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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