436 ELEMENTARY CHEMICAL MICROSCOPY 



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. 



A frequently employed method consists in stretching coarse 

 canvas tightly upon the lap and charging it with the abrasive 

 powder mixed with water to a thin paste. This paste is spread 

 upon the canvas by means of a flat brush as often as required. 

 The lap should revolve at not much less than 1000 R.P.M. 

 Whenever papers are emp]oyed it is best to go over their sur- 

 faces 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 

 " Hubert " l papers are the best and most uniform of grain. The 

 most useful are numbers, ooo, oo, o and i, the last named being 

 the coarsest. 



For the final polishing rouge, alumina, alundum or emery are 

 usually employed. When suspended in a large volume of water 

 the polishing powders must be of sufficient fineness to remain in 

 suspension for fully fifty minutes. In work of the highest class 

 fifty minutes is too short a time. In the Cornell University 

 laboratories emery has given excellent results especially with 

 soft alloys and is preferred to rouge or alumina. 



The finest obtainable commercial " emery flour " is placed 

 in a ball-mill for forty eight-hours or more, and is then levigated 

 in a LeChatelier apparatus. The water carrying over the finest 

 particles is received in tall cylinders, set aside for fifteen to thirty 

 minutes and if any deposition has taken place the supernatant 

 liquid with particles in suspension is set aside for one or more 



1 These imported papers can be obtained from Montgomery & Co., 105 Fulton 

 Street, New York City. 



