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 ^ 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., 63 ( 1914 ), 33 . 
