THEORY OF CONDUCTION. 189 



they are bad conductors of caloric, and probably 

 worse than water, in proportion to the difference 

 between their specific gravities. 



It has also been proved in the preceding 

 chapter, that all the lightest known bodies are 

 composed of large proportions of caloric, (which 

 repels its own particles,) compared with the 

 quantity of ponderable matter that all vegeta- 

 ble and animal compounds are composed chiefly 

 of oxygen, hydrogen, carbon, and nitrogen, in 

 various ratios. 



It has been further shewn, that the attraction 

 of all bodies for caloric, increases in proportion as 

 they are deprived of it. If then it be true, that 

 the conducting power of bodies is only a modifi- 

 cation of their attraction for caloric, (a proposi- 

 tion that cannot be doubted,) it becomes obvious 

 that the conducting power of bodies is inti- 

 mately connected with the relative proportions 

 of aethereal and ponderable matter of which they 

 are composed. Other things being equal, it is 

 attracted most strongly by those bodies which 

 contain the least of it, and repelled by those 

 which are full of it. The atoms of gases being 

 surrounded by extensive atmospheres of caloric, 

 (that repels its own particles,) have very little 

 affinity for caloric, and are therefore bad con- 

 ductors. The same thing is true, though in a 

 less degree, of all the lighter liquids and solids. 



On the other hand, the atoms of the metals 



