1905.] 



Slip- Bands in Metallic, 



559 



obtain a satisfactory polish, however, the ordinary method of polishing 

 with emery papers, followed by rouge, had to be slightly modified ; 

 not only is it difficult to polish electro-deposited copper by means of 

 rouge, but it was found that this polishing medium rapidly eroded 

 a deep groove between the copper and iron, thus defeating the object 

 of the method. After some trials, a satisfactory polishing medium 

 for this and other purposes where surface erosion is undesirable was 

 found in calcined oxide of magnesium. The powder, as purchased 

 under the description of " chemically pure light oxide of magnesia," 

 was used by the author without further treatment, but still better 

 results could no doubt be obtained by using an elutriated product. 

 The magnesia powder is used in much the same way as rouge; a 

 revolving disc of metal is covered with several layers of fine well- 

 washed calico, and fed with a thin paste of magnesia and water ; it is, 

 however, essential that only distilled water should be used, as the 

 magnesia becomes gritty in the presence of dissolved salts. In any 

 case, its use requires some care, as such a magnesia disc, if allowed to 

 dry, becomes hard and useless. 



The section, when satisfactorily polished by means of magnesia, is 

 not yet ready for detailed examination ; even when the grinding on 

 emery has been carried so far that very little polishing is required, 

 it is found that a considerable amount of metal has been smeared or 

 dragged over the surface, more or less obliterating the true boundary 

 line which it is desired to examine. In order to overcome this 

 obstacle, it is advisable so to arrange matters that the last rubbing on 

 emery paper shall be done in a direction approximately parallel to 

 the boundary of the two metals ; the direction of rubbing during the 

 final polishing should then be at right-angles to the boundary, and 

 the specimen should be so held on the revolving disc that the un- 

 avoidable tendency to drag or smear will be such as to draw the iron 

 over the copper on the side where the boundary is to be examined. 



Fig. 1 illustrates this point on an exaggerated scale ; the diagram 



FIG. 1. 



Copper 



Iron 



Copper 



represents the face of the section, AB representing the boundary between 

 iron and copper which gives the sectional view of the previously 



