LESSONS IN CHEMISTRY. 159 



coating on the zinc. A knife blade placed in the solution is 

 quickly covered with copper. Objects are plated with copper by 

 placing them in a solution of copper sulphate and connecting 

 them with a galvanic current, when the sulphate is decomposed 

 and the copper deposited on the article. There are oxides, 

 sulphides, chlorides, carbonates and arsenites of copper, but 

 perhaps the most interesting are the blue and green malachites, 

 carbonates of copper which are highly prized for their beauty. 



The metals generally are solids, but mercury is a liquid at 

 ordinary temperatures, having none of the properties usually 

 ascribed to metals except its brilliant luster and high specific 

 gravity. The pure mercury is sometimes found in nature, but it 

 more commonly occurs as a sulphide in the mineral cinnabar. 



MERCURY (Hg) is obtained from cinnabar by the heat, which 

 burns out the sulphur, vaporizing the mercury, which is con- 

 densed in brick chambers or flues. It forms alloys, called amal- 

 gams, with most of the other metals, especially with gold and 

 silver. This property makes mercury a valuable agent in sepa- 

 rating gold and silver from their ores. 



Mercury is used in making thermometers and barometers, but 

 its chief use is in silvering mirrors. The silvering of a mirror 

 consists of an amalgam of tin. A sheet of tin foil is laid on a 

 table and rubbed over with mercury, and then another thin 

 layer of mercury is poured over it. The glass is then carefully 

 slid over the foil so as to push off some of the superfluous mer- 

 cury, together with impurities; then heavy weights are laid on 

 the glass to squeeze out any excess of mercury, and in a short 

 time the amalgam adheres firmly to the glass. 



The mercurous chloride is the calomel used in medicine, and the 

 mercuric chloride is the active poison, corrosive sublimate. An 

 oxide of mercury is an ingredient of red precipitate ointment, and 

 the pigment vermillion is the sulphide. The vapor of mercury, and 

 nearly all its compounds, are poisonous, and all those who work 

 with mercury in any of its forms suffer from its effects. 



SILVER (Ag) is a white, malleable, ductile metal which is not 

 easily tarnished or corroded. Pure silver is often found in na- 



