EMISSION OF LIGHT AND HEAT. 61 



any such radiance, that there can not be any. It is con- 

 ceivable, certainly, that the two kinds of radiance may be 

 absorbed and afterward emitted together, and that the 

 hand is much more sensitive to the one kind than the eye 

 is to the other ; in other words, that the radiance, acting 

 as heat, will produce the sensation of warmth in the nerves 

 of feeling while its intensity is yet low, and yet will not, 

 acting as light, produce the sensation of vision in the nerves 

 of sight until its intensity is very high. 



However this may be in the case of radiance of low in- 

 tensity, we know that, when the radiance is of high in- 

 tensity, the heat and light bear a very intimate relation to 

 each other, so much so that the degree of heat is expressed 

 often by the kind and intensity of the light that is emitted. 

 Blacksmiths and machinists say "red hot" and "white hot" 

 to indicate different degrees of temperature, and also "cher- 

 ry-red" and a "low red in the dark." 



There is a curious difference between solids and gases 

 at high temperatures in respect to their power of emitting 

 light. When any substance is so intensely hot that it emits 

 bright light, we say that it is incandescent / when a gas is 

 incandescent, it forms flame. Young persons, often, in look- 

 ing at a flickering flame blazing up from the fire, or at that 

 rising from a candle or a lamp, wonder what it is. Now it 

 is simply incandescent gas a kind of inflammable air called 

 hydrogen gas, which, in burning that is, in combining with 

 the oxygen of the air is heated to such a degree as to be- 

 come incandescent. 



Burning is simply a chemical action. It is usually the 

 combining of some combustible with the chemical sub- 

 stance called oxygen. There are certain very curious con- 

 siderations connected with the fact that so much heat is 

 developed by the combination of oxygen with combustible 

 substances, but I have not space to explain them here. All 



