28 CALORIC. 



will also boil with much less heat on lofty moun- 

 tains than in the valleys. 



Bodies differ very much with respect to the fa- 

 cility with which heat passes through them. Those 

 which transmit caloric easily are called conductors 

 of caloric ; and, according to the power of doing 

 so, they are termed good or bad conductors. 

 Those which do not transmit heat at all, or with 

 great difficulty, are called non-conductors. How- 

 ever, it should be observed, that, perhaps, no 

 substances are absolutely non-conductors ; but 

 liquids and gases admit the passage of caloric 

 through them with such great difficulty, that, for 

 practical purposes, this division is found useful. 

 Liquids carry heat chiefly by transportation ; the 

 part of the liquid that is heated rises to the sur- 

 face, and gives place to another which is warmed 

 in its turn, and so on until the whole has been 

 heated. 



Many very important applications of this prin- 

 ciple have been made by Count Rumford to ceco- 

 nomical purposes. He showed that a stratum of 

 confined air was one of the best modes of prevent- 

 ing the escape of heat. 



The best conductors of heat are metals, and the 

 best non-conductors are fluids and porous sub- 

 stances. Charcoal is an excellent non-conductor. 



Heat may be excited by mere friction ; and this, 

 probably, was the earliest mode of obtaining it to 

 procure fire. It is still practised among uncivilized 

 nations. For this purpose they take two pieces of 

 dry wood, one about eight or nine inches long, 

 and the other piece quite flat. They cut a blunt 

 point upon the first, and, pressing it upon the 

 other, they whirl it round very quickly, holding it 



