258 
NOTES AND MEMORANDA. 
in the flame of a spirit lamp ; when nearly dropping it is touched to 
the glass over the dot. This ensures a central position for the object 
when mounted. The slide is then heated over the spirit lamp until 
the cement is liquid and the specimen placed on the melted shellac, 
with the best surface uppermost. It is then gently pressed down and 
the slide set aside to cool. It has then only to be labelled and placed 
in the cabinet. Pulverulent minerals, small crystals, concentrated 
washings, &g., are best mounted in cells and covered with thin glass 
in the usual manner. In some cases the dust from powdered minerals 
adheres to the glass cover if loosely mounted. In such cases I find it 
better to coat the slide within the turned circle with gum arabic, in 
rather thick solution. When dry, if breathed upon, the powdered 
mineral will attach itself to the gummed surface. The glass cover 
may then be replaced and cemented. Those minerals of which rocks 
are composed, as, for example, albite, orthoclase, mica, labradorite, 
epidote, quartz, &c., should not only be mounted as opaque objects, 
but also in thin sections, to admit of their being examined by polarized 
light. The study of rocks by the aid of the microscope is growing 
yearly in favour, and no geologist will now decide on the character of 
a specimen without first submitting it to the optical test. To fully 
understand the rocks, which are composite, the student must familiarize 
himself with the optical properties of the minerals composing them. 
He must learn to distinguish on the first turn of the polarizer the 
difference between quartz and felspar, and to decide as quickly if the 
felspar be orthoclase or albite. This can only be done by careful 
study of minerals. For opaque minerals I find that the best illumi- 
nation to be that produced by the parabolic illuminator, although the 
large bull's-eye condenser will be found to produce good effects when 
the former apparatus is not at hand. The advantage of the illu- 
minator is, that the light is thrown downward into the cavities of the 
specimen, which are often filled with beautiful crystals. In trans- 
parent sections in which opaque minerals are imbedded, the parabola 
is indispensable. The most beautiful effects are often obtained by its 
use. Thin sections should also be examined by polarized light ; the 
details of the inner structure are by this means often brought out in 
an unexpected manner. The microscope can hardly be dispensed with 
in determinative mineralogy. If the mineralogist is called upon to 
decide upon the fusibility of a mineral and finds the result doubtful, 
he has only to strongly heat a thin fragment in the blowpipe flame, 
and place it in the field of his instrument. If at all fusible the thin 
edges will be seen to be rounded. A fragment of a mineral containing 
alumina, if wet with nitrate of cobalt, and strongly heated, will often 
show the prominent parts tinged blue under the microscope, when the 
unassisted eye fails to distinguish the reaction. 
