1825.] Mr. Powell on Terrestrial Light and Heat. 365 



Incandescent iron. Distance 6 inchesi 



Exp. 2. 



(12.) For the sake of coniparison, I here again repeated the 

 observation of the light with the case : the indications were: 



Rise in the first 30 seconds . , 



Case. Case and two screens. 



(2) (1) (2) 



11 .... 8° 9 



(I) 

 8° 



(13.) The general inference from these experiments is, that 

 observing the progress of the radiation from a hot mass of metal, 

 beginning with the heat of luminosity, we tind the radiant heafe 

 increasing, and the heating power of the light distinct from it 

 increasing also, but the former in a less ratio than the latter. 



Thus it would appear that tliis same law is followed in all the 

 different cases of luminous hot bodies here considered, in propor- 

 tion to the density of the flame, to the completeness of combus- 

 tion, to the coalescing of several flames, and to the degree of 

 ignition in metal. 



The heating power of light increases in a higher ratio than the 

 simple radiant heat which accompanies it. 



(14.) The fact which I conceive is established in the first part 

 of my experiments, viz. that in the radiation from luminous 

 bodies, simple radiant heat exists distinct from the light and its 

 heat, appears to me of some importance in regard to the validity 

 of that theory which asserts that heat is merely light in a state 

 of combination. According to that theory, as the temperature 

 of a body is raised, it begins to give out the ** igneous fluid" in 

 a free radiant state : this at first is simple radiant heat, but by 

 degrees its properties and intensity become altered, and it begins 

 to act upon our organs with au illuminating efl'ect ; but is liable 



