MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 331 



B. ANISOTROPIC. 



Hexagonal. Iodide; 1 secondary arsenate; sec- 

 ondary phosphate. 

 Tetragonal. 



Orthorhombic. Chromate; nitrate; nitrite; sul- 

 phate; potassium-silver iodide. 

 Monoclinic. 

 Triclinic. - Bichromate. 



DETECTION. 



A. By Means of Hydrochloric Acid. 



Apply the reagent by Method III A, page 254, to the test 

 drop previously acidulated with nitric acid. 



If silver is present an immediate precipitate should result. 

 Examine under the microscope. Silver chloride is so insoluble 

 in water that it is thrown down as an amorphous mass. If the 

 precipitate is wholly crystalline, either silver is absent or else 

 present in very small amount. In order to identify silver in an 

 amorphous precipitate it is necessary to recrystallize it. Before 

 so doing it is always advisable, and often necessary, to first 

 remove the solution from the precipitate and wash the latter. If 

 the hydrochloric acid has been carefully added and the drop not 

 stirred, it is easy to draw off the clear solution from the curdy, 

 heavy precipitate of silver chloride. When the amount of pre- 

 cipitate is very small it is best to have recourse to the centrifuge 

 to accomplish the separation. After removing the supernatant 

 liquid, wash the precipitate once or twice with hot water acidified 

 with nitric acid. The washed precipitate is then recrystallized 

 from concentrated hydrochloric acid, or from ammonium hy- 

 droxide. 



To the precipitate of silver chloride, at the corner of a slide, 

 add a drop or two of concentrated hydrochloric acid, and heat 

 the preparation over the micro-flame. If the precipitate is not 

 completely dissolved, rapidly draw off the hot acid, without 

 exercising any great care. On cooling, tiny crystals of silver 

 chloride separate. Octahedral crystals predominate. 

 1 Upon heating, Agl becomes isometric. 



