and Laboratory Methods. 



19-23 



Fig. 5. — Same Steel as Fig. 4 heated to 6o0° C. 



Diluted nitric acid is used as an etching medium in case of hardened steel. 

 With increase in hardness and working there is an increase in the fineness of the 

 micro-structure of steel, to de- 

 velop which it is best to use 

 the more dilute solution of 

 nitric acid. At the same time, 

 the finer structures are the 

 more difficult to develop and 

 take a much shorter time to 

 etch ; hence it is particularly 

 true that several short appli- 

 cations of the etch are more 

 likely to produce the result 

 sought for than one prolonged 

 etching would. Examination 

 between successive applica- 

 tions shows how the etching 

 is progressing. 



To etch with nitric acid 

 pour a small quantity of the 

 proper dilution into a suitable 

 dish and, holding the cleaned 



micro-section, face down, with pincers or a straight-nosed crucible tongs, dip 

 the face to be etched at least twice into the etching fluid, immediately remove 

 and let the action continue with face of section turned up, so that the action 

 may be watched. When it is complete (two seconds is often enough) hold in 

 stream of water and dry as described before. If examination shows etching to 



be incomplete repeat the pro- 

 cess ; if too deeply etched, re- 

 peat the polishing. 



Various devices for temp- 

 orarily mounting specimens, 

 especially for those of irregular 

 shape, are supplied by dealers, 

 but none has been found more 

 convenient than a small one 

 and one-half inch square of 

 ground glass upon which is a 

 lump of beeswax the size of a 

 hazel nut. It need not neces- 

 sarily be ground glass, but 

 generally a n identification- 

 mark is needed and then the 

 ground surface is very con- 

 venient to write on. 

 Pjq g To mount an irregular 



