318 ELEMENTARY CHEMICAL MICROSCOPY 



for identification upon the faint pink tint of sodium silicofluoride 

 as well as upon its crystal form. The same caution holds good 

 for "Bakelite" varnish --obtain one not highly colored if pos- 

 sible and coat the glass slide with only a thin film. In coating 

 glass slides with any protective varnish always carry the coating 

 over the edges. 



Glass slides varnished with Canada balsam dissolved in chloro- 

 form or xylene and subsequently dried in an oven at a slightly 

 higher temperature than that of the room can also be used, but 

 are not so convenient as the methods given above. 



Rathgen has recently called attention to an entirely different 

 manner of employing fluorides in microchemical reactions. He 

 has shown l that a very sensitive and characteristic reaction for 

 aluminum may be obtained by mixing the finely powdered ma- 

 terial with several times its weight of ammonium fluoride in a 

 platinum cup or tiny platinum crucible, to which is then added 

 four or five drops of -sulphuric acid and the whole heated gently 

 until all volatile fluorine compounds have been expelled; the heat 

 is next slowly raised to drive off the sulphuric acid and the cup 

 finally brought for a moment to a low red. After cooling, the 

 residue is transferred to an object slide by means of a drop of 

 water and a tiny brush. Aluminum gives tiny six-sided crystals 

 and hexagonal plates. 



EXPERIMENTS. 



Experiments involving the use of fluorides will be found outlined in Chapter 

 XIV under the elements Sodium, Barium. 



1 Zeit. anal. Chem., 53 (1914), 33. 



