354 



PROFESSOR J. A. EWING AND MR. W. ROSENHAIN 



the finest jeweller's rouge is suitable, but in special cases we resorted to the use <>f 

 peroxide of iron obtained by precipitation from a solution of pure ferric chloride. 



The polishing machine is shown in diagram-section in fig. 1. A is the spindle ot 

 an electro-motor carrying a small driving disc B, fitted with a rubber ring to increase 

 the driving friction. The polishing disc C is horizontal and has a vertical axis 

 running in a bearing in the casting D. The under side of the polishing disc bears 

 ujxm the driving wheel B and takes motion from it. When it is desired to st<>|> 

 temporarily the polishing disc is raised out of contact with the driving wheel. The 



Fig. 1. 



casting D is held to the base-board by a bolt passing through a slot which allows the 

 disc to l>e brought nearer to the motor, so that it may bear upon the driving wheel at 

 any desired distance from its axis, thus giving a means of varying the speed. More 

 usually, however, the speed was varied by regulating the current in the motor. 



With metals which are very liable to tarnish, wet polishing has to be avoided ; in 

 some cases dry rouge, and in others rouge moistened with a little paraffin, may be 

 used the latter we found particularly useful in polishing copper. But in some metals, 

 especially the more fusible ones, such as lead, zinc, and tin, it is difficult to obtain a 

 satisfactory surface by any method of polishing. In most of these cases, however, a 

 method of obtaining a good surface entirely without polishing becomes available. 

 This consists in pouring the molten metal on a smooth body, such as glass or polished 

 steel, in contact with which it is allowed to solidify. We have, as a rule, used glass 

 for this purpose, and in spite of frequent failures, due to fracture of the glass or to 

 less obvious causes, we find that it is generally practicable to get a good specimen in 

 this way with much less trouble than is required to produce a specimen of iron or 

 steel by the ordinary process of polishing. This method of " glass casting " cannot well 

 be applied to metals which oxidize at temperatures below their melting points, and for 

 metals which have a very high melting point a smooth body other than glass must be 

 used. The metals most readily treated by casting against a smooth surface arc 

 gold, bismuth, cadmium, lead, tin, zinc, and fusible alloys. 



In the examination of lead another method of obtaining a good surface, without 

 either melting or polishing, was also used. A face of the specimen was freshly cut to 

 remove the tarnish, and was then pressed against a smooth surface of plate-glass. 

 Whenever a sufficient pressure could be reached without breaking the glass a very 



