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 pohshing, 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 looo R.P.M. 
Whenever papers are employed 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 ” ^ 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. 
