MERCURY. 179 



which takes place ; on cooling, and, more quickly, on pressing 

 or rubbing the yellow powder, it reassumes the original con- 

 dition and the red color. 



Mercuric sulphate, HgS0 4 . When mercury is heated with 

 strong sulphuric acid (the presence of nitric acid facilitates the 

 formation) chemical action takes place between the two sub- 

 stances, sulphur dioxide being liberated and mercuric sulphate 

 formed, which is obtained as a heavy, white, crystalline powder: 



Hg + 2H 2 S0 4 = HgS0 4 + 2H 2 O + SO 2 . 



Yellow subsulphate of mercury, Hydrargyri subsulphas flavus, 

 HgS0 4 .2HgO = 727.1 (Basic mercuric sulphate, Tarpeth mineral). 

 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, HgS0 4 .2HgO, it may be looked upon 

 as mercuric sulphate in combination w T ith mercuric oxide. It is 

 a heavy, lemon-yellow, tasteless, insoluble powder. 



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

 triturated with a sufficient quantity of mercury, direct combina- 

 tion takes place, and the rnercurous salt is formed : 



H g S0 4 + H g = Hg 2 S0 4 . 



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

 curic nitrate, Hg2N"O 3 , may both be obtained as white salts by 

 dissolving mercury in nitric acid. The relative quantities of the 

 two substances present determine whether rnercurous or mer- 

 curic 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 especially on heating. Both salts are 

 white and soluble in water. 



Mercuric sulphide, HgS = 231.7. This compound has been 

 mentioned as the chief ore of mercury, occurring crystallized as 

 cinnabar, and has a red color. The same compound may, how- 

 ever, be obtained by passing hydrosulphuric acid gas through 

 mercuric solutions, when at lirst a white precipitate is formed 



