MICROCHEMICAL REACTIONS OF ZINC 355 



most satisfactory tests available for the identification of copper. 

 The change in color due to the solid solution of the copper 

 salt Cu(CNS) 2 -Hg(CNS) 2 (?), in the zinc salt is a most inter- 

 esting one and one for which no really satisfactory explanation 

 is yet at hand. 



The cobalt salt enters into the zinc salt in solid solution to 

 yield light blue crystals. With very small amounts the color 

 is exceedingly faint and the crystal form unchanged, but as the 

 proportion of cobalt increases, the skeleton crystals of the zinc 

 salt become deeper and deeper blue, simpler, less feathery, and 

 gradually assume the color and appearance of the normal cobalt 

 mercuric thiocyanate. As in the case of the copper-zinc com- 

 pound, these blue crystals are doubtless cases of solid solution, 

 but the theory of isomorphous mixture is more tenable in this 

 case than in that where copper is present. 



Cobalt alone yields deep blue-black orthorhombic prisms, 

 Co(CNS)2-Hg(CNS)2, usually imperfectly developed and unit- 

 ing to form star-like clumps and radiating masses. This con- 

 stitutes a valuable method for differentiating cobalt from nickel, 

 since nickel yields no double thiocyanate crystals under the 

 usual conditions which obtain in microchemical testing. 



Small amounts of zinc in the presence of much cobalt cannot 

 be detected by this reagent. 



Inorganic salts of cadmium yields Cd(CNS)2-Hg(CNS)2 in 

 brilliant colorless, probably orthorhombic prisms, usually several 

 times as long as broad but the appearance of these prisms varies 

 with the conditions which obtain at the time of their formation, 

 as, for example, the concentration, depth of the test drop, amount 

 of reagent added, acidity, etc. These variations are, however, 

 not of a kind to render the test doubtful, long prisms, either 

 singly or in groups being the rule. 



Even a small amount of cadmium destroys the feathery and 

 branched character of the skeletons of the zinc-mercury thio- 

 cyanate, owing to the formation of mixed crystals, and there 

 generally results crystallites of the shape of an arrowhead. 

 Small amounts of zinc in the presence of much cadmium will 

 usually escape detection. 



