CHAPTER XV. 
PREPARING OPAQUE OBJECTS FOR THE MICROSCOPIC 
STUDY OF INTERNAL STRUCTURE. 
In order that alloys and many other similarly constituted 
materials may be properly studied and their internal struc¬ 
tures ascertained it is usually essential that large or small pieces 
be ground down to a plane surface which may be so placed 
under the microscope as to lie at right angles to the optic axis 
of the instrument. It is further necessary that this plane sur¬ 
face shall be so smooth as to show no striations due to grinding, 
otherwise these parallel or irregular streaks will confuse the 
observer. Removal of the streaks is accomplished by polishing 
or, in other words, grinding with an abrasive so fine that the 
scratches made are so close together and so shallow that they 
will not be resolved by the objectives used in the microscopic 
examination. If these polished specimens are subjected to the 
action of various solvents, it will be found that in non-homo- 
geneous materials, certain components are easily dissolved and 
certain others are resistant. The specimen thus treated, is said 
to have been etched, and when the etched surface is examined a 
more or less marked crystalline structure is visible. Through 
the judicious selection of the proper etching liquids we are able 
to bring into view different components or phases and thus trace 
the changes in structure through changes in percentage compo¬ 
sition, or through changes in the temperatures to which the 
specimens have been submitted. 
Or instead of submitting the polished surface to the action of 
a corrosive liquid, we can rub it upon a thick, soft cloth charged 
with a fine abrasive powder. The softer components will thus 
be more rapidly worn away than the harder; again we obtain 
evidence of a more or less marked crystalline structure. The 
specimen is no longer spoken of as having been etched, but 
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