166 PRACTICAL LESSONS IN SCIENCE. 



water, as well as in disinfecting or deodorizing decaying animal 

 and vegetable substances. 



CHROMIUM (Cr) is found in nature chiefly as chrome iron ore, 

 Fe 2 Cr 2 4 . When this ore is heated with potassium carbonate and 

 potassium nitrate, potassium chromate is formed. And this salt 

 treated with nitric acid yields potassium dichromate, K 2 Cr. 2 O 7 , 

 the most important form of chromium. It occurs as beautiful 

 red tabular crystals, which dissolve in about ten parts of water. 

 It is sometimes used in electrical batteries, and is a good oxidiz- 

 ing agent. Lead chromate is the chrome yellow used in painting 

 and calico printing. 



BISMUTH (Bi) is a grayish brittle metal of crystalline form. 

 It is used in some alloys as type metal, giving them the property 

 of expanding slightly in the mold so that they are forced into 

 the finest lines. Two parts of bismuth, one of lead and one of 

 tin forms an alloy which melts below the temperature of boiling 

 water. The nitrate of bismuth is used in medicine. 



LEAD (Pb) is a soft grayish metal which is useful in many 

 ways. Its most common form in nature is the sulphide which 

 occurs in connection with sulphides of copper, iron, zinc and 

 silver, with quartz, barium sulphate, calciun fluroide and some- 

 times with bismuth and antimony. Sulphide of lead or galena 

 crystallizes in cubes, having a bright metallic luster. Lead is 

 obtained from galena by heating it with iron, when the latter 

 combines with the sulphur and the lead is set free. Or the sul- 

 phide is roasted in the air until some of it is converted into lead 

 oxide and the lead sulphate, then heated without the access of 

 air when the lead and sulphur are set free as represented in the 

 following equations : PbS-f 2PbO=3Pb-f SO 2 and PbS+PbS0 4 = 

 2Pb-f-2S0 4 . 



Lead is malleable, and in the form of sheets is used to line 

 tanks, cover roofs, make pipes, etc. Lead oxide, PbO is litharge 

 which heated in the air becomes red lead, Pb 3 4 . The red lead 

 is used in storage batteries, where it takes up oxygen becoming 

 lead peroxide Pb0 2 . The red oxide is used as a paint, and in the 

 manufacture of glass and matches and as a glaze for pottery. 



