Fire and Heat 95 



Into two test tubes nearly full of water put two equal sized lumps of 

 ice, one weighted with lead so that it will sink to the bottom of the 

 tube. Apply heat simultaneously to the top of the one containing 

 the weighted ice and to the bottom of the other. In which does the 

 ice melt first? What does this indicate as to the heat conductivity 

 of water? If it were as good a con- 

 ductor as iron, would the ice melt 

 in both at about the same time? If 

 heat is applied long enough, in which 

 would the water boil first? If water 

 were heated by conduction only, it 

 would boil thoroughly and just as 

 promptly no matter where heat might 

 be applied. ' 



Conductivity of substances. 

 Some substances in common use, 



i j- it, FIG. 29. Experiment to show 



arranged according to tneir con- that water conducts hea t. What 

 ductivity of heat, are here given, does this show as to its rate of con- 

 Silver is the best conductor of all 



metals, and following it in order are copper, gold, aluminum, 

 and iron ; then glass, brick, stone, water, asbestos, cotton fiber, 

 wood, paper, cork, woolen fiber, silk, fur, feathers, sawdust, 

 and air. Water is one of the best conductors of ordinary 

 liquids, an important fact in many of its uses. Air, which is 

 typical of the gases, is the poorest conductor in the list. To 

 summarize: metals are better heat conductors than ordinary 

 liquids, and liquids are superior conductors to the gases. 



Exercise : The materials of which clothing is made have a wide 

 range in their conductivity of heat. This is suggestive in selecting 

 materials for hot and cold weather. Cotton goods, for example, are 

 not only fair conductors of heat, but are also permeable to air or allow 

 free circulation. Are they better adapted to hot weather or cold 

 weather use? In extreme cold, would they check sufficiently the 

 escape of heat from the body? Materials such as woolen goods, 

 furs, feathers, and eider down hold in their minute interstices quan- 

 tities of still air. Such materials with their inclosed mass of air form 

 thus a kind of blanket only slightly permeable to air or allowing a 

 very slight circulation. When used as clothing or covering, they 



