148 HEAT. 



Liquors are but slight conductors of heat. 



As before mentioned, the metals are the best conduc- 

 tors of heat ; next come stony substances ; some of 

 which are, however, better conductors than others. Bricks 

 are worse conductors of heat than stones, except some 

 varieties of sand stone; brick houses, therefore, are warmer 

 in winter, and cooler in summer, than stone houses. 



Glass is a bad conductor of heat; this causes it to 

 break when suddenly heated, in consequence of one 

 surface being expanded before the other can become 

 heated so as to expand also. Cold, of course, produces 

 the same effect by contracting one surface before the 

 other can have parted with its heat. Glass tumblers 

 which have thick sides and bottoms, are very liable to 

 break when hot liquids are poured into them, for the 

 same reason. The purer the glass the less liable it is to 

 break, because it is more compact, and therefore expands 

 more equally. For the same reason crockeryware is 

 liable to be broken. Hence also, the practice of glass 

 manufacturers, who place the ware after it is blown and 

 shaped, in a hot oven to cool gradually. This is called 

 annealing. 



Dried woods and charcoal are very bad conductors of 

 heat ; hence the use of the latter in making instruments 

 for keeping provisions, &/c, in hot weather, called refri- 

 gerators. Dried bass-wood has only about four times as 

 much conducting power as water. Pitch-pine, and 

 spruce are better conductors than white pine. 



Count Rumford made numerous experiments on the 

 conducting power of the different substances which are 

 used for clothing. He found that the worst conductors 

 were hare's fur, and eider down. Next came beaver, 

 raw silk, ravellings of manufactured silk, wool, cotton, 

 and linen lint, and the more open the texture of the 

 substance, the worse conductor it was. For this reason 

 blankets are warmer than fulled cloth of the same fine- 

 ness ; eider down comfortables, than those made of cotton 

 or wool. The warmest clothing is such as has the finest 

 texture and the longest nap. All of these substances 

 have a large quantity of air combined with them, and air 

 being a bad conductor, the more air they contain among 

 their parts the warmer they are. 



There are numerous instances in the arts, where 



