CHAP, in.] ON THE CONDUCTIBILITY OF HEAT. 



363 



which is a metal tube, further surrounded by a metallic network or 

 trellis. The air required for combustion enters laterally, by openings 



FIG. 248. Section of one of Combe's lamps. 



situated below the flame, which it reaches after having passed two 

 thicknesses of metallic meshes, 



IV. VARIOUS DOMESTIC APPLICATIONS OF HEAT. 



Certain customs based on the feeble conducting power of certain 

 substances are well worth a passing reference. 



Why are the hands of tools and utensils that we put on the 

 fire made of wood ? Why do we cover the handles of kettles with 

 cane, or flat irons with leather or cloth? Because wood, leather, 

 and cloth are very feeble conductors. It is for the same reason that 

 these substances feel warmer than the metals, or than marble, when 

 they are all in the same neighbourhood, where all the objects have one 

 and the same temperature (less than that of the hand). The floor of a 

 room does not feel so cold .as the pavement, because the wood conducts 

 heat less than brick, and a floor of pine feels less cold than one of oak 

 for the same reason. In all these examples the contact of the hand 



