128 ANSWERS TO PRACTICAL QUESTIONS 



33. In welding iron the surfaces to be joined are some- 

 times sprinkled with sand. Explain. 



The silica acts as a flux with the oxide upon the sur- 

 face and lays bare the metal for welding. 



34. What is the difference between an alloy and an amal- 

 gam ? 



An amalgam is composed of mercury and some other 

 metal. An alloy consists of any metals whatever. 



35. Steel -articles are blued to protect from rusting, by 

 heating in a sand-bath. Explain. 



A thin coating of oxide is. formed on the surface of the 

 metal. 



36. Give the rational formula for copperas and white lead. 



1. FeSO 4 = FeO,SO 3 . 



2. PbCO 3 = PbO,CO 2 . 



37. Why is Hg used for filling thermometers! 



(See Philosophy \ page 235.) 



Because it is fluid at all ordinary temperatures. 



38. What oxide is formed by the combustion 0/"Na, K, 

 Zn, S, Fe, Pb, Cu, P, etc. ? Which are bases ? Acids ? 

 Give the common name of each. 



(i). Na 2 O is formed when Na oxidizes in dry air, or 

 oxygen at a low temperature. This takes up water with 

 great avidity, forming HNaO (NaHO), sodium hydroxide. 

 Na 2 O 2 is made when Na is heated to 200 C. HNaO is 

 the caustic soda of commerce, and is an alkaline base. 



