ON THB NATURi. OF HEAT. QSSt 



opaque*. It is from this circumstance, that opaque bodies 

 only are heated by the sun's rays; in intercepting th*tp» 

 they receive the heating power, the vibration of these rays* 

 Such bodies act upon solar heat somewhat in the same 

 manner ns all bodies act upon culinary heat. 



From this view of the subject it appears, that air is a very Air a quick 

 quick conductor of heat; Berthollet has remarked this j^^^^^ 

 factf* nevertheless air is employed in the arts as a bad 

 conductor : this circumstance requires some explanation. 



Different bodies are susceptible^ in difterent, degrees, of Conductinj^ 

 undergoing a change in their vibration ; and, having suffered P°^^''='^ ^^ ^^' 

 a change in their vibration, they convey this change to dis- 

 tant parts with different degrees of celerity, Caeteris pari- 

 bus, those bodies, which are most susceptible of change in 

 vibration, induce the least change in other bodies; and, coe- 

 teris paribus, those bodies, whiciiconvey the changes thej may 

 have suffered with most celerity, produce the greatest change 

 in other bodies. Thus the conductor, which occasions the 

 greatest change in temperature, is that vvhicb.unites the proper- 

 ties of celerity of conducting power and little susceptibility of 

 change in vibration : and thus, although air conducts vibra- 

 tion with much celerity, yet, from its high susceptibility of 

 change in vibration, its effect in augmenting or reducing the 

 temperature of bodies is by no means great. It appears 

 that the terms good and bad conductors are involved in some 

 ambiguity. 



Radiation of Cold. Radiation of 



This phenomenon appears to me to be the most decisive cold. 

 in demonstrating the true nature of caloric; it deserves per- 

 haps the appellation of experimentura crucis. Effect of a con. 



A concave mirror has the property of concentrating the cave mirror. 

 rays of vibration proceeding from a source properly opposed 

 to it. In a similaf manner the vibrations of air constituting 

 sound are converged in an eliptical chamber. The opera- 

 tion of mirrors does not however increase the intensity of the 



* It is necessary to remark, that I have considered the theory of light 

 of Huygens and Euler as the most probable j a few observations on this 

 subject may probably at some future time be tiansmitted to the Philoso- 

 phical Journal. 



t Murray, Vol, J, p. 374. 



vibration 



