Monthly Mlcr'nscopican 
Journal, April 1, 1869. J 
of some Precious Stones. 
221 
flat Water-of-Ayr, or other very fine and even-grained stone. It is 
very difficult to grind small crystals if they be merely held in the 
fingers, and almost impossible to make the surfaces flat; but a 
simple contrivance renders it easy and ensures a good result. The 
specimen should be fastened near one end of a piece of glass, about 
one inch broad and two long, and one or more strips of glass, one 
over the other, fixed at the opposite end, so as to be about as thick 
as the specimen. By this contrivance we easily get a good bearing 
on the zinc plate, and can grind and polish the specimens so that 
the face is quite flat. A cement made of shell lac melted with Canada 
balsam is very convenient, since it is hard enough and yet suffi- 
ciently fusible. After polishing one side, the specimen should be 
taken off and the polished surface turned towards the glass ; and one 
or more pieces of glass fixed at the opposite end, so that they may 
be of almost exactly the same thickness as the specimen after it has 
been properly ground down. By looking through it at a candle, any 
deviation from parallelism in the opposite faces can be recognized 
by the apparent change in the position of the flame ; and, if neces- 
sary, the error may be corrected by grinding more away from either 
the glass or the mineral. It may then be taken off by the aid of 
heat, the greater part of the cement removed mechanically, the rest 
dissolved away by digesting in alcohol or other suitable solvent, 
and the specimen mounted on a slide with Canada balsam. In the 
case of gems this process gives well-polished surfaces; but, since 
softer minerals do not take a good polish and are easily scratched, 
it is much the best to mount over them with balsam a piece of thin 
glass cut of the proper size and shape. 
Tlie thickness of the sections must depend to some extent on 
the character of their structure. I think that in many cases they 
are cut too thin. By far the best plan is to examine them as soon 
as they are sufficiently thin to be partially transparent, and to judge 
from what is seen whether they should be left thick or made thinner. 
If they contain comparatively large fluid cavities, or enclose crystals 
which require only a low magnifying power, they would be spoiled 
by being cut thin : whereas, if the structure requires a high 
power, a thick section might be objectionable. I have prepared 
thick sections for some purposes, and can nevertheless use high 
powers to examine the minute fluid-cavities lying near the surface ; 
and since, moreover, we can reduce their thickness when found 
necessary, I am decidedly in favour of leaving them rather too thick 
than otherwise. 
Probably no mineral could show more interesting phenomena 
than the fluid-cavities in sapphire. Figs. 1, 2, 3, and -l ; but they 
are unfortunately so rare, that very few persons could hope to 
obtain such beautiful specimens as those belonging to Mr. Butler, 
who has had unusual opportunity for procuring them, and made 
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