LESSONS IN CHEMISTRY. 167 



One part of litharge, and ten parts of brick dust with enough 

 linseed oil to form a paste makes a cement often used by build- 

 ers in repairing broken stone. Acetate of lead, or sugar of lead, 

 is formed by dissolving litharge in vinegar. Lead carbonate, 

 PbC0 3 is the well-known and much-used white lead. 



Lead resists the action of most acids, but when exposed to the 

 air the surface is soon covered by the lead oxide, which with a 

 little moisture becomes lead hydrate, which, combined with car- 

 bon dioxide of the air, becomes lead carbonate. The salts of lead 

 are in general poisonous, and lead in all forms should be used with 

 care. Nearly all natural waters act on lead, forming dangerous 

 compounds. Lead is a constituent of several alloys, as type 

 metal, and solder; bird shot are an alloy of arsenic and lead, and 

 some bullets are an alloy of lead and antinfony, the alloys being 

 harder than lead. 



TIN (Sn) is a soft, malleable white metal, having something of 

 the appearance of silver. The principal ore of tin is the stannic 

 oxide, Sn0 2 , from which the metal is obtained by heating it with 

 coal. Tin does not tarnish in the air at ordinary temperatures, 

 and is largely used as a coating for other metals, as copper and 

 iron. It is also an ingredient of several alloys, as bronze, solder, 

 pewter and type metal. The stannic oxide, when melted with 

 caustic soda, forms the sodium stannate, Na 2 Sn0 3 . This sub- 

 stance is largely used as a mordant by calico printers. Stannous 

 and stannic chlorides are also used in calico printing. Stannic 

 sulphide, SnS 2 , is known as mosaic gold, or bronze powder. 



GOLD (Au) is a beautiful yellow metal, surpassing all others in 

 malleability and ductility. It may be beaten into sheets so thin 

 that 282,000 of them would make a pile only one inch high. 

 Gold is less affected by exposure to the air than any other metal, 

 and is used as a coating of other metals for their protection and 

 to increase their beauty. Gold is usually found free in the veins 

 of rocks, or in the sands resulting from the disintegration of the 

 rocks. It is sometimes so associated with iron, sulphur and cop- 

 per that it is very difficult of reduction. There are oxides and 

 chlorides of gold, but they are of little importance. Gold is so 



