Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/81

Rh M E T - Chlorine. All metals, when treated with chlorine gas at the proper temperatures, pass into chlorides. In some cases the chlorine is taken up in two instalments, a lower chloride being produced first, to pass ultimately into a higher chloride. Iron, for instance, is converted first into Fed.,, ultimately into Fe 2 Cl c, which practically means a mixture of the two chlorides, or pure Fe.,Cl 6 as a final product. Of the several products, the chlorides of gold and platinum (AuCl 3 and PtCl 4 ) are the only ones which when heated beyond their temperature of formation dissociate into metal and chlorine. The ultimate chlorina- tion product of copper, CuCl 2, when heated to redness, decomposes into the lower chloride, Cu 2 Cl 2, and chlorine. All the rest, when heated by themselves, volatilize, some at lower, others at higher temperatures. Of the several individual chlorides, the following are liquids or solids, volatile enough to be distilled from out of glass vessels : AsCl 3, SbCl 3 , SnCl 4 , BiCl 3 , HgCL, the chlorides of arsenic, antimony, tin, bismuth, mercury re spectively. The following are readily volatilized in a current of chlorine, at a red heat : A1 2 C1, Cr 2 Cl G , Fe 2 Cl G , the chlorides of aluminium, chromium, iron. The follow ing, though volatile at higher temperatures, are not vola tilized at dull redness : KC1, NaCl, LiCl, NiCl,, CoCl , MnCl 2, ZnCl 2 , MgCl. 2, PbCl 2 , AgCl, the chlorides of potassium, sodium, lithium, nickel, cobalt, manganese, zinc, magnesium, lead, silver. Somewhat less volatile than the last-named group are the chlorides (MC1 2 ) of barium, strontium, and calcium. Metallic chlorides, as a class, are readily soluble in water. The following are the most important exceptions: chloride of silver, AgCl, and subchloride of mercury, Hg.,Cl.&amp;gt;, are absolutely insoluble ; chloride of lead, PbCl 2 , and&quot; subchloride of copper, Cu 2 Cl 2, are very sparingly soluble in water. The chlorides AsCl 3, SbCl 3 , BiCl 3 , are at once decomposed by (liquid) water, with formation of oxide (As. 2 O 3 ) or oxychlorides (SbCIO, BiCIO) and hydro chloric acid. The chlorides MgCL,, A1 2 C1, Cr 2 Cl 6 , Fe 2 Cl suffer a similar decomposition when evaporated with water in the heat. The same holds in a limited sense for ZnCl 2 , CoCl 2, NiCl 2 , and even CaCl 2. All chlorides, except those of silver and mercury (and, of course, those of gold and platinum), are oxidized by steam at high temperatures, with elimination of hydrochloric acid. The above statements concerning the volatilities and solubilities of metallic chlorides form the basis of a number of important analytical methods for the separation of the respective metals. For the characters of metals as chemical elements the reader is referred to the article CHEMISTRY and to the special articles on the different metals. (w. D.) METAL-WORK. Among the many stages in the de velopment of primeval man, none can have been of greater moment in his struggle for existence than the discovery of the metals, and the means of working them. The names generally given to the three prehistoric periods of man s life on the earth the Stone, the Bronze, and the Iron age imply the vast importance of the progressive steps from the flint knife to the bronze celt, and lastly to the keen-edged elastic iron weapon or tool. The length of time during which each of these ages lasted must of course have been different in every country and race in the world. The Digger Indians of South California have even now not progressed beyond the Stone Age ; while some of the tribes of Central Africa are acquainted with the use of copper and bronze, though they are unable to smelt or work iron. The metals chiefly used have been gold, silver, copper and tin (the last two generally mixed, forming an alloy called bronze), iron, and lead. The peculiarities of these E T 71 various metals have naturally marked out each of them for special uses and methods of treatment. The durability and the extraordinary ductility and pliancy of gold, its power of being subdivided, drawn out, or flattened into wire or leaf of almost infinite fineness, have led to its being used for works where great minuteness and delicacy of execution were required ; while its beauty and rarity have, for the most part, limited its use to objects of adornment and luxury, as distinct from those of utility. In a lesser degree most of the qualities of gold are shared by silver, and consequently the treatment of these two metals has always been very similar, though the greater abundance of the latter metal has allowed it to be used on a larger scale and for a greater variety of purposes. Bronze is an alloy of copper and tin in varying propor tions, the proportion of tin being from 8 to 20 per cent. The great fluidity of bronze when melted, the slightness of its contraction on solidifying, together with its density and hardness, make it especially suitable for casting, and allow of its taking the impress of the mould with extreme sharp ness and delicacy. In the form of plate it can be tempered and annealed till its elasticity and toughness are much increased, and it can then be formed into almost any shape under the hammer and punch. By other methods of treatment, known to the ancient Egyptians, Greeks, and others, but now forgotten, it could be hardened and formed into knife and razor edges of the utmost keenness. In many specimens of ancient bronze small quantities of silver, lead, and zinc have been found, but their presence is probably accidental. In modern times, after the discovery of zinc, an alloy of copper and zinc called brass has been much used, chiefly for the sake of its cheapness as compared with bronze. In beauty, durability, and delicacy of surface it is very inferior to bronze, and, though of some commercial importance, has been of but little use in the production of works of art. To some extent copper was used in an almost pure state during mediaeval times, especially from the 12th to the 15th century, mainly for objects of ecclesiastical use, such as pyxes, monstrances, reliquaries, and croziers, partly on account of its softness under the tool, and also because it was slightly easier to apply enamel and gilding to pure copper than to bronze (see fig. 1). In the mediaeval period it was used to some extent in the shape of thin sheeting for roofs, as at St Mark s, Venice ; while during the 16th and 17th centuries it was largely employed for ornamental domestic vessels of various sorts. Iron i The abundance in which iron is found in so many places, its great strength, its remarkable ductility and malleability in a red-hot state, and the ease with which two heated surfaces of iron can be welded together under the hammer combine to make it specially suitable for works on a large scale where strength with lightness are required things such as screens, window-grills, orna mental hinges, and the like. In its hot plastic state iron can be formed and modelled under the hammer to almost any degree of refinement, while its great strength allows it to be beaten out into leaves and ornaments of almost paper-like thinness and delicacy. With repeated hammering, drawing out, and annealing, it gains much in strength and toughness, and the addition of a very minute quantity ofjjarbon converts i Some recent analyses of the iron of prehistoric weapons have brought to light the interesting fact that many of these earliest specimens of iron manufacture contain a considerable percentage nickel This special alloy does not occur in any known iron ores, but is invariably found in meteoric iron. It thus appears that iron was manufactured from meteorolites which hail fallen to the earth ir an almost pure metallic state, possibly long before prehistoric man had learnt how to dig for and smelt iron in any of the forms oJ which are found on this planet.