the higher Crustacea.
- ↑ The word “Law” is used in this summary merely as a convenient heading for the statement of a more or less general proposition.
