128 S. W. SMITH ON THE MICROSTRUCTUEE OF METALS AND ALLOYS. 



effected. The specimen is washed and examined as soon as the 

 brightness of the surface is dulled. It may then be further 

 etched, if, on examination, the structure is found to be imperfectly 

 developed. Over-etching is to be avoided. 



Other methods of treating the polished surface, in order to 

 develop the structure, have been suggested from time to time. 

 The " polish attack " of M. Osmond yields excellent results in 

 examining sections of steel. The section is gently rubbed on 

 damp parchment, fixed to a piece of planed wood and moistened 

 with an infusion of liquorice root. A small amount of precipitated 

 calcium sulphate is also placed on the parchment, and latterly 

 M. Osmond advises the substitution of a 2-per-cent. solution of 

 ammonium nitrate for the liquorice infusion. Another method 

 by which the structure is developed, is by heating the specimen 

 in air to a temperature below redness, by which means the 

 different degree of oxidation of the constituents renders the 

 structure apparent. 



It should be mentioned that soft alloys, such as those of lead, 

 which, on account of their low melting point, are easily prepared, 

 are difficult to polish, since the softer constituents in polishing 

 spread over and obliterate the harder portions. To avoid this, it 

 has recently been suggested that the metal or alloy be poured or 

 cast on to a sheet of mica which forms the bottom of a small 

 mould. In this way a brilliant surface is obtained without 

 polishing, and it is merely necessary to etch the surface and 

 examine it. 



Examination of the Sj^ecimen. — Owing, of course, to the opacity 

 of the section, the metallographist is limited to oblique or vertical 

 illumination in examining it. Otherwise, for ordinary purposes 

 no special accessories are necessary for work on metals and alloys. 

 The specimen, which may often be of irregular shape, is mounted 

 on a glass slide with wax or some suitable material, care being 

 taken that the polished surface is parallel to the slide. In 

 examining specimens under low-power objectives, the Sorby-Beck 

 reflector is largely used, although a cover-glass, supported at an 

 angle of forty-five degrees between the objective and the surface, 

 with a small black screen behind it, works well. 



For high powers with which vertical illumination only is 

 possible, the Beck illuminator, which is screwed between the 

 objective and the microscope tube, works admirably, although it 



