242 On the Freparation of Rock Sections. [Sffi aSTmS 
f in. in thickness, making both the stone and plate-glass tolerably- 
hot, so that the balsam may sink into the pores, or softer parts of 
the section, which, upon cooling, remains firmly attached to the 
glass ; — care must of course be taken not to employ a heat so ele- 
vated as to risk the expulsion of any liquid which might possibly 
be contained in fluid-cavities in the specimen. 
This small square of plate-glass now serves as a handle when 
grinding down the other surface upon the lapidary's plate, or by 
hand, as before described, until the section, which should be exa- 
mined from time to time, proves to be sufficiently translucent for 
use under the microscope. It is then finished in precisely the same 
manner as the former face, with water alone, upon a fine Turkey 
hone, and is ready for mounting upon its permanent slide. The 
mounting is done simply by warming the plate-glass square until 
the balsam is liquefied, when the rock section is carefully transferred 
on to the gently heated glass slide ; afterwards the upper surface of 
the section is moistened with a drop of turpentine, and a thin glass 
cover fixed on the top of it with Canada balsam as usual. 
The glass slides which I use are all 1 J in. square, a size I con- 
sider more convenient than the usual one of 3 by 1 in., both as 
being less liable to crack right across the centre — to its utter 
destruction — as commonly occurs when the latter are accidentally 
let fall, and more particularly as permitting, in the case of very 
delicate or friable sections, that the section be finished upon the 
slide itself, without the necessity of any subsequent removal, as 
already recommended in ordinary cases. In such cases, in order to 
prevent the slide being scratched when grinding, it is only requi- 
site to cement to each of its four corners a small piece of thin micro- 
scopic cover-glass or of sheet-zinc previous to commencing ; these 
corners — which likewise ensure that the section is ground more flat 
and uniform in thickness — are removed after the operation. 
Hard rocks are, as a rule, easier to prepare than soft or porous 
ones, which latter should first be soaked in turpentine, then in 
soft Canada balsam, and afterwards heated until quite hard. Kocks 
containing minerals of very difierent degrees of hardness, as, for 
example, quartz with calcite, &c., should be ground very carefully 
and slowly upon a close-grained stone or plate, so that whilst the 
hardest parts are being ground down the soft ones may not be worn 
away or destroyed at a greater rate. 
The thickness of such rock sections must naturally vary greatly, 
according as the mineral components of the rock itself are more or 
less opaque ; some of the more compact and opaque rocks necessi- 
tate sections not more than toVo^J^ of an inch in thickness, but 
ordinarily one-tenth of this, or -^-^ih. of an inch, is sufficient, and 
in some cases no more than a quarter of this latter thickness is 
required. 
