PREPARING HARD SPECIMENS 
437 
days to sediment. The water is then drawn off from the deposit. 
This final deposit is mixed with a little distilled water and trans¬ 
ferred to a stock bottle. For use a little of the stock suspension 
is added to distilled water, introduced into an atomizer and 
sprayed upon the cloth-covered lap. , 
It is best to polish the specimen in two directions. The cloth 
of the revolving lap must never be allowed to become dry during 
polishing, nor on the other hand should it be too wet. 
General Methods for Preparing Hard Specimens. — Grind to a 
plane surface upon the proper wheel, using a high speed and 
holding the specimen so that it just barely touches the rotating 
surface. If pressed too hard against the wheel there will be 
deep scoring and too much heating. Observe great care to 
prevent the specimen from turning in the fingers. A properly 
rough-ground specimen should show alt the striations parallel and 
of approximately the same depth. Next bevel or round the 
edges of the specimen around the ground surface, then apply 
the specimen to a finer-grained wheel or to a lap fed with finer- 
grained powder, grinding so that the striations are at right angles 
to the first. Continue grinding until when examined with a 
low magnification no vestiges of the first striations remain. If 
now the striations are very shallow, polishing may be begun; 
if not shallow, grind with a third finer abrasive; again grinding 
at right angles to the direction last taken and continuing until 
all trace of the preceding grinding has disappeared. Polishing 
is carried out in like manner, using finer and finer powders 
moistened to a pasty consistency with water or oil or other 
suitable vehicle. When oil, vaseline or a similar Vehicle has been 
employed in the grinding, especially when dealing with materials 
which have a tendency to adsorb the grease, as for example 
certain rocks, earthenwares, terra-cottas, porcelains, cements 
and concretes, etc., it will be found that polishing proceeds with 
far greater speeds and with much better surfaces when the pol¬ 
ishing powders are suspended in a solvent for greases and oils, 
than when water is employed. The best of these are alcohols, 
ethers and light petroleum products or mixtures of them. 
With each change in fineness, polish at right angles to the 
