260 



ON THE KATURE OP ttSAT, 



Motton of 

 heat. 



If the transition be sudden, powerful light and heat are oc- 

 casioned ; if it be slow and equable, a continual spring of 

 light and heat is formed. In the same manner, if the mo- 

 tion of air be rapid, a sound is produced as powerful as the 

 heat and light, in the first instance ; if the motion of the air 

 be slow and equable, the sound produced is smooth and un- 

 interrupted. 



Motion of Caloric. 



It has been said, that the best conductors of caloric re- 

 ceive and part with this power the most rapidly. This is 

 precisely what our hypothesis would have led us to expect, 

 a priori. It is to be remarked, that the action of hot and 

 cold bodies upon each other is reciprocal. The heating and 

 cooling of bodies is, according to our opinion, the same 

 operation; both are reducible to the effecting a change in 

 the state of vibration ; and different substances are suscep- 

 tible of this change in different degrees ; those which are 

 most so are the most easily heated, and the most readily 

 cooled. 



This explanation applies equally well to the absorption and 

 radiation of heat and cold ; which are perhaps greater diffi- 

 culties in the opposite opinion, than even the circumstance 

 with respect to conducting power. 



It was formerly stated, that radiant heat Is extremely dif- 

 ferent, according as it comes from the sun, or from a source 

 of heat upon Earth ; I wish however to state this difference 

 somewhat more distinctly. 

 From the sun. That the heat of the sun is transmissible through and re- 

 frangible by transparent media, is abundantly proved ; t^e 

 : refrangibility of the heat accompanying the coloured ptrt 

 of the prismatic spectrum, and of the invisible rays of so- 

 lar heat, is shown in the 13th and jyth experimentJ of 

 Dr. Herschel*; and the familiar use of burning lenses de- 

 monstrates the refraction of the caloric of the undecomposed 

 solar ray. 



On the other hand, that culinary heat is not transmi«ible 

 through any solid body is very decisively proved by thi va- 

 luable experiments contained in Mr. Leslie's third chapter: 

 see the inquiry. ' 



• PhU. Trans, fer 1800: or Journal, 4to Series, Vxrf. IV, p. ^^4, 36fi. 



/ The 



Radiant heat. 



From a fire. 



