516 SMITH'S INTERMEDIATE CHEMISTRY 



such as silver and gold, as well as traces of sulphides, are not 

 ionized. They fall to the bottom of the vat, as a sort of heavy 

 mud. At the cathode copper ions alone are discharged, and 

 deposited, because copper is the least active of the metals present 

 in ionic form. In this way copper, 99.8 per cent pure, can be 

 obtained, and gold and silver are recovered from the mud. 



Nickel Plating. Here the bath contains an ammoniacal 

 solution of ammonium-nickel sulphate (p. 501), and a plate of 

 nickel forms the anode. The article to be plated is carefully 

 cleaned, to secure a uniform deposit, and is suspended as cathode 

 in the vat. The surface of the deposit is afterwards burnished. 



MERCURY Hg 



Metallurgy. Mercury occurs both free and as mercuric sul- 

 phide HgS, cinnabar. Most of the ore comes from California and 

 Spain. 



The ore is roasted, sulphur dioxide escapes, and the vapor of 

 mercury is condensed in long, tortuous flues. 



Properties. Mercury is a liquid at ordinary temperatures, 

 hence its name, quicksilver (i.e. live silver). It freezes at 40, 

 and boils, giving an invisible, non-conducting vapor, at 357. The 

 vapor density shows the molecules to be monotomic, as indeed are 

 the molecules of all metals of which the densities have been meas- 

 ured. 



The metal has a silvery metallic luster, which is not affected by 

 the air, and a high specific gravity. 



Mercury dissolves other metals, forming alloys or amalgams. 



Mercury, when moderately heated, combines with oxygen, form- 

 ing mercuric oxide (red), but the action is reversible, and the 

 oxide is decomposed by stronger heating (p. 15). It combines 

 readily with sulphur and the halogens. Dilute hydrochloric and 

 sulphuric adds are not attacked by mercury. Concentrated 

 nitric acid attacks and dissolves it readily. 



