THE METALLOGRAPHY OF METEOKIC IR.01N" 95 



Under no circumstances should buffing be employed, as it produces 

 a superficial dragging of the metal. 



Macroetching. — Macroetching, which is much deeper than that 

 used for microscopic work, may be done with a variety of etchants. 

 Nital (5 percent nitric acid in alcohol) is most satisfactory, as it does 

 not cause the discoloration around troilite inclusions which takes 

 place if an aqueous solution of the acid is used. When the iron is 

 free from troilite 5 to 8 percent of nitric acid in water is satisfactory. 

 The author has used with good results ammonium persulphate (2 

 grams to 10 cc. of water, freshly prepared). 



The etchant is best applied with a soft brush (an old shaving brush 

 serves well) ; it should be spread plentifully and evenly over the sur- 

 face and kept constantly in motion, until the maximum distinctness 

 of structure is developed. Very thorough washing under a stream 

 of water, preferably warm, is necessary to avert rust which often 

 forms in a short time — particularly if the iron contains cracks or 

 interstices. The specimen should then be dried and preferably kept 

 warm for some time. 



If a specimen is to be kept for any considerable time after macro- 

 etching, the surface should be covered with a thin coat of lacquer 

 or spar varnish. Until thus protected, it should not be touched by 

 the fingers, else stains are likely to be produced. 



When a specimen is to be macroetched for permanent display, 

 using nitric acid and water, it is advisable to fill any cracks or inter- 

 stices with shellac before etching, to prevent the penetration of 

 acid into the mass. This, however, cannot be done if microetching 

 is to be done later, because the alcoholic etching solutions would 

 dissolve the shellac. 



Macro-observation. — The macroetched surface should be examined 

 with a hand glass or under a microscope at low magnification (10 

 to 25 diameters). If it is an octahedrite, the width of the bands 

 may be measured with an eye-piece micrometer. If a hexahedrite, 

 the character of Neumann lines, grain boundaries, etc., may be noted. 

 Inclusions of schreibersite or of cohenite are easily identified by 

 their resistance to a needle scratch, although the one is not easily 

 distinguished from the other. 



Troilite inclusions can be located and identified by a sulphur 

 print, which is best taken after polishing and before etching. Troilite 

 may thus be distinguished from graphite, which does not show on a 

 sulphur print; it is often associated with troilite and might escape 

 observation. If the sample includes part of the natural surface of 

 the mass, the surface should be examined at its edges for possible 

 intrusions of magnetic oxide or invasions of hydroxide. 



Finally, the specimen should be photographed with ordinary day- 



