CONDUCTION 



97 



Poor Conductors. Water is a poor conductor of heat, as may be 

 shown by nearly filling a test tube with water, and, with a piece of 

 wire, fastening some ice at the bottom of the tube. Heat the 

 test tube near the top. Soon the water in the top will boil, while 

 ice remains in the bottom of the tube. You will then have boiling 

 water at one end and ice water at the other. Carefully notice on 

 what part of the side you heated the tube. Prove that water con- 

 ducts heat very poorly. 



Air is another poor conductor, especially dry, dead air. By 

 dead air we mean air not in motion. Damp air does not conduct 



01 



FIG. 74. 



FIG. 75. 



heat very rapidly unless in motion. Often a person may remain 

 in damp, still air without any great discomfort, after the moisture 

 in contact with the body has been heated. 



Water under a bridge will not freeze for some time after the 

 water in the open has frozen. Shawls with many openings filled with 

 dead air are very warm. Birds often ruffle up their feathers on a cold 

 day to get plenty of dead air among the spaces between the feathers. 



To prevent the air from circulating, charcoal, sawdust, and 

 other porous substances are often loosely packed into spaces between 

 walls. Leather, paper, fur, felt, woolen cloth, animal wool and 

 many other solids are poor conductors. 



Why is the porcelain enamel lining used at A? Fig. 76. 



Why is the inside lining at B made of wood? 



