388 ELEMENTARY CHEMICAL MICROSCOPY 



Laps may be either horizontally or vertically driven. The 

 beginner will find that satisfactory surfaces are obtained easier 

 upon the horizontal lap but it is open to the objection that it 

 does not readily clear itself and any dust or dirt falling upon 

 it or any large particle of abrasive will be apt to deeply groove 

 the specimen. The vertical lap on the other hand is difficult to 

 keep charged with pasty abrasive or thin suspensions of abrasive 

 and polishing powders. 



In the case of soft alloys, facing to a smooth surface is most 

 easily accomplished by means of files, rough dressing with a 

 10 or i2-inch bastard cut file and passing to an 8 or lo-inch 

 single cut. With moderately soft materials such as brass, 

 laying a single cut mill file flat upon the work bench and pushing 

 the specimen down the file against the cutting edges will be 

 found to yield good smooth surfaces with less practice and skill 

 than by holding the specimen in- a vise and pushing the file. 

 The specimen should be pushed lengthwise of the file with gentle 

 pressure until it reaches the tang end, then lifted off; the file 

 turned edgewise and struck a sharp blow upon the bench to 

 remove filings, again laid flat and the specimen again laid upon 

 the file and gently pushed toward the tang end, and the process 

 repeated until a small plane surface is obtained. Specimens 

 should never be rubbed back and forth upon an abrasive surface, 

 for it is then almost impossible to keep the striations parallel, 

 a matter of not a little importance. 



In order to facilitate smoothing and polishing, the edges of a 

 specimen should always be slightly beveled or rounded during 

 the roughing. Unless this precaution is taken the beginner will 

 find it difficult to avoid cutting, tearing or destroying the fabric 

 carrying the polishing powder. 



After surfacing with wheel or file the specimens are smoothed 

 upon laps fed with very fine abrasive powder or upon laps or 

 blocks upon which abrasive paper has been smoothly glued. 

 Whenever such papers are employed it is best to go over their 

 surfaces with a low-power magnifier and reject any sheets which 

 show isolated large particles of the abrasive covering. Of the 

 fine-grained abrasive papers tried by the author, the French 



