424 ELEMENTARY CHEMICAL MICROSCOPY 
a slide bearing a drop of silver nitrate. If no tiny prismatic 
crystals are obtained and no clouding of the silver nitrate, cyan¬ 
ides are absent. If a clouding of the drop results, make a fresh 
test, this time substituting for the sulphuric acid, a saturated 
solution of primary sodium carbonate; hydrocyanic acid will 
be set free and will give a characteristic silver cyanide. 
h. Set free the vapors of the acid and expose to them a drop 
of sodium picrate. A blood red solution results. 
CYANATES. 
a. To a drop of concentrated solution add at the center, a 
tiny crystal of cobalt acetate. The crystal will be immediately 
surrounded by a deep blue colored zone and a blue amorphous 
precipitate. The blue zone increases in diameter and eventually 
may reach the circumference of the drop. Upon evaporation 
deep blue tetragonal dendrites, and tabular and prismatic crys¬ 
tals of a compound corresponding to the formula K2Co(CNO)4 
will appear. Note that to obtain this compound the cyanate 
must be in excess. With sulphocyanates tested thus a deep 
blue liquid is obtained on evaporation, but the blue dendrites 
which may separate have a different habit. 
Cyanides yield no blue, but a brown color instead. Even a 
small amount of cyanide will prevent the blue zone, but the 
crystal will be blue surrounded by a yellow or brown zone. 
h. Treat a drop with dilute sulphuric acid in the distilling 
apparatus. Fig. 153, page 293. Evaporate very gently almost 
to dryness; add a few fibers of freshly ignited asbestos and 
proceed to test for ammonia. See Ammonium, Method A , page 
332. With sulphuric acid cyanates yield carbon dioxide and 
ammonium sulphate. 
Precaution. — Always make a blank test upon the reagents 
to be sure of their freedom from ammonium slats. 
FERRICYANIDES. 
a. Give ojf Vapors when heated with sulphuric acid which 
produce silver cyanide. See Cyanides, a, page 423. 
b. To the test drop add sodium acetate, then apply a solution 
