S. W. SMITH ON THE MICROSTRUCTURE OF METALS AND ALLOYS. 127 



whom microscopy owes so much) was led from the study of the 

 microstructures of crystals, minerals, and rocks to the structures 

 of meteorites and then to those of iron and steel. It is to the 

 latter that, up to the present, the greater portion of metallo- 

 graphic work has been devoted with the object of solving industrial 

 problems connected with the use of steel for various purposes. 

 The work is, however, rapidly spreading to the study of alloys of 

 metals of every conceivable combination. It will be instructive 

 now, to consider briefly the technology of the subject. 



Pre]Xt')'atio7i of the Specime7i for Examination. — It is usual, in 

 commencing a systematic study of the alloys of two metals, to 

 examine first the metal of the highest melting or '' freezing" 

 poir-i, and then pass up the series by examining alloys each con- 

 taining, say, 5 per cent, more of the second metal than the last, 

 until finally pure metal is reached at the other end of the series. 

 The preparation of these alloys is effected by merely melting the 

 mixture together in a clay pot under a laj^er of powdered carbon 

 to prevent oxidation. By means of a hack-saw a portion is then 

 cut off, of convenient size for polishing and subsequent mountino- 

 on the stage of the microscope. One face of the specimen is then 

 filed up to give an even surface. This surface is further polished 

 by rubbing on emery papers of gradually increasing fineness, the 

 scratches produced by rubbing in one direction on one paper, 

 being effaced by rubbing on the next in a direction at right 

 angles to the last. After this treatment on four or five emery 

 papers, a final brilliant polish is attained by rubbing with 

 jewellers' rouge on cloth moistened with water stretched over a 

 piece of board. The operation of polishing is in actual practice 

 rendered less tedious by the use of mechanical arrangements for 

 rotating wheels, on which the various emery papers are fastened. 

 Having obtained a well polished surface, free from scratches, it 

 is necessary to reveal the structure of the metal by etching the 

 surface, except in certain cases in which polishing alone reveals 

 the structure. This surface etching is effected in various ways, 

 according to the nature of the specimen dealt with. As a rule it 

 is sufiicient to dip the surface for a few seconds in dilute nitric 

 acid, and then wash in water or methylated spirit, finally drpng 

 the surface by gently wiping with chamois leather moistened with 

 benzole. In many cases strong acid is necessary, and it may even 

 be necessary to heat the liquid before satisfactory etching is 



