Page:EB1911 - Volume 18.djvu/222

 used to tint the constituents. Thin coloured films may often be produced by the oxidation of the specimen when heated in air. This, as a means of developing the structure, in the case of the copper alloys is specially useful. Tinted crystals may thus be distinguished from the investing layer caused by the presence of a minute quantity of another constituent. The temper colours produced by heating iron or steel in air are well known. Carbide of iron is less oxidizable than the iron with which it is intimately associated, and it assumes a brown tint, while the iron has reached the blue stage. These coloured films may be fixed by covering with thin films of gelatine.

In some cases the alloy may be attacked electrolytically by exposing it for a few minutes to a weak electric current in a bath of very dilute sulphuric acid. Certain organic bodies give very satisfactory results. The Japanese, for instance, produce most remarkable effects by simple reagents of which an infusion of certain forms of grass is a not unimportant constituent. In the case of iron and steel a freshly prepared infusion of liquorice root has been found to be most useful for colouring certain constituents of steel. Osmond, who was the first to use this reagent, insisted that it should be freshly prepared and always used under identical conditions as regards age and concentration. His method of applying it was to rub the specimen on parchment moistened with it, but he has subsequently modified this “polish attack” by substituting a 2% solution of ammonium nitrate for the liquorice infusion. In each case a small quantity of freshly precipitated calcium sulphate is used on the parchment to assist the polishing.



Appliances used in Micrography.—The method of using the microscope in connexion with a camera for photographic purposes will now be considered. Every micrographer has his own views as to the form of an installation to be adopted, and it will therefore be well to give an illustration of a definite apparatus which has been found to give satisfactory results. It consists of a microscope A with a firm base placed in a horizontal position. The microscope can be connected by a tube B with the expanded camera CC, at the end of which is the usual frame to receive the photographic plate. A practised observer can focus on a plate of clear glass by the aid of a subsidiary low-power microscope lens. If a semi-transparent plate is employed it should be as fine as possible. The surface of the table is cut in such a way near H that the observer who is seated may conveniently examine the object on the stage of the microscope, the portion B turning aside for this purpose. The subsequent focusing is effected by a rod, FFF, and gearing attached to the fine adjustment of the microscope, GA; flap J when raised forms the support of the lamp used for illumination. As an illuminant an arc light has many advantages, as the exposure of the plate used will seldom exceed 10 seconds. The filament of a Nernst lamp can be used as the source of light; though not so brilliant as the arc it possesses the great advantage of perfect immobility. For the best results, especially with high powers, the source of light must be small, so that its image can be focussed on to the surface of the object; this advantage is possessed by both of these illuminants. Next in value comes the acetylene flame, and an incandescent lamp or a gas lamp with a mantle will give good results, but with much longer exposure. Actual illumination is best effected by a Beck vertical illuminator or a Zeiss prism. It is necessary that the lens used for concentrating the light on the illuminator should be an achromatic one, as colour effects cause trouble in photographing the objects. For lower powers the Lieberkuhn parabolic illuminator is useful. Certain groups of alloys show better under oblique illumination, which may be effected by the aid of a good condensing lens, the angle of incidence being limited by the distance of the object from the objective in the case of high magnification. As regards objectives, the most useful are the Zeiss 2 mm., 4 mm. and 24 mm.; two other useful objectives for low powers being 35 mm. and 70 mm., both of which are projecting objectives. A projecting eye-piece, preferably of low power, should be employed with all but the two latter objectives. The immersion lens, the Zeiss 2 mm., is used with specially thickened cedar oil, and if the distance from the objective to the plate is 7 feet, magnifications of over 2000 diameters can easily be obtained. As regards sensitized plates, excellent results have been obtained with Lumiére plates sensitive to yellow and green. The various brands of “process” plates are very serviceable where the contrasts on the specimen are not great. Some reproductions of photo-micrographs of metals and alloys will be found in the plate accompanying the article .

METALLURGY, the art of extracting metals from their ores; the term being customarily restricted to commercial as opposed to laboratory methods. It is convenient to treat electrical processes of extraction as forming the subjects of and (qq.v.). The following table enumerates in the order of their importance the metals which our subject at present is understood to include; the second column gives the chemical characters of the ores utilized, italics indicating those of subordinate importance. The term “oxide” includes carbonate, hydrate, and, when marked with*, silicate.