368 ELEMENTARY CHEMICAL MICROSCOPY 
factory tests for mercury. At the moment the potassium iodide 
strikes the drop a white or pinkish cloud appears, rapidly chang- 
ing to yellow then to brilHant red. The mercuric iodide Hgl2 
first formed is very soluble in excess of the reagent forming the 
soluble compound Hgl2 • 2 KI. The precipitate therefore appears 
as an ever-widening circle about the fragment of solid reagent 
until the latter is completely dissolved. If the outer edge of the 
brilliant red circle is now examined with a moderately high power 
it will .be seen to consist of tiny ruby red rhombs and rods to¬ 
gether with more or less spherical masses and imperfect rosettes. 
Precautions must be taken to avoid adding an excess of reagent; 
otherwise no permanent separation will take place. In order to 
avoid the possibihty of error it is always well to add a fragment 
of copper sulphate, which will take up the excess of iodide and 
cause the separation of the mercuric salt. 
Precautions. 
A few very stable complex salts of mercury usually fail to yield 
a test for mercury with potassium iodide. If therefore no test is 
obtained for mercury, boil the unknown with strong nitric acid, 
evaporate almost to dryness, dilute with water and test again. 
EXPERIMENTS. 
See under Lead, Method A, page 371. 
D. Mercuric Salts can he Detected through the Formation of 
Double Thiocyanates. 
This test is the reverse of that employed for the detection of 
Zinc (Method A, page 353); of Copper (Method A, page 385); 
or of Cobalt (Method A, page 412), to which the student is 
referred for details. 
Add to a small test drop (which must not contain much free 
mineral acid) a fragment of potassium thiocyanate about the 
size of a pinhead. Stir until dissolved. Place next this drop a 
tiny drop of water in which is dissolved a very little zinc sul¬ 
phate. Cause the test drop to flow into the zinc solution. Char¬ 
acteristic crystals of zinc-mercury thiocyanate will appear. 
Instead of zinc sulphate, copper sulphate or cobalt nitrate 
may be employed. 
