PREPARING OPAQUE OBJECTS 391 



crystalon or similar abrasives, but also does not so deeply score 

 the specimen and therefore the time lost in grinding is usually 

 gained in polishing. 



For grinding, the lap head should rotate quite slowly, one or 

 two hundred revolutions per minute being the maximum for 

 ordinary work. In polishing a somewhat higher speed may be 

 employed with advantage. Polish with fine rouge and complete 

 the finish with " putty powder." 



Etching. This step has for its object the development of 

 the crystalline structure of the specimen. It is based upon the 

 principle of submitting the polished specimen to the action of a 

 corrosive liquid of such a nature as to dissolve some components 

 more rapidly than others. 



The surface to be treated being a mirror surface, free from all 

 striations, it follows that the slightest attack by an etching 

 liquid will be easily seen by means of the microscope. 



Suppose, for example, we have an alloy consisting of a single 

 crystalline phase and an eutectic. Two systems of attack 

 would reveal the nature of its structure; a reagent could be 

 employed which would dissolve the eutectic leaving the crys- 

 talline phase unattacked, or another reagent could be selected 

 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 in terpre table. Too long immersion 

 leads to uneven etching, to crystal sections with badly eroded 

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



