326 ELEMENTARY CHEMICAL MICROSCOPY 



In the absence of ammonium silicofluoride, pure silicon dioxide 

 and ammonium fluoride can be added to the acidified drop of 

 the solution to be examined. 



Precautions. 



Neither ammonium silicofluoride nor ammonium fluoride 

 should ever be employed without having first been tested for the 

 presence of sodium. If the reagents are found to be impure, it 

 is necessary to sublime them in a platinum crucible, or receive 

 the sublimate on platinum foil held over the material heated in 

 a platinum cup. 



In the presence of much calcium the crystals of sodium silico- 

 fluoride may become distinct hexagonal prisms instead of hexag- 

 onal plates, a fact which must be borne in mind when working 

 with material of unknown composition. 



The silicofluoride test is one of the most valuable at our com- 

 mand in testing silicates for sodium, in which case we need add 

 only hydrofluoric acid or ammonium fluoride and sulphuric acid. 



The addition of sodium and a fluoride gives us a test for Si, 

 Ti or B. 



Remember that glass slides cannot be used in this test for 

 sodium; that only low-power (i inch) objectives of great work- 

 ing distance should be employed, and even then the front lens 

 should always be protected in some way, as, for example, with a 

 small cover glass held in place with glycerine, oil or other suitable 

 substance. The preparation should be examined as rapidly as 

 possible, and must be quickly removed from the stage. When 

 the microscope is provided with a nosepiece, it is advisable to 

 remove the objectives not in use before examining any prepa- 

 rations liable to give off hydrofluoric acid or volatile fluorine 

 compounds. The objective must always be thoroughly cleaned 

 after any such tests. 



EXPERIMENTS. 



a. Test, as directed above, salts of Na in both neutral and acid solutions. 



b. In order to better appreciate the reasons for employing celluloid slips, place 

 a drop of water on a glass slide, acidulate (but add no Na), then add the reagents 

 and examine the preparation. 



c. Try to obtain crystals of K 2 SiF 6 from KC1. 



