MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 321 



hot cause the bichromate to flow into the test drop. Mercurous 

 salts yield characteristic crystals. Mercuric salts do not. 1 



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 HNOs 

 present in both cold and hot solutions. 



C. Add to a Drop of the Material a Tiny Fragment of Potas- 

 sium Iodide. See Method ///, page 252. 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- 



1 Bichromate added to hot unacidified HgCl2 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 HgCl 2 K 2 Cr 2 O 7 . Ammonium 

 bichromate gives orthorhombic six-sided prisms of the compound HgCl 2 '3(NH4) 

 Cr 2 O 7 . 



