MICROCHEMICAL REACTIONS OF MERCURY 
367 
hot cause the bichromate to flow into the test drop. Mercurous 
salts yield characteristic crystals. Mercuric salts do not.^ 
There are generally formed with mercurous salts a number of 
different compounds. There first separates a dark red granular 
precipitate, soon changing into dark red crosses, bundles of irregu¬ 
lar crystals and peculiar dendrites and skeleton masses. Later 
yellow crystallites appear. 
In any given test the appearance of the precipitate both as 
to crystal form and color will depend upon the concentration of 
the drops, the degree of acidity and the temperature. 
Mercuric salts give no such precipitates and no crystalline 
compounds will appear unless the preparation is allowed to 
evaporate practically to dryness. There will then appear light 
yellow feathery dendritic and radiating branching moss-like 
masses. 
Lead yields slender yellow monoclinic prisms, seldom grouped 
in masses. This element unless present in excess does not appear 
to seriously interfere with the test for mercury. 
Silver separates in dark red pleochroic plates and scales which 
may often mask the mercury compounds. 
EXPERIMENTS. 
Test as above both mercurous and mercuric salts with and without HNO3 
present in both cold and hot solutions. 
C. Add to a Drop of the Material a Tiny Fragment of Potas¬ 
sium Iodide. — See Method III, page 300. Mercuric salts yield 
vermilion colored mercuric iodide; mercurous salts a heavy bright 
yellow amorphous precipitate somewhat resembling lead iodide 
in color but instead of being in plates always agglutinated in a 
formless mass. 
With mercuric salts we obtain one of the best and most satis- 
‘ Bichromate added to hot unacidified HgCL solutions causes the separation 
on cooling of hard star-like masses of crystals. According to Millon (Ann. chim. 
phys. (3) 18 , 388) this compound has the formula HgCb • K2Cr207. Ammonium 
bichromate gives orthorhombic six-sided prisms of the compound HgCb • 3(NH4)2 
Cr207. 
