SILVER MERCUHY. 341 



When mercuric iodide is dissolved in potassium iodide solution, a complex 

 salt is formed, HgI 2 .2KI, or K 2 HgI 4 , from which many of the usual reagents fail 

 to precipitate the mercury. For example, caustic potash has no effect on the 

 compound, and such an alkaline solution is known as Nessler's reagent. This 

 reagent gives a yellow color with traces of ammonia, and a brown precipitate 

 with larger amounts : 



2HgI 2 + 4NH 3 Hg 2 NI + 3NHJ. 



Nessler's reagent is used in water analysis for detecting traces of ammonia. 

 The compound, Hg 2 NI, is called dimercur-ammonium iodide. 



Mercuric sulphate, Hg-SO 4 . When mercury is heated with strong 

 sulphuric acid (the presence of nitric acid facilitates the formation) 

 chemical action takes place between the two substances, sulphur 

 dioxide being liberated and mercuric sulphate formed, which upon 

 evaporation of the solution is obtained as a heavy, white, crystalline 

 powder : 



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



Yellow mercuric subsulphate, Hg-SO 4 .(Hg-O) 2 (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, 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 : 



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



Nitrates of mercury. Mercurous nitrate, HgNO 3 , 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 contain- 

 ing free acid. Water alone causes decomposition of the nitrates, 

 similar to that of the sulphates, resulting in the formation of insoluble 

 basic salts, the composition of which depends on the relative propor- 

 tions of the mercury salt and water used. 



Experiment 46. 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- 



