200 COMMON SUBSTANCES 



in making mortar and plaster. Ca is a base-forming 

 element, the base being calcium hydrate, Ca(OH) 2 . 



232. Mercury. The element mercury (Hg) is a metal, 

 which is a liquid at ordinary temperatures. It is some- 

 times called quicksilver. It occurs free in rocks, and 

 also as the sulphide (HgS). The metallic Hg is a heavy 

 liquid, 13.6 times heavier than water. It is used in ther- 

 mometers and barometers ; also in forming the reflecting 

 surface of mirrors. It dissolves several metals, and for 

 this reason it is used in separating silver and gold from 

 their ores. 



Hg forms several salts, among them two chlorides, 

 HgCl and HgCl 2 . HgCl, mercurous chloride, is 

 called calomel and is used in medicine. Mercuric chlo- 

 ride, HgCl 2 , is called corrosive sublimate; it is used as a 

 disinfectant to kill bacteria. Hg is a poison, as are some 

 of its compounds. 



233. Other Metallic Elements. The metals copper 

 (Cu), lead (Pb), tin (Sn), zinc (Zn), silver (Ag), and gold 

 (Au) are familiarly known to us. Cu, Ag, and Au occur 

 free in nature ; Cu and Sn occur as oxides ; Cu, Pb, 

 Zn, and Ag occur as sulphides, and Zn as a carbonate. 

 These metals are in common use, and we can easily think 

 of the uses of each. Cu and Zn form sulphates that are 

 common, and Pb forms lead carbonate, ^PbCO 3 , known 

 as white lead and used in making paint. Silver easily 

 forms the sulphide Ag 2 S ; as there is nearly always a 

 little S in the air, and always some given off in the per- 

 spiration from the body, silver articles become coated 

 with Ag 2 S and are said to " tarnish." 



