THE METALLOGRlAiPHY OF METEORIC IRON 101 



more satisfactory for distinguishing cohenite. In a few cases chromic 

 acid was used, with negative results, to prove the absence of carbide. 



Effects on fine structures. — Reference has been made elsewhere 

 to the fact that the attack of any solvent etching reagent is stronger 

 along the interfaces of any two components. As a result, fine struc- 

 tures are quickly affected by any ordinary acid etchant. Thus 

 dense plessite and ataxites darken with even very light etching, and 

 the same is true of kamacite when it possesses an acicular gamma- 

 alpha transformation structure. (See Chapter XI.) For the same 

 reason an imperfectly transformed gamma-alpha aggregate, when 

 very fine, may appear black and unresolved even at high magnification. 



It is to be noted that the action of neutral sodium picrate on 

 phosphide, and of alkaline sodium picrate on carbide, is not a true 

 etch but only the deposition of a colored film. Therefore there is 

 no attack by these reagents along interfaces, and a surface even when 

 filled with fine structures of kamacite and taenite shows no effect 

 unless it contains phosphide or carbide. 



Thus when phosphide is distributed in minute particles, their 

 nature is not conclusively determined by etching with picral or nital, 

 because they would appear black with any solvent etchant regard- 

 less of their nature. When etched with neutral sodium picrate, 

 however, they appear black if phosphide; otherwise they would be 

 invisible. 



Preserving specimens. — Specimens polished and etched for metal- 

 lographic study often tarnish or rust very quickly, and the only sure 

 way to preserve them is to keep them in a desiccator as long as they 

 are being worked upon. After they are no longer needed for imme- 

 diate use, they should be preserved by lacquering. 



With a light coat of lacquer specimens can still be studied quite 

 satisfactorily under the microscope, although it should be removed 

 for photographic work. That is quickly done with a bit of cotton 

 wet in amyl acetate. 



Microexamination. — For the study of microetched specimens a 

 microscope with vertical illumination is used. The most satisfactory 

 instruments are of the types made especially for metallographic work, 

 but a vertical illuminator can readUy be applied to an ordinary 

 microscope. A yellow-green filter should be used. 



The most useful amplification is 80 to 100 diameters (16 mm. 

 objective), but 400 to 500 diameters should be available. For the 

 latter magnification a 5.5 mm. objective is preferable to 4 mm. 

 because of the more convenient working distance between the front 

 lens and the object. 



Higher powers are used at times in the study of artificial irons, 

 requiring the use of immersion objectives; but the technique of their 



