154 ELEMENTS OF APPLIED MICRISCOPY. 
detach with a hammer a chip of suitible form without 
using a special section-cutter. The chip or section is 
then ground down on an emery-wheel or by hand, first 
on an iron or copper plate covered with No. 120 emery 
and water, then on a glass plate with flour of emery. 
The slice thus prepared is cemented to a piece of glass 
about one-inch square. Canada balsam may be used 
as a cement, the specimen being pressed down into the 
warm balsam, with care that bubbles are not included, 
the slide being then heated till the balsam becomes 
hard and firm. After cementing, the free surface of the 
section is ground down until the required: degree of 
transparency has been attained, the glass serving as 
a holder and support. A good section should not be 
over 0.05 mm. in thickness. 
Certain opaque minerals are observed by reflected light, 
and in many cases it is convenient to examine mineral 
powders directly. They may be mounted in water for 
the study of the form of the particles, and in glycerin 
or some similar highly refractive substance for internal 
structure. 
The color and form of the crystalline or amorphous 
constituents of a rock are first observed under the micro- 
scope. Appearances of high or low relief indicate differ- 
ences of refraction. The presence of cleavage lin:s 
and the occurrence of inclusions of gaseous, liquid, or 
glassy character should be noted. The types of crystals 
present may be made out, care being taken to interpret 
correctly the peculiar appearances due to the plane in 
which the section studied may happen to lie. Even the 
