440 ELEMENTARY CHEMICAL MICROSCOPY 



which would first dissolve the crystals leaving the eutectic. 

 Whenever it is possible, specimens should be etched by both 

 systems, for then the probability of misinterpretation of appear- 

 ances is much reduced. The development of the structure of a 

 specimen so as to render its microscopic study successful requires 

 considerable practice. 



Small specimens are grasped in rubber-tipped or cloth-covered 

 (binding tape) forceps and dipped, polished surface down, or 

 polished surface sidewise, into the etching liquid; immediately 

 removed, washed in running water, dried with lens paper and 

 examined. If the structure has not been sufficiently developed, 

 it is again dipped and again washed and examined. This process 

 is repeated until the etching is sufficiently deep to make the 

 crystal phase or phases interpretable. Too long immersion 

 leads to uneven etching, to crystal sections with badly eroded 

 edges and often to serious pitting. With many of our etching 

 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 often obtained upon the completion 

 of the polishing a thin film of the softer components more or 

 less completely 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 



