UTILITY OF THE COXDUCTIXG POWER OF BODIES. 261 



affect the wax. The conducting powers may be judged 

 of, likewise, Avith considerable accuracy in cold weather, 

 by merely placing the hand upon the different substances. 

 The best conductors will feel coldest, because they with- 

 draw 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 PEIXCIPLE. 



A knowledge of this property is often very useful. For 

 instance, it is found that hard and compact kinds of wood, 

 as bead), maple, and ebony, conduct heat nearly twice as 

 rapidly as light and porous sorts, like pine and bass-wood. 

 Hence, doors and partitions made of light wood make a 

 warmer house than those that are more heavy and com- 

 pact. Pine or bass-w^ood would, in this respect, be better 

 than oak or ash. 



Porous substances of all kinds are the poorest conduct- 

 ors ; sawdust, for example, being much less so than the 

 wood that produced it. For this reason, sawdust has 

 been used as a coating around the boilers of locomotives, 

 to keep in the heat, and for the walls of ice-houses, to ex- 

 clude 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 made to fill the same 

 space. Ashes, being more porous, are found to be still 

 better. Tan, which is similar to sawdust, is well adapted 

 to filling in the walls of stables and poultry -houses, where 

 more than usual warmth in winter is required. Confined 

 air is a very poor conductor of heat ; hence the advantage 

 of double walls and double windows, provided there are 

 no crevices for the escape of the confined air. This prin- 

 ciple has been lately applied in the manufacture of hollow 

 hricJc for buildinor the walls of dwellinofs. 



