MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 309 



other complex salts formed, there will always be formed some of 

 the typical uncolored zinc mercury sulphocyanate. 



Copper alone yields beautiful branching dendrites and radiat- 

 ing masses of acicular prisms, yellowish green in color. The 

 reaction is sensitive and beautiful and constitutes one of the 

 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 -H 2 O, 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 sulphocyanate. As in the case of the copper-zinc 

 compound, these blue crystals are doubtless cases of solid solu- 

 tion, 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 sulphocyanate crystals under the 

 conditions which obtain in microchemical testing. 



Small amounts of zinc in the presence of much cobalt cannot 

 be detected by this reagent. 



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 



