392 ELEMENTARY CHEMICAL MICROSCOPY 



liquids gases are formed; the tiny gas bubbles clinging to the 

 surface, if not at once dislodged, prevent a uniform attack and 

 a specimen is obtained of no value for study. The only course 

 left open is to regrind and polish anew. 



In cases where much gas is evolved better specimens may 

 often be obtained by dipping a small wad of absorbent cotton 

 into the etching liquid and gently brushing the wet cotton upon 

 the surface, washing in running water from time to time. In 

 other cases stretching a piece of soft clean chamois leather upon 

 a board, moistening with the reagent and rubbing the specimens 

 lightly upon this surface will give good results. 



With most alloys there is obtained upon the completion of the 

 polishing a thin film of the softer components more or less com- 

 pletely covering the surface, due to surface flow during the 

 mechanical treatment. Not infrequently this surface film is 

 of such a character that after etching the appearance of the 

 etched surface is such as to entirely mislead the investigator. 

 With some alloys dipping for a few seconds in exceedingly dilute 

 acid (sulphuric is best) will remove the film, yet not appreciably 

 etch the preparation. This often essential step requires con- 

 siderable practice in order to duly appraise the time of exposure 

 to the acid to just dissolve the surface film and yet not attack 

 the polished surface. 



The following are a few of the most generally useful of etching 

 reagents. For the development of certain specific structures 

 the student must consult the literature dealing with these prob- 

 lems. 



Ammonium Hydroxide + Hydrogen Peroxide. 1 Immerse the 

 alloy in ammonium hydroxide diluted to such a strength (1:4) 

 that the alloy is not rapidly etched. Add hydrogen peroxide 

 from a pipette drop wise. This method gives better results 

 than mixing the reagents before the specimen is immersed. 

 Great care must be observed to avoid too rapid an attack and 

 too deep etching. Excellent for alloys high in copper. 



Ferric Chloride. Prepare a hot, almost saturated solution 

 of ferric chloride ; filter, and add an equal volume of concentrated 

 1 Ramsay, Chem. N., 87 (1903), 291. 



