1-1'52 Journal of Applied Microscopy 



Avoid adding too much reagent. This, however, is a matter of little import- 

 ance when zinc alone is present, but it is quite necessary when dealing with 

 mixtures. 



Neither magnesium nor aluminum interfere with this test, save that when 

 magnesium is present in 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. 



The reagent gives reactions with zinc, cadmium, copper, cobalt and indium. 

 These reactions are among the most interesting and elegant of micro-chemistry 

 and leave little to be desired. 



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

 white and of the form shown in Fig. 68 ; but if copper is present in minute 

 amount, the crystals of the zinc salt are colored chocolate brown without under- 

 going 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. 



This change in color of the zinc salt brought about by the presence of copper 

 is a most interesting one. The zinc compound — Zn(CNS) 2 • Hg(CNS)2 — con- 

 tains no water of crystallization, while the copper salt normally separates as — 

 Cu(CNS)2 • Hg(CNS)2 ' ^2^ — ^nd being hydrated is greenish in color. The 

 presence of water of crystallization in salts of copper seems to determine their color. 

 The removal of the water leads to the production of a brown or almost color- 

 less body. The nature of this change is not yet thoroughly understood. It 

 seems probable that in the case of the brown and black copper-zinc-mercury 

 sulphocyanates we have to deal with a case of solid solution, although it is also 

 conceivable that an anhydrous copper-mercury double salt may exist in the pres- 

 ence of the zinc compound, isomorphous with the latter, yet incapable of exist- 

 ing alone. 



In the presence of cobalt, the zinc salt is colored blue, the intensity of the 

 coloration depending upon the amount of cobalt present. With very small amounts 

 the color is exceedingly faint and the crystal form unchanged, but as the pro- 

 portion of cobalt increases, the skeleton crystals of the zinc s.alt 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 solution, 

 but the theory of isomorphous mixture is more tenable in this case than in that 

 where copper is present. 



Small amounts of zinc in the presence of much cobalt cannot be detected by 

 this reagent. 



Cadmium gives long prismatic crystals (Fig. 71), which are more soluble 

 than the zinc salt. Even a small amount of cadmium destroys the feathery and 

 branched character of the skeletons of the zinc-mercury sulphocyanate, owing 



