392 ELEMENTARY CHEMICAL MICROSCOPY 
plumbo-fluorides the salts have the composition M2RF6, where 
M is an alkali metal and R may be Zr, Si or Pb. 
Crystalline double fluorides of aluminum with copper, nickel 
and zinc have been described, but these are too soluble to appear 
under the conditions which usually obtain in an analysis. 
Precautions. 
Employ only neutral solutions. 
Always have an excess of ammonium fluoride, for if not a 
compound of different formula results appearing as very tiny 
rods, worthless as an identity test for aluminum. 
Salts of lithium, sodium and iron must be absent. 
The presence of silicon and analogous elements will generally 
seriously complicate matters, and may ruin the test, owing to 
the formation of silico-fluorides, etc. (See ammonium silico- 
fluoride tests, under sodium and barium.) Aluminum silico- 
fluoride is gelatinous, and does not crystallize. 
Testing for aluminum with ammonium fluoride generally yields 
results a trifle quicker than Method A, but the delicacy of the 
reaction is very little greater. Moreover, Method B is subject 
to many complications and interferences, and there is always 
danger, in spite of great care, of damaging objectives by the 
corrosive vapors arising from the test drop, since objectives of 
moderate power and therefore short working distance must be 
employed. For these reasons, testing with ammonium fluoride 
cannot be considered as being as satisfactory as the cesium 
sulphate method. One of the chief reasons for inserting the test 
in this series is the fact that crystals of ammonium alumino- 
fluoride may occasionally appear when ammonium fluoride is 
being employed for other purposes, and the presence of alu¬ 
minum is not suspected. 
The method of testing for aluminum by heating with ammo¬ 
nium fluoride in a platinum cup has been described under 
Method XV, page 318 (q.v.). The results thus obtained are in 
most cases somewhat more reliable than those given above but 
require more time, patience and care. 
