Reflections on Heat. 169 



periments which have been performed in order to de- 

 termine the nature of heat, I will limit myself in this 

 memoir to some of the principal results of these re- 

 searches. 



A very remarkable phenomenon, and one which must 

 have been noticed as soon as men had any acquaintance 

 with fire, is the radiation from solid bodies as soon as 

 they become very warm. 



When a solid body a bar of iron, for example is 

 at about the temperature of the surrounding air, we 

 do not see or perceive anything which indicates that it 

 possesses a radiating surface ; but if we heat it strongly 

 in the glowing fire of a forge, the body changes color, 

 becomes at first red, then white, is visible in the dark, 

 lights up surrounding objects, and warms in a sensible 

 degree all objects which are struck by the rays which 

 it emits in all directions. 



If we allow it to cool slowly in the undisturbed air 

 of a dark room, we see that it changes color again ; from 

 white it becomes red, then a darker red; the light which 

 it gives forth gradually diminishes ; the intensity of its 

 calorific rays becomes less at the same time, and soon 

 it ceases to shed light round about it. 



It continues, however, to emit from its surface calo- 

 rific rays for some time after it has ceased to be lumi- 

 nous, as may be perceived by holding the hand near it. 



The calorific rays which very warm bodies send off 

 from their surfaces pass through the transparent air 

 without heating it, nor do they heat sensibly those 

 bodies at whose surfaces they are reflected. 



These very important facts, which ought not to be 

 forgotten, have been established by the results of a 

 large number of experiments. 



