SILVER-MERCURY. 201 



Yellow mercuric subsulphate, Hydrargyri subsulphas flavus, 

 HgrSO 4 .(HgO) 2 = 727.4 (Basic mercuric sulphate, Turpeth mineral, 

 Mercuric oxy-sulphate). When mercuric sulphate, prepared as directed 

 above, is thrown into boiling water, it is decomposed into an acid 

 salt which remains in solution, and a basic salt which is precipitated. 

 As shown by its composition, HgSO 4 .(HgO) 2 , it may be looked upon 

 as mercuric sulphate in combination with mercuric oxide. It is a 

 heavy, lemon-yellow, tasteless powder, almost insoluble in water. 



Mercurous sulphate, Hg 2 SO 4 . When mercuric sulphate is triturated 

 with a sufficient quantity of mercury, direct combination takes place, 

 and the mercurous salt is formed : 



HgS0 4 + Hg = H g2 S0 4 . 



Nitrates of mercury. Mercurous nitrate, Hg 2 (NO 3 ) 2 , and Mer- 

 curic nitrate, Hg(NO 3 ) 2 , may both be obtained as white salts by dis- 

 solving mercury in nitric, acid. The relative quantities of the two 

 substances present determine whether mercurous or mercuric nitrate 

 be formed. If mercury is present in excess the mercurous salt, if nitric 

 acid is present in excess the mercuric salt, is formed, the latter espe- 

 cially on heating. Both salts are white and soluble in water. 



Experiment 36. Heat gently a small globule (about 1 gramme) of mercury 

 with 2 c.c. of nitric acid until red fumes cease to escape. If some of the mer- 

 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 : 



6Hg + 8HN0 3 = 3[H g2 (N0 3 y + 4H 2 O + 2NO; 

 3[Hg 2 (N0 3 ) 2 ] + 8HN0 3 = 6[Hg(NO 8 ) a ] + 4H 2 O + 2NO. 



Mercuric sulphide, Hg-S = 231.8. This compound has been 

 mentioned as the chief ore of mercury, occurring crystallized as cin- 

 nabar, which has a red color (Plate IV., 2). The same compound 

 may, however, be obtained by passing hydrosulphuric acid gas 

 through mercuric solutions, when at first a white precipitate is 

 formed (a double compound of the sulphide of mercury in combina- 

 tion with the mercuric salt), which soon turns black (Plate IV., 1): 



HgCl 2 + H 2 S = 2HC1 + HgS. 



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

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



