CONDUCTION OF HEAT. 25 



which have been subsequently published.* Their method was 

 to transmit known quantities of the gases, heated to 212 in an 

 uniform current, through a serpentine tube, surrounded by 

 water, the temperature of which was observed, by a delicate 

 thermometer at the beginning and end of the process. Their 

 results are as follows : 



Specific heat of gases referred to water. 



specific heat. specific beat, 



same weight. same weight. 



Water 1000 Nitrogen 275 



Air 26? Nitrous oxide 23J 



Hydrogen 3294 Olefiant gas 421 



Carbonic acid 221 Carbonic oxide 288 



Oxygen 236 Steam 847 



It will be observed, that the capacity for heat of steam is less 

 than that of an equal weight of water. Hence, the specific heat 

 of a body may change with its physical state. Delaroche and 

 Berard likewise observed that the capacity of a gas is increased 

 by its rarefaction. When the volume of a gas is doubled, by 

 withdrawing half the pressure upon it, its specific heat is not 

 quite so much as doubled. This is the reason why a gas be- 

 comes cold in expanding. In the expanded state it requires 

 more heat to sustain it at its former temperature, from the 

 augmentation which has occurred in its capacity. Air expanded 

 into double its volume is cooled 40 or 45 degrees ; and it has its 

 temperature raised to that extent by compression into half its 

 volume ; suddenly condensed to one-fifth of its volume by a 

 piston in a small brass cylinder, so much heat is evolved as to 

 cause the ignition of a readily inflammable substance, such as 

 tinder. 



COMMUNICATION OF HEAT BY CONDUCTION AND RADIATION. 



I. Conduction. When one extremity of a bar of iron is 

 plunged into a fire, the heat passes through the bar in a gradual 

 manner, being communicated from particle to particle, and after 

 passing through the whole length of the bar, may arrive at the 

 other extremity. Heat, when conveyed in this way, is said to 

 be conducted. 



* Armales de Chitnie, t. 75 ; or Annals of Philosophy, vol. ii. 



'.-. V . 



