COPPER AND MERCURY 517 



Uses. Mercury is used in filling thermometers and l)arom- 

 eters. Sodium amalgam is used in the laboratory and tnezinc 

 plates of batteries are amalgamated superficially to protect them^ 

 when the battery is not in use. Dentists fill teeth with mixtures 

 of mercury with silver, copper, cadmium and other metals, which 

 quickly set to a solid amalgam. The pulverized ores of gold and 

 silver, mixed with water, are allowed to trickle over thin layers of 

 mercury. The latter dissolve the particles of the precious metals, 

 while the sand passes on. 



Compounds of Mercury. As in the case of copper, there are 

 two sets of compounds mercurous Hg 1 and mercuric Hg n . All 

 the common salts are completely volatile, with or without decom- 

 position, when strongly heated (vapor poisonous!). 



Mercuric chloride or corrosive sublimate HgCl 2 is made by 

 subliming a mixture of mercuric sulphate and sodium chloride. 

 It is a white substance, soluble in water. Hydrogen sulphide 

 precipitates mercuric sulphide HgS (black) from the solution. 

 Mercuric chloride is a violent poison. Albumen (white of eggs) 

 forms insoluble compounds with it, and is used as an antidote. 



Mercurous chloride or calomel HgCl is precipitated as an 

 amorphous white powder when a chloride is added to a solution of 

 a mercurous salt. It is used in medicine to stimulate all organs 

 producing secretions. Mercuric fulminate Hg(ONC)2 is used in 

 percussion caps (p. 483). 



Baser metals precipitate mercury from solutions of its salts. 

 The grey deposit is best seen on a clean strip of copper foil : 



Cu + Hg++ -> Cu++ + Hg I. 



Exercises. 1. Make an equation: (a) for the oxidation of 

 ethyl alcohol by heated cupric oxide; (b) for the precipitation of 

 cupric sulphide from cupric sulphate solution. 



2. When we electrolyzed sodium chloride solution (p. 139), 

 hydrogen was liberated at the cathode. What principle, used in 



