236 HEAT. 



conducting powers. It will speedily melt on the cop- 

 per rod ; soon after, on the rod of iron ; glass will re- 

 quire longer time ; stone or eathenware still longer ; 

 while on a rod of wood it will scarcely melt at all. 

 These rods should he laid horizontally, that the hot air 

 rising from the sand may not affect the wax. The 

 conducting powers may be judged of likewise, with 

 considerable accuracy in cold weather, by merely plac- 

 ing the hand upon the different substances. The best 

 conductors will feel coldest, because they withdraw the 

 heat most rapidly from the hand. Iron will feel colder 

 than stone ; stone colder than brick ; wood still less so ; 

 and feathers and down least of all, although the real 

 temperature of all may be precisely the same. 



UTILITY OF THIS PRINCIPLE. 



A knowledge of this property is often very useful. 

 For instance, it is found that hard and compact kinds 

 of wood, as beach, maple, and ebony, conduct heat 

 nearly twice as rapidly as light and porous sorts like 

 pine and basswood. Hence doors and partitions made 

 of light wood make a warmer house than those that are 

 more heavy and compact. Pine or basswood would, 

 in this respect, be better than oak or ash. 



Porous substances of all kinds are the poorest con- 

 ductors ; saw-dust, for example, being much less so 

 than the wood that produced it. For this reason, saw- 

 dust has been used as a coating around the boilers of 

 locomotives to keep in the heat, and for the walls of 

 ice-houses to exclude it. Sand, filled in between the 

 double walls of a dwelling, renders it much warmer in 

 winter and cooler in summer than if sandstone were 



