3QO ELEMENTARY CHEMICAL MICROSCOPY 



lap. A beautiful mirror surface should have been obtained with 

 no signs of striations when examined with a microscope of the 

 same power as will be employed after etching. Wash the speci- 

 men carefully, and dry by gently pressing with lens paper. 

 Never rub when drying and always avoid touching the polished 

 surface with the unprotected fingers. 



If oil has been used as the vehicle, wash first with gasoline or 

 benzene, and follow with alcohol and ether. 



General Methods for Preparing Soft Specimens. The beginner 

 should never attempt to grind and polish soft specimens upon a 

 rotating wheel or lap. Even the roughing is best done with a 

 file or by rubbing upon abrasive paper or cloth glued upon 

 blocks of wood. Great care must be observed in rubbing the 

 specimen so that it shall never turn. The lines of abrasion must 

 be kept parallel. Every few minutes the block should be turned 

 on edge and struck upon the bench with a sharp blow in order to 

 clear it from loose particles and dust; if this is not done deep 

 scoring of the surface is sure to follow. When passing from one 

 abrasive to a finer one, turn the specimen to a position at right 

 angles to the other 'and rub very gently until every trace of the 

 former scratches has disappeared. The polishing is carried out 

 in the same manner upon close-textured soft cloth stretched 

 upon blocks and covered with a thin paste of rouge or alumina, 

 ending up with the finest possible floated rouge. It will be found 

 convenient to pass from a grain of 220 to F, to FF, to FFF, then 

 to ordinary rouge or "rotten stone" and finally end up with the 

 finest particles obtained by washing and decanting the finest 

 commercial rouge. Wash and dry the specimen with lens paper. 

 But even lens paper will scratch the surface of soft alloys or 

 other soft material. 



When dealing with very soft materials, after washing with 

 water, shake off the last drops and pour absolute alcohol over the 

 polished surface, shake, repeat the operation and then remove 

 the last traces of alcohol with a few drops of ether. 



Grinding Hard Friable Material like Glass or Porcelain. - 

 Employ lap heads of block tin fed with emery powder and water 

 or turpentine. Emery does not cut as fast as carborundum, 



