356 ELEMENTARY CHEMICAL MICROSCOPY 



Nickel yields no crystalline precipitate until a very high con- 

 centration is reached, when yellowish disks and spherulites 

 appear. Much nickel in the presence of zinc modifies the appear- 

 ance of the crystals of the double zinc salt, in the same manner 

 as cadmium. With much nickel and very little zinc only spher- 

 ulites are obtained. 



The presence of both copper and cobalt in a solution contain- 

 ing zinc gives rise to the formation of mixed crystals of very 

 peculiar color and form. These peculiarities are accentuated 

 when cadmium is also present. The experienced worker thus 

 will have little difficulty in detecting a number of elements in 

 one single operation. 



Manganous salts in excessively concentrated solutions con- 

 taining a trace of free sulphuric acid yield crystals closely resem- 

 bling those of the cadmium double salt. 



Ferrous compounds, if only in very small amount, do not 

 interfere with the formation of the typical crystals of the zinc 

 salt but in high per cent there will usually be obtained radiating 

 groups or feathery dendrites closely resembling the copper salt. 



Ferric salts always yield a pink or red color and have no effect 

 upon the zinc compound until a concentration is reached such 

 that a deep blood red color appears. Under such conditions the 

 zinc-mercury thiocyanate first separates as a deep reddish brown 

 salt, jet black by transmitted light, yet still retaining the typical 

 feathery dendritic form, but in a few seconds these undergo 

 a sudden and remarkable change into masses of curving branch- 

 ing filiform crystals. This is especially marked in test drops 

 containing sodium or ammonium acetate. 



Lead, unless present in large amount, usually seems to have 

 little or no effect on the zinc reaction. Under some conditions 

 it seems to interfere, however, and it is, therefore, always best 

 to first remove the lead by means of dilute sulphuric acid. Add 

 the acid, decant or filter; evaporate the clear solution to dry- 

 ness; fume off the free sulphuric acid; dissolve in water; add 

 ammonium acetate, and test as above. 



Silver gives with the reagent a white amorphous precipitate, 

 soon crystallizing in the form of small, thin, slender prisms 



