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 a certain range of temperatures situated close to its boiling point, combines slowly with oxygen into the red oxide, which, however, breaks up again at higher temperatures. All other metals, when heated in oxygen or air, are converted, more or less readily, into stable oxides. Potassium, for example, yields peroxide, K2O2 or K2O4; sodium gives Na2O2; the barium-group metals, as well as magnesium, cadmium, zinc, lead, copper, are converted into their monoxides MeO. Bismuth and antimony give (the latter very readily) sesquioxide (Bi2O3 and Sb2O3, the latter being capable of passing into Sb2O4). Aluminium, when pure and kept out of contact with siliceous matter, is only oxidized at a white heat, and then very slowly, into alumina, Al2O3. Tin, at high temperatures, passes slowly into oxide, SnO2.

Sulphur.—Amongst the better known metals, gold and aluminium are the only ones which, when heated with sulphur or in sulphur vapour remain unchanged. All the rest, under these circumstances, are converted into sulphides. The metals of the alkalis and alkaline earths, also magnesium, burn in sulphur vapour as they do in oxygen. Of the heavy metals, copper is the one which exhibits by far the greatest avidity for sulphur, its subsulphide Cu2S being the stablest of all heavy metallic sulphides in opposition to dry reactions.

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 FeCl2, ultimately into FeCl3, which practically means a mixture of the two chlorides, or pure FeCl3 as a final product. Of the several products, the chlorides of gold and platinum (AuCl3 and PtCl4) are the only ones which when heated beyond their temperature of formation dissociate into metal and chlorine. The ultimate chlorination product of copper, CuCl2, when heated to redness, decomposes into the lower chloride, CuCl, 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 glass vessels: AsCl3, SbCl3, SnCl4, BiCl3 HgCl2, the chlorides of arsenic, antimony, tin, bismuth, mercury respectively. The following are readily volatilized in a current of chlorine, at a red heat: AlCl3, CrCl3, FeCl3, the chlorides of aluminium, chromium, iron. The following, though volatile at higher temperatures, are not volatilized at dul redness: KCl, NaCl, LiCl, NiCl2, CoCl2, MnCl2, ZnCl2, MgCl2, PbCl2, 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 (MCl2) of barium, strontium and calcium.

Metallic chlorides, as a class, are readily soluble in water. The following, are the most important exceptions: silver chloride, AgCl, and mercurous chloride, HgCl, are absolutely insoluble; lead chloride, PbCl2, and cuprous chloride, CuCl, are very sparingly soluble in water. The chlorides AsCl3, SbCl3, BiCl3, are at once decomposed by (liquid) water, with formation of oxide (As2O3) or oxychlorides (SbOCl, BiOCl) and hydrochloric acid. The chlorides MgCl2, AlCl3, CrCl3, FeCl3, suffer a similar decomposition when evaporated with water in the heat. The same holds in a limited sense for ZnCl2, CoCl2, NiCl2, and even CaCl2. 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.

For the characters of metals as chemical elements see the special articles on the different metals.

See generally A. Rossing Geschichte der Metalle (1901); B. Neumann, Die Metalle (1904); also treatises on chemistry.

METALLOGRAPHY.—The examination of metals and alloys by the aid of the microscope has assumed much importance in comparatively recent years, and it might at first be considered to be a natural development of the use of the microscope in determining the constitution of rocks, a study to which the name petrography has been given. It would appear, however, that it is an extension of the study of the structure of meteoric irons. There can be no question that in the main it was originated by Dr H. C. Sorby, who in 1864 gave the British Association an account of his work. Following the work of Sorby came that of Professor A. Martens of Charlottenburg, presenting many features of originality. F. Osmond has obtained results in connexion with iron and steel which are of the highest interest. A list of the more important papers by these and other workers will be found in the appended bibliography.

Preparation of the Specimen.—Experience alone can enable the operator to determine what portion of a mass of metal or alloy will afford a trustworthy sample of the whole. In studying a series of binary alloys it has been found advantageous in certain cases to obtain one section which will show in a general way the variation in structure from one end of the series to the other. This has been effected by pouring the lighter constituent carefully on the surface of the heavier constituent, and allowing solidification to take place. A section through the culot so obtained will show a gradation in structure from pure metal on one side to pure metal on the other. A thin slice of metal is usually cut by means of a hack-saw driven by mechanism. The thickness of the piece should not be less than in. and in order that it may be firmly held between the fingers it should not be less than 1 in. square. The preliminary stages of polishing are effected by emery paper placed preferably on wooden disks capable of being revolved at a high rate of speed. The finest grade of emery paper that can be obtained is used towards the end of the operation. Before use the finer papers should be rubbed with a hard steel surface to remove any coarse particles. The completion of the operation of polishing is generally effected on wet cloth or parchment covered with a small amount of carefully washed jeweller’s rouge. Various mechanical appliances are employed to minimize the labour and time required for the polishing. These usually consist of a series of interchangeable revolving disks, each of which is covered with emery paper, cloth or parchment, according to the particular stage of polishing for which it is required. In the case of brittle alloys and of alloys having a very soft constituent, which during polishing tends to spread over and obliterate the harder constituents, polishing is in many cases altogether avoided by casting the alloy on the surface of glass or mica. In this way, with a little care, a perfect surface is obtained, and it is only necessary to develop the structure by suitable etching. In adopting this method, however, instances have occurred in which the removal of the cast surface has shown a structure differing considerably from the original.

Polishing in Bas-Relief.—If the polishing be completed with fine rouge on a sheet of wet parchment, placed upon a comparatively soft base such as a piece of deal, certain soft constituents of an alloy may often be eroded in such a manner as to leave the hardest portions in relief. For the later stages of polishing H. L. Le Chatelier recommends the use of alumina obtained by the calcination of ammonium alum; and for the final polish of soft metals, chromium oxide.

Although in some cases a pattern becomes visible after polishing, yet more frequently a mirror-like surface is produced in which no pattern can be detected, or if there is a pattern it is blurred, as if seen through a veil or mist. This is due to a thin layer of metal which has been dragged, or smeared, uniformly over the whole surface by the friction of the polishing process. Such a surface layer is formed in all cases of polishing, and the peculiar lustre of burnished silver or steel is probably due to this layer. But to the metallographist it is an inconvenience, as it conceals scratches left by imperfect polishing, and also hides the pattern. It is therefore desirable to conduct the polishing so as to make this layer as thin as possible: it is claimed for alumina that it can be so used as to produce a much thinner surface layer than that due to the employment of rouge. The surface layer is very readily removed by appropriate liquid reagents, and, the true surface of the metal having been laid bare, the etching reagent acts differently on the individual substances in the alloy and the pattern can thus be emphasized to any required extent. Osmond divides etching reagents into three classes—acids, halogens and salts. As regards acids, water containing from 2 to 10% of hydrochromic acid is useful. It is made by mixing 10 grams of potassium bichromate with 10 grams of sulphuric acid in 100 grams of water. The use of nitric acid requires much experience. It is frequently employed in the examination of steels, Sir W. C. Roberts-Austen preferred a 1% solution in alcohol, but many workers use concentrated acid, and effect the etching by allowing a stream of water to dilute the film of acid left on the surface of the specimen after dipping it. Of the halogens, iodine is the most useful. A solution in alcohol is applied, so that a single drop covers half a square inch of surface. The specimen is then washed with alcohol, and dried with a piece of fine linen or chamois leather. Tincture of iodine also affords a means of identifying lead in certain alloys by the formation of a yellow iodide of lead, while the vapour of iodine has in certain cases been