228 OUTLINES OF NATURAL PHILOSOPHY. 



be the time in seconds, reckoned from the moment 

 when the difference of the temperature was D; 

 and if A be the remaining difference of temperature 



at the end 



of the time t, we have D (1 i Y A, 

 ^ ' 



and f- 



log n log (n 1) 



If this law is rigorously observed, no body can ever 

 perfectly acquire the temperature of the medium in 

 which it is placed, and this may be the fact, speak- 

 ing with mathematical exactness, though the dif- 

 ference of the two temperatures may soon become 

 so small as to escape observation. In respect of us, 

 the bodies are then of the same temperature. 



319- The diffusion of heat through a fluid, is 

 promoted by a hydros tatical principle, the heat 

 rarifying the fluid, and so producing a motion and 

 mixture of parts, by which the heat is communi- 

 cated more rapidly through the whole, than it 

 could be through a mass of which the parts were 

 immovable in respect of one another. 



The .communication of heat in this way, is so rapid f 

 that it renders the ordinary slow progress of acqui- 

 ring and losing heat by contact almost impercep- 

 tible, and has given rise to the error, that heat has 

 no tendency to pass through fluids, except in con- 

 sequence of the mixture of their parts. 



320. Heat 



