6 MINERALOGY AND LITHOLOGY. 
tion of these principles is somewhat different when they are used in 
microscopic study,—and that, although a number of treatises have been 
written upon microscopic mineralogy, they are none of them written 
in English,—that a short description is here given of the methods of 
preparing specimens for examination, and of a microscope for examining 
them; to which is added a short statement in regard to what is to be 
observed, and of those points in which microscopic examination differs 
from the ordinary optical examination of minerals. 
When minerals occur in isolated crystals or masses, or when as con- 
stituents of rocks they are in grains or crystals of some size, all their 
physical properties are easily observed, and they can be subjected 
to chemical examination; but when minerals occur in complex, fine- 
grained aggregations, intimately intermingled, as they do in many rocks, 
none of the physical properties of the minerals can be observed, neither 
can they be isolated for blow-pipe examination. Hence, when a rock 
is fine-grained or compact in its structure, and the individual crys- 
tals are either invisible or unrecognizable, if we would study their 
nature, means must be devised to separate and identify them. To 
accomplish this result, many methods have been devised. A rock is 
sometimes pulverized, and any magnetic constituent that it contains Is 
withdrawn with a magnet. Sometimes in the powder a heavy constitu- 
ent is separated by virtue of its superior specific gravity; sometimes the 
ready solubility of a mineral in acid allows of its separation from the 
more insoluble constituents. These and other properties of minerals 
have often been taken advantage of to effect their separation ; but the 
most effective method of discovering the nature and composition of com- 
pact rocks is, to make very thin sections of them, and examine the sec- 
tions with a microscope, aided by certain contrivances for modifying the 
light. This is referred to at the outset, because this method is not 
merely employed in rock study, but it is now an essential feature of the 
study of minerals. By its help, the purity or impurity of minerals is es- 
tablished, as well as the nature and character of their impurities. More- 
over, the optical properties of many minerals can be most readily studied 
microscopically ; and the many other useful applications of this method 
are constantly increasing. As the method of preparing these sections 
of minerals and rocks for observation is not described in books that 
