446 APPLIED SCIENCE 



of tin by smelting. Because of its high price and its low 

 tensile strength (about two tons per square inch), tin is com- 

 paratively little used. As one of the constituents of gun-metal 

 or bronze, however, it is of great value. Since it is not acted 

 upon by salts and weak acids in the cold nor by animal or 

 vegetable juices and also since it resists oxidation, tin is often 

 applied to other metals as a protective covering. 



In making tin plate, sheet iron is thoroughly cleaned by 

 acid baths, then greased with melted tallow or palm oil to 

 prevent contact with the air, and dipped in a bath of molten 

 tin. A layer of palm oil or other grease also covers the melted 

 tin. After the bath a thin layer of tin sticks to the sheet 

 iron. With this addition the sheet iron is passed through 

 rollers to squeeze off superfluous metal and perfect the coating. 

 This plate is made into tin cans to hold oil, paint, fruit, 

 vegetables, fish, etc. It is also used for roofing and for the 

 manufacture of kitchen utensils. 



518. Uses and Chemical Properties of Lead. Lead is 

 seldom found in a pure state, but usually as the carbonate 

 (PbCO 3 ), the sulphate (PbSO 4 ), or the sulphide galena 

 (PbS). To obtain lead it is necessary to free the ore of its 

 combining elements. The processes of stamping, washing, 

 and smelting are used for this purpose. 



The ore put into the furnace should consist of from five 

 to eight different kinds, as a mixture of ores produces better 

 lead. This charge is first roasted to oxidize the sulphur or 

 arsenic contained in the ore to sulphur dioxide and arsenic 

 oxide. After the remaining ore has been fused, a layer of 

 slag or refuse forms to the depth of two or three inches. The 

 slag is drawn off, and then the molten lead is allowed to 

 run into a pan where it is skimmed and ladled into molds. 



