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 
