186 RADIATION IN THE SOLAR SYSTEM. 



which there are only 400 in an inch, down to the short waves found 

 by Schumann in the radiation given off by liydrogen under the 

 influence of the electric dischar<>e, waves of which there are a quarter 

 of a million in an inch. No doubt the rano-e will be extended. 



Eadiant eneroy consists of a mixture of any or all of these wave 

 lengths, but the eye is only sensitive at the most to a little more than 

 one octave in the nine or more. 



This radiation is emitted not only by in.candescent bodies such as 

 the sun, the electric arc, or flames. All bodies are pouring out radiant, 

 energy, however hot or cold they may be. In this room we see 

 things by the radiation which they reflect from the daylight. But, 

 besides this borrowed radiation, every surface in the room is send- 

 ing out radiation of its own. Energy is pouring forth from Avails, 

 ceiling, floor, rushing about with the speed of light, striking against 

 the opi)osite surfaces, and being reflected, scattered, and absorbed. 

 And though this radiation does not affect our eyes, it is of the utmost 

 importance in keeping us warm. Could it be stopped, Ave should 

 soon be driven out by the intense cold, or remain to be frozen to 

 death. 



As the temperature of a body is raised, the stream of radiation it 

 pours out increases in quantity. But it also changes in (}uality. 

 Prol)ably the surface ahvays sends out waves of all lengths from 

 the longv^st to the shortest, but at first, Avhen it is cold, the long Avaves 

 alone ai-e appreciable. As it gets hotter, though all the AvaA^es become 

 more intense, the shorter ones increase most in intensity, and ulti- 

 mately they become so prominent that they affect our sense of sight, 

 and then Ave say that the body is red or Avhite hot. 



The quality of the stream depends on the nature of the surface, 

 some surfaces sending out more than others at the same temperature. 

 But the stream is the greatest from a surface Avhich is, Avhen cohl, 

 quite l)lack. Its blackness means that it entirely absorbs Avhate\'er 

 radiation falls upon it, and such a surface Avhen heated sends out 

 radiation of vvovy kind, and for a giA^en temperature each kind of 

 radiation is present to the full extent — that is. no surface sends out 

 more of a giA'en wave length than a black sui'face at a giA^en tempera- 

 ture. 



A very simple experiment shows that a black surface is a better 

 radiator, or poni-s out more energy Avhen hot, than a surface Avhich 

 ch)es not absorb fully, l.'ut reflects nnu-h of the radiation which falls 

 upon it. If a platinum foil Avith some black marks on it be heated 

 to redness, the marks, black Avhen cold, are much brighter than the 

 surromiding metal when hot; they are, in fact, i)ouring out nnu'h 

 more visible i-adiation than the metal. 



It is with these black surfaces that I am concerned to-day. But, 

 inasnuich as it seems absurd to call them bhick Avhen they are Avhite 



