Relations of Air, Beat, and Cold. 451 



by the streaming of the heated air, for they suffer no de- 

 rancrement from the most violent agitation ot their medmm. 

 The air must therefore, without changing its place, disse- 

 minate the impressions that it receives ot heat, by a sort 

 of undulatorv commotion, or a series or alternating pulsa- 

 tions, like those by which it transmits the impulse ot 

 soum'i. The portion of air next the hot surhce, suddenly 

 acquiring heat from its vicinity, expands proportionally, 

 and benns the chain of pulsations. In again contractmg, 

 thisaenal shell surrenders its surplus heat to the one im- 

 mediately before it, and which is now in the act ot expansion ; 

 and thus the tide of heat rolls onwards, and spreads ilselt 

 on all sides. These vibratory impressions are not strictly 

 darted in radiating lines, but each successive pulse, as in the 

 case too of sound, presses to gain an equal diffusion. Dit- 

 ferent obstructions may, therefore, cause the unc|ulations 

 of heat to deflect considerably from their course. Thus, it 

 successive rings of pasteboard be fashioned into the twisted 

 form of a cornucopia, and its wide mouth presented at 

 some distance to the fire, a strong heat will, ui spite oi the 

 aradual inflection of the tube, be accumulated at Us narrow 

 end : in the same manner probably, as waves, flowing froni 

 an open bay into a narrow harbour, now contracted and 

 bent aside, yet without being reflected, rise into furious 



' ""eut the same pulsatory system will enable the atmosphere 

 to transmit likewise the impressions of cold. The shell ot 

 air adjacent to a frigid surface, becoming suddenly chilled, 

 suffers a corresponding contraction, and which must excite 

 a concatenated train of pulsations. This coniraction is 

 followed by an immediate expansion, which withdraws a 

 portion of heat from ihe next succeeding shell, itself now 

 in the act of contracting; and the tide of apparent cold, or 

 rather of deficient heat, shoots forwards, with diffusive 

 sweep. The energy of transmission is subject, in this case 

 also/to the same modifications from the nature ot the af- 

 fected surface. Thus, a troblet filled with pieces of broken- 

 ice or still better with a frigorific mixture composed ot 

 ,now and salt, will, at a moderate distance, yet excite a 

 rh.Hinir sensation; but a silver pot, filled with a similar 

 mixtu.^, will not cool the hand, till ,1 has become profusely 

 covered with dew, and therefore now presents a non-metalliQ 



"""X't the same quality by which a surface propels the 



hot or cold pulses, equally fits it, under other circumstances, 



to receive their impressions. If a vitreous surtace sends 



p f 2 loriD 



