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. 



^his 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; HNa 2 P0 4 ; 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 (but never upon the 

 stage of the microscope) or hasten the reaction by gentle warming. 



Sodium silicofluoride Na 2 SiF 6 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 



