150 AGRICULTURAL CHEMISTRY 



Some of the zinc salts are used in the preparation of 

 paints, while the metal itself is used in many ways as in 

 the preparation of alloys, solder, and galvanized iron. 



193. Galvanized Iron. Iron is galvanized by being cov- 

 ered with a layer of zinc. Galvanized iron is extensively 

 used for water pipes because it does not rust so readily as 

 ordinary iron. When heated, however, the zinc coating 

 of the galvanized iron is removed. 



194. Occurrence of Tin. Tin is found in nature largely 

 in the form of the oxid, SnO 2 , and, to a less extent, in 

 combination with other metals. The oxid is heated in a 

 furnace with charcoal, and the molten tin cast into bars. 



195. Tin Salts. Tin forms two series of salts, stan- 

 nous and stannic. In the former, the element is bivalent, 

 and in the latter, it is tetravalent. Stannous and stannic 

 chlorid, the sulfid, oxid and hydroxid are among the 

 more common tin salts. They are used in the arts in 

 various ways as pigments and as mordants in the coloring 

 of cloth. Tin forms a number of alloys and is extensively 

 used for roofing and other purposes. Ordinary tinware 

 is simply iron-coated with a layer of tin. 



196. Occurrence of Lead. Lead is found principally 

 in the form of sulfid (galena). It is also found in combi- 

 nation with silver and other metals, and, in the process 

 of refining of silver, is separated as a by-product. 



197. Oxids of Lead. There are four oxids of lead, 

 namely : lead monoxid, PbO, lead peroxid, PbO 2 , lead 

 suboxid, Pb 2 O, and lead sesquioxid, Pb 2 O 3 . The sub- 



