ELEMENTARY CHEMICAL MICROSCOPY 



For the qualitative examination of simple salts and alloys it 

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

 better to employ the carbonate test first, then corroborate with 

 the sulphocyanate 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 1 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 sulphocyanate 

 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 chocolate brown without undergoing any change of 

 form. These brown 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 proportion of copper completely changes the appearance 

 of the crystals, and jet black spheres and botryoidal masses 

 result. Finally a point is reached where crystals of copper 

 mercuric sulphocyanate 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 



1 This salt is generally given as anhydrous. Recent work seems to throw some 

 doubt upon this and to indicate the presence of one molecule or less of water of 

 hydra tion. 



