130 
BULLETIN OF THE LABORATORIES 
I, and i-2. Incomplete crystals are barrel-shaped, presence pf cal- 
cium increases the size. 
3. Fliw-silicate of calcium occurs in peculiar, long lanceolate 
crystals, often in rosettes. The angles and edges are not sharp. 
Monoclinic, easily soluble in water. 
4. Fluo-silicaie of magnesium appears in rhombohedrons the angles 
of which are truncated by oR, and in combinations of R . i-2 or 
R. i-2 . oR. It often appears in rhombohedrons deformed in one angle 
or in cruciate or pectinate forms. 
Ferric and maganesic compounds can hardly be distinguished from 
magnesium fluo-silicate, and strontium compounds resemble fluo- 
silicate of calcium. 
5. Lithia compounds appear in obtuse hexagonal pyramids, while 
fluo-silicate of barium occurs in excessively minute needles. 
The fluorides of iron, magnesium, and maganese may be dis- 
tinguished by subjecting - them to the action of chlorine gas, which 
changes the color of the iron compound to citron yellow, of the 
manganese to reddish, while the fluoride of magnesia remains colorless. 
Behrens gives the following method which, when carefully followed, 
is exceedingly delicate : — 
The sample to be studied is isolated and pulverized and is placed in 
a covered platinum capsule not more than i cm. in diameter into which 
a few drops of fluoric acid, fluoride of ammonium, and concentrated 
hydrochloric acid have been placed, evaporate, and, if necessary, 
repeat the operation. The dried mass resulting is evaporated with 
sulphuric acid nearly to dryness, when gray fumes are formed. Add 
water and again evaporate so that for each milligram of the powder a 
centigram of fluid is produced. A drop of this fluid is now placed 
on the slide with a capillary pij^ette and, after protecting the objective, 
is examined for calcium. If calcium is presant, evaporation produces 
crystals of gypsum (I . i-f ; i .) On the margin of the drop charac- 
teristic swallow-tail twins may be found. .0005 mg. of lime can be 
detected in this way. 
A drop of platinum chloride is now added and, if potassium was 
present, strongly refractive octahedrons or trillings or quadrillings 
appear. 
Sodium is detected by cerium sulphate, a saturated solution of which 
is placed on the slide near the drop to be examined and connected 
with it by a capillary glass thread. In the drop of the reagent there 
