CHEMISTRT. 



35 



dense bodies are generally the best conductors of caloric: 

 the metals are better conductors than wood or glass ; porous 

 bodies conduct with less facility than dense ones. Snow is 

 porous, and therefore a poor conductor of caloric, this is why 

 the ground freezes less when covered with snow, than when it 

 is naked. 



The 'different conducting power of bodies is illustrated by a 

 familiar example: on a cold winter morning we find the 

 hearthstone intensely cold to the feet, while the woolen carpet 

 is warm : now as as they are both exposed to the same tem- 

 perature, the different sensation produced must depend on the 

 different conducting power of the two bodies, the one con- 

 ducting off the heat of the body so rapidly as to produce 

 the feeling of coldness, and the other conducting but very 

 slightly. 



By specific caloric is understood, that quantity which is 

 peculiar to each body : when one body is found to possess a 

 greater amount of caloric than another of equal weight, it is 

 said to possess a greater capacity for caloric. The reason 

 why different substances possess different capacities for caloric, 

 is not precisely known. Bodies least dense appear generally 

 to possess the greatest capacity for caloric, while those more 

 dense possess the least. Hydrogen gas, the lightest of all 

 known bodies, is said to possess this capacity in the greatest 

 degree. 



When a piece of cold iron is hammered for a few minutes, 

 it becomes hot: when sulphuric acid is mixed with a liquid 

 less heavy and dense, as water or alcohol, the mixture becomes 

 hot; when ice melts and becomes water, it absorbs heat, or 

 becomes colder, as is shown by a thermometer. The heat 

 which is developed in the last case, and which was before 

 inappreciable to the senses, is called latent heat. 



All heated bodies are constantly emitting or throwing off 

 caloric; this is called radiant caloric, because it is radiated 



