CONDUCTION. 31 



wool, cotton, fur, and feathers, slowest of all. Liquids 

 and gases, as will be hereafter seen, are non-conduct- 

 ors of heat. The superior conducting power of metals 

 is shown in the rapidity with which an iron wire, one; 

 end of which is held in the flame of a lamp, grows hot 

 at the other end. A splinter of wood, or a pipe-stem, 

 is heated from end to end much less rapidly, while 

 scarcely any heat would be communicated along a roll 

 of cotton cloth, one end of which was inflamed. 



58. ILLUSTRATION. The difference of 



How may the - m 



conducting conducting power in metals and earths may 

 alL f be ie ii- be illustrated by fastening 

 lustrated? together by a wire, as repre- 

 sented in the figure, an iron nail and a 

 bit of pipe-stem of equal length, and 

 heating them over a spirit lamp. The end of a match 

 having been fastened with thread to each, it is found 

 that the heat will travel along the nail and inflame 

 the match at its end long before the other match is ig- 

 nited. 



59. PROTECTION FROM THE CENTRAL FIRE 



How are we 



protected from OF THE EARTH. We are protected from the 

 the central heat centra i ne at of the earth by the non-con- 



of the earth ? * 



ducting power of the rocks and soil which 

 form its outer crust. So a crust forms after a time over 

 the streams of lava which flow from volcanoes ; but, 

 owing to its non-conducting power, the lava below re- 

 mains liquid for years. 



60. CONDUCTION FROM ONE BODY TO AN- 



Wh en does con- mi i Ji 



duction take THER - This takes place most rapidly 

 place most ra- the more perfect the contact between the 



pidly ? 



two. Conduction from air or a gas to a 



