and the Economy of Fuel. 45 



Those bodies which are the worst conductors, or 

 rather the best non-conductors of heat, are best adapted 

 for forming coverings for confining heat. 



All the metals are remarkably good conductors of 

 heat ; wood, and in general all light, dry, and spongy 

 bodies are non-conductors. Glass, though a very hard 

 and compact body, is a non-conductor. Mercury, water, 

 and liquids of all kinds, are conductors ; but air, and in 

 general all elastic fluids, steam even not excepted, are 

 non-conductors. 



Some experiments which I have lately made, and 

 which have not yet been published, have induced me to 

 suspect that water, mercury, and all other non-elastic 

 fluids, do not permit heat to pass through them from 

 particle to particle, as it undoubtedly passes through 

 solid bodies, but that their apparent conducting powers 

 depend essentially upon the extreme mobility of their 

 parts; in short, that they rather transport heat than 

 allow it a passage. But I will not anticipate a subject 

 which I propose to treat more fully at some future 

 period. 



The conducting power of any solid body in one solid 

 mass is much greater than that of the same body 

 reduced to a powder, or divided into many smaller 

 pieces. An iron bar, or an iron plate, for instance, is a 

 much better conductor of heat than iron filings ; and 

 sawdust is a better non-conductor than wood. Dry 

 wood-ashes is a better non-conductor than either ; and 

 very dry charcoal reduced to a fine powder is one of the 

 best non-conductors known ; and as charcoal is perfectly 

 incombustible when confined in a space where fresh air 

 can have no access, it is admirably well calculated for 

 forming a barrier for confining heat, where the heat to 

 be confined is intense. 



