218 WELLS'S NATUEAL PHILOSOPHY. 



conductors, and the -woolen carpet and hoarth-rug very bad conductors. 

 The action of the two substances is as follows: — As soon as the hearth- 

 stone has absorbed a portion of heat from the feet, it instantly disposes 

 of it by conducting it away, and calls for a fresh supply ; and this ac- 

 tion will continue until the stone and the foot have established an equili- 

 brium of temperature between them. The carpet and the rug also absorb 

 heat from the feet in like manner, but they conduct or convey it away so 

 slowly, that its loss is hardly perceptible. 



Most varieties of wood are bad conductors of heat ; hence, though one end 

 oi a stick is blazing, the other end may be quite cold. Cooking vessels, for 

 this reason, are often fiirnished with wooden handles, which conduct the heat 

 of the vessel too slowly to render its influx into our hands painful. For tho 

 same reason we use paper or woolen kettle- holders. 



To what extent ^^^- Bodies in the gaseous, or aeriform con- 

 bodieslonXct dition aro more imperfect conductors of heat 

 ^'^*'^ than liquids. Common air, especially, is one 



of the worst conductors of heat with which we are ac- 

 quainted. 



How is air ^97. Air is, however, readily heated hy con- 



heatedf vection. Tlius, when a portion of air by com- 



ing in contact with a heated body has heat imparted to 

 it, it expands, and becoming relatively lighter than thtr 

 other portions around it, rises upward in a current, carry^ 

 ing the heat with it ; other colder air succeeds, and (being 

 heated in a similar way) ascends also. A series of cur- 

 rents are thus formed, which are called " convective cur- 

 rents." 



In this way air, which is a bad conductor, rapidly reduces the temperature 

 of a heated substance. If the air which encases the heated substance were 

 to remain perfectly motionless, it would soon become, by contact, of the same 

 temperature as the body itself, and the withdrawal of heat would be checked : 

 but as the external air is never perfectly at rest, fresh and colder portions 

 continually replace and succeed those which have become in any degree 

 heated, and thus the abstraction of heat goes on. 



For this reason a windy day always feels colder than a calm day of the 

 same temperature, because in the former case the particles of air pass over 

 us more rapidly, and every fresh particle takes some portion of heat. 



How may the 498. Tho couductiug power of all bodies is 

 Towe'r^dies diminished by pulverizing them, or dividing 

 bedin.inishcd? i[yQ^-^ into fine filaments. 



Thus saw-dust, when not too much compressed, is one of the most perfect 



