814 CONDUCTIVITY OF SOLIDS. 



and it takes therefore a considerable time before an 

 appreciable effect is produced at some distance from the 

 source of heat. 



Among the metals silver and copper are the best con- 

 ductors, lead is the worst. In general, all other inorganic 

 substances are not so good conductors of heat as metals. 

 Glass is a very bad conductor. The worst conductors 

 of all solid bodies are porous bodies, derived from 

 the animal or vegetable kingdom, such as wood, fibrous 

 substances, feathers, furs, &c. 



Bad conductors are used for preserving the tempera- 

 ture of bodies, that is, either to prevent abstraction of 

 heat by colder bodies in their neighbourhood, or acces- 

 sion of heat from without if the body is at a lower 

 temperature than those in its vicinity. Thus warm 

 clothes, furs, &c, hinder our body from losing heat to 

 the cold air by which it is surrounded in winter; houses 

 with double walls, having between them badly conduct- 

 ing materials, such as straw, sawdust, ashes, &c., are 



used for keeping ice in the summer, 

 for they prevent access of heat 

 from the air without. 



Liquids are bad conductors, 

 with the single exception of the 

 liquid metal mercury. It is only 

 FIG. 398 ( real size). by convection,- that is, by causing 

 a circulation of their particles in the manner described 

 on page 777, that the mass of a liquid can be heated 

 throughout in a comparatively short time. When 

 liquids are heated near the surface, the heated particles 

 remain at the surface, while the heavier and colder 

 particles continue at rest where they are; no circulation 



