44 Of the Management of Fire 



than the wood, but incomparably less so than the pin or 

 nail of metal ; and among all the various bodies that can 

 be tried in this manner, no two of them will be found to 

 give a passage to heat through their substances with 

 exactly the same degree of facility.*' 



To confine heat is nothing more than to prevent its 

 escape out of the hot body in which it exists, and in 

 which it is required to be retained ; and this can only be 

 done by surrounding the hot body by some covering 

 composed of a substance through which heat cannot 

 pass, or through which it passes with great difficulty. If 

 a covering could be found perfectly impervious to heat, 

 there is reason to believe that a hot body, completely 

 surrounded by it, would remain hot for ever ; but we are 

 acquainted with no such substance, nor is it probable 

 that any such exists. 



Those bodies in which heat passes freely or rapidly 

 are called conductors of heat ; those in which it makes 

 its way with great difficulty or very slowly, non-conduct- 

 ors, or bad conductors of heat. The epithets, good, 

 bad, indifferent, excellent, etc., are applied indifferently 

 to conductors and to non-conductors. A good con- 

 ductor, for instance, is one in which heat passes very 

 freely ; a good non-conductor is one in which it passes 

 with great difficulty ; and an indifferent conductor may 

 likewise be called, without any impropriety, an indifferent 

 non-conductor. 



* To show the relative conducting power of the different metals, Doctor 

 Ingenhouz contrived a very pretty experiment. He took equal cylinders of the 

 different metals (being straight pieces of stout wire, drawn through the same 

 hole, and of the same length), and, dipping them into melted wax, covered them 

 with a thin coating of the wax. - He then held one end of each of these cylin- 

 ders in boiling water, and observed how far the coating of wax was melted by 

 the heat communicated through the metal, and with what celerity the heat 

 passed.. 



