468 ON IMPRESSIONS OF COLD 



The progress of heat through different bodies, depends- 

 on their peculiar constitution, and varies extremely in its rate. 

 To examine this phenomenon more closely, it will be proper 

 to distinguish the conducting media into the three classes of 

 — solid — liquid — and gaseous, 



1. In the case of a solid conductor, if one end of a rod be 

 heated, the effect will gradually advance, yet not with a con- 

 tinuous flow, to the other end. If the rod were conceived to 

 be distinguished into elementary spaces, each of these in the 

 progress of communication must experience, within imper- 

 ceptible limits, an alternate accession and reduction of heat, 

 and must consequently first expand and then contract. In 

 fact, it absolutely could not receive and deliver heat at the 

 same instant of time, for a contemporaneous expansion and 

 contraction — the necessary result — would imply an- absurdity. 

 But contraction naturally succeeds to expansion, as a part of 

 the same vibratory impression. Those elementary conducting 

 spaces, must therefore undergo an alternating hot and cold 

 fit j so that the communication of heat through a solid sub- 

 stance, is performed by a series of minute articulate motions, 

 resembling the progressive vermicular action by which the 

 creeping insects are enabled to advance along the ground. 

 The celerity of the transfer of heat through any solid is hence 

 determined by the extent of such articulations, and the rapid 

 succession of the internal oscillations ; which again depends on 

 the elasticity of the conducting substance, modified by the 

 influence of its density. 



2. In the communication of Heat through liquids, an auxili- 

 ary principle, originating in the internal mobility of the me- 

 dium, concurs to accelerate its progress. The warmed portions 

 of the fluid, acquiring expansion, rise upwards by their conse- 

 quent 



