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. 
J. 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 ; 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 
