324 ELEMENTARY CHEMICAL MICROSCOPY 
Stir for a few moments. The reagent drop should now slowly 
clear up, and a perfectly clear solution should result. If, how¬ 
ever, the quantity of bismuth nitrate employed has been exces¬ 
sive, a residue remains; it is then necessary to decant the clear 
liquid. On another slide, or better on platinum foil, heat with 
dilute sulphuric acid a few particles of the substance to be 
tested. Drive off the excess of acid; cool and stir to provoke 
crystallization. If the drop refuses to crystallize, add more of 
the substance and heat again. A drop of the reagent prepared 
as above is placed at the corner of a slide, and to it is added, at 
the center, without stirring, a little of the moist mass of the 
material to be tested, taken from the platinum foil. Warm the 
preparation gently by holding it for a second or two about one 
centimeter above the micro-flame. Cool rapidly and examine 
at once. 
This reaction is more valuable for potassium than for sodium 
and constitutes one of the best microchemical tests for bismuth. 
EXPERIMENTS. 
Test for Na in NaCl; HNa2P04; in mixture of salts of Na and K and in mix¬ 
tures of salts of Na and Ca. 
C. By Means of Ammonium Silicofluoride. 
See precautions given under Method XV, page 316. 
To the drop of the neutral, or at the most only slightly acid 
solution of the material to be tested, add a fragment of ammo¬ 
nium silicofluoride. Allow to stand some time {hut never upon the 
stage of the microscope) or hasten the reaction by gentle warming. 
Sodium silicofluoride Na2SiF6 separates in the form of six- 
sided plates or prisms belonging to the hexagonal (?) system. 
Unless the crystals are excessively thin they appear with trans¬ 
mitted light to have a very faint rosy tint. They polarize only 
feebly. 
The corresponding potassium salt of like formula is much 
more soluble, separates only from decidedly concentrated solu¬ 
tions, and crystallizes in small, colorless cubes, octahedra and 
combinations of these two, or in dodecahedra (isometric). A 
