Mar.. 1889. micro-chemical examination of minerals. 
61 
mastery of the method by comparing known felspars placed 
simultaneously in the flame, on opposite sides of it, and 
observed together. In this way the distinction of anorthite 
from labradorite becomes quite easy, and the soda percentage 
of an ortlioclase may be estimated with considerable approach 
to accuracy. 
The second method I have to speak of is of somewhat 
limited application in certain ways, although it is applicable 
to other minerals than the felspars. 
v 
It is that adopted by Dr. E. Boricky, of Prague, who 
explains that he could not use Bzabo’s method because there 
was no gas supply at his disposal. He decomposes the 
mineral with a dilute solution of fluosilicic acid on a glass 
slide, which has been evenly covered with a coating of hard 
Canada balsam, and after allowing 24 hours for the 
decomposition, allows the drop of reagent to evaporate as far 
as it will, in a dry, slightly warm place, and examines the 
forms of the fluosilicates which are left. Borne of these are 
highly characteristic—the potash salt crystallises in brilliant 
octolieura, variously modified; while the soda compound, 
which is somewhat more soluble, and is therefore not quite so 
readily crystallised, separates in short, stout, hexagonal 
prisms. Unfortunately the determination of lime is not so 
certain, the forms due to it being less definite and liable to 
modification in presence of soda. The fluosilicates of 
protoxide of iron and of magnesia have the same crystalline 
form, and are both quite soluble in water, so that they are not 
very easily obtained, and can only be distinguished from each 
other by exposing them to the action of either the vapour of 
sulphide of ammonium, which blackens the iron salt, or of 
chlorine, which reddens it. 
On the whole, I think this method mav be said to be 
superseded by the processes to which we now come. Details 
of the tests differ, but the principle is—decomposition of the 
mineral by either hydrochloric, sulphuric or hydrofluoric 
acid, according to the necessities of the case, and the 
subsequent examination, mostly by precipitation methods, 
of the solution thus obtained. 
The decomposition may be effected directly on a section 
picked out by the microscope and isolated by means of 
a hole of suitable size, made either in a cover glass or where 
hydrofluoric acid is to be used in a piece of platinum foil. 
This is cemented on to the section with balsam, the hole 
must be cleared of balsam, and thoroughly cleaned with 
spirits of wine, and the mineral grain thus exposed can be 
subjected to the action of the acid, which should be led into the 
