342 METALS AND THEIR COMBINATIONS. 



cury remains undissolved, the solution will deposit crystals of mercurous 

 nitrate on cooling. Use some of the solution, after being diluted with much 

 water, for mercurous tests. Use another portion as follows : Heat the solution, 

 or some of the crystals, with about an equal weight of nitric acid until no more 

 red fumes escape. Add to a few drops of the diluted liquid a little hydro- 

 chloric acid, which, if the conversion of the mercurous into mercuric salt has 

 been complete, will give no precipitate. If, however, one should be formed, the 

 solution is heated with more nitric acid until no precipitate is formed by hydro- 

 chloric acid, when the solution is evaporated and set aside for crystallization. 

 The respective changes may be represented by the following equations : 



3H g + 4HN0 3 == 3HgNO 3 + 2H 2 O + NO ; 

 3HgNO 3 + 4HN0 3 = 3Hg(NO 3 ) 2 -f 2H 2 O + NO. 



Mercuric sulphide, HgS. This compound has been mentioned as 

 the chief ore of mercury, occurring crystallized as cinnabar, which 

 has a red color (Plate IV., 2). The same compound may, however, 

 be obtained by passing hydrogen sulphide through mercuric solutions, 

 when at first a white precipitate is formed (a double compound of the 

 sulphide of mercury in combination with the mercuric salt), which 

 soon turns black (Plate IV., 1) : 



HgCl 2 + H-jS = 2HC1 + HgS. 



The black, amorphous, mercuric sulphide may be converted into the 

 red, crystallized variety by sublimation, and is then the preparation 

 known as red sulphide of mercury, cinnabar, or vermilion. It forms 

 brilliant dark-red crystalline masses, or a fine bright scarlet powder, 

 which is insoluble in water, hydrochloric or nitric acid, but soluble 

 in nitro-hydrochloric acid. 



Mercuric and mercurous sulphides may be made also by triturating 

 the elements mercury and sulphur in the proper proportions, when 

 they combine directly. 



Ammoniated mercury, Hydrargyrum ammoniatum, NH^HgCl 

 = 249.61 ( White, precipitate, Mercuric-ammonium chloride). This com- 

 pound is made by pouring solution of mercuric chloride into ammonia 

 water, when a white precipitate falls, which is washed with highly 

 diluted ammonia water and dried at a low temperature : 



HgCl 2 + 2NH 4 OH == NH 2 HgCl + NH 4 C1 + 2H 2 O. 



As shown by the composition of this compound, it may be re- 

 garded as ammonium chloride, NH 4 C1, in which two atoms of hydro- 

 gen have been replaced by one atom of the bivalent mercury. The 

 mercuric-ammonium salts are of a different type from those combina- 



