354 ELEMENTARY CHEMICAL MICROSCOPY 



leaves little to be desired, but in the analysis of minerals, it is 

 better to employ the carbonate test first, then corroborate with 

 the thiocyanate reagent. 



Upon adding a rather concentrated solution of the reagent to 

 a dilute solution of the metals listed above the following results 

 are obtained : 



Zinc yields an almost instantaneous precipitation of the com- 

 pound Zn(CNS)2.-Hg(CNS)2 in pure white feathery crosses and 

 branching feathery aggregates. These skeleton crystals, when 

 thick, appear black by transmitted light and snow white by 

 reflected light. The normal crystal of the double thiocyanate 

 of zinc and mercury is said to be a right-angled prism of the 

 orthorhombic system, but under the conditions which obtain in 

 ordinary practice, only skeleton and dendritic forms will be seen. 



Neither magnesium nor aluminum interfere with this test, 

 save that when magnesium is present in very large amount, the 

 separation of the zinc salt is retarded, and that aluminum under 

 similar conditions renders the skeleton crystals of the zinc salt 

 somewhat less feathery. 



When zinc alone is present the crystals, as has been stated 

 above, are snow white and of the form described; but if copper 

 is present in minute amount, the crystals of the zinc salt are 

 colored lavender or brown without undergoing any change of 

 form. These crystals begin to appear after the white ones have 

 separated. More copper than sufficient to yield the brown tint 

 produces black crystals of modified form; still a greater pro- 

 portion of copper completely changes the appearance of the crys- 

 tals, and jet black spheres and botryoidal masses result. Finally 

 a point is reached where crystals of copper mercuric thiocyanate 

 predominate, accompanied by the black crystals just mentioned. 

 In all cases, however, because of the much lower solubility of 

 the zinc compound than that of the other complex salts formed, 

 there will always be formed some of the typical uncolored zinc 

 mercury thiocyanate. 



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 



