CONDUCTION. 



35 



power, and again is non-conducting because of the air 

 between the particles. 



67. FIRE-PROOF SAFES. These are constructed on the 

 same principle, the space be- 

 tween the double walls being 

 filled with gypsum, or some 

 other non-conducting material. 

 They are used as repositories 

 of valuable papers and other 

 property, for greater security in 

 case of fire. 



68. WHY BODIES FEEL COLD OR HOT. 



Why do bodies . 



feel cold or A metallic door-knob feels colder than the 

 kot ? wood to which it is fastened, although it 



cannot actually be so. It is because the metal is the 

 best conductor, and carries off the heat of the hand 

 more rapidly. If a piece of metal and wood be placed 

 in a hot oven till both become equally hot, as they must 

 by long exposure to the same heat, the metal will feel 

 hotter than the wood. It is because the metal, by its 

 greater conducting power, supplies heat more rapidly 

 to its own surface to be taken away by the hand. 



69. SIMPLE TEST OF CONDUCTING POWER. 



Give a simple 



test for deter- Among bodies equally hot, the colder a 



Inducting 6 bod y feels j the better conductor it is. That 

 power of 'a body this is generally the case is evident from 

 the last paragraph. On applying this test, we find the 

 metallic lamp-stand, cooler, and therefore a better con- 

 ductor than the table cover on which it stands. In 

 an oven, or other place where the heat is greater than 

 that of our bodies, the test would be reversed. For 



