66 HEAT OR CALORIC. 



Iron filings are worse conductors than an iron bar of the same 

 weight ; saw dust is worse than the solid wood. Rumford. The 

 cause probably is the intervention of air between their parts ; air being 

 a very bad conductor. 



(e.) Stones are next to metals. 



Crystalline stones conduct better than mechanical aggregates, e. g. 

 trap better than sandstone ; the difference is evident to the touch, 

 and it appears also, from their widely different power of condensing 

 the atmospherical vapor ; a trap rock will be wet from this cause, 

 while one of sandstone will be dry. 



Earth and sand conduct worse than stones. At the siege of Gib- 

 raltar, in the American war, red hot balls were carried from the fur- 

 naces to the bastions, in wooden wheelbarrows, by merely placing 

 a layer of sand beneath them. 



(/*.) Bricks are worse conductors than stones. 



Because they are full of pores containing air ; they are used to im- 

 pede the escape of heat, as in the lining of chemical furnaces of iron,* 

 which, while they are melting brass or cast iron within, can be safely 

 touched by the hand without. 



A hot brick or plank, wrapped in flannel, retains its heat a long time ; 

 it is used for warming the feet, in winter travelling, and in sickness. 



(g.) Glass is a very bad conductor. 



When thick, it cracks from sudden heating or cooling, but, if thin, 

 it bears sudden changes of temperature very well. The reason is. 

 that being a bad conductor, when one side is hot, it swells, and the 

 colder side is strained, and often gives way. 



(h.) Dry wood is a bad conductor. 



Hence, it is used for handles of metallic instruments, as of ladles, 

 soldering irons, tea and coffee pots, gridirons,f &c. It is also a bad 

 conductor of electricity. " Common bone, whale bone, ivory and 

 porcelain," are very imperfect conductors, especially when compar- 

 ed with metals. 



(i.) Charcoal is a very bad conductor. It may be held by the fin- 

 gers, within an inch or less, of the part which is red hot ; it is used 

 in wine coolers, with double sides, to prevent the entrance of heat, 

 and it is mixed with clay and other materials for bricks and crucibles, 



(/.) Feathers, silk, wool, hair, and down, are still worse con- 

 ductors. Hence they are so effectual in preserving animal heat, both 

 in the animals naturally invested with them, and in the human race 

 who wear them for clothes. They are not naturally warm, but pre- 



* And in the iron furnaces now used in this country, for burning anthracite coal. 



t Worsted, being a very bad conductor, workmen who have occasion to handle 

 substances which are either hotter or colder than is agreeable, frequently wear 

 gloves made of this substance. L. u. K. 



At Wallingford, Con. pewter tea pots are now made, with hollow metallic handles, 

 and they do not often become inconveniently hot. because they contain imprisoned air 



