186 RADIATION IN THE SOLAR SYSTEM. 



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

 ])j Schumann in the radiation given off by hydrogen under the 

 influence of the electric discharge, waves of which there are a quarter 

 of a million in an inch. No doubt the range will be extended. 



Eadiant energy consists of a mixture of any or all of these waA'e 

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

 one octane in the nine or moi'e. 



This radiation is emitted not only by incandescent bodies such as 

 the sun, the electric arc, or flames. All bodies are ])onring out radiant 

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

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

 besides this borrowed I'adiation, every surface in the room is send- 

 ing out radiation of its oAvn. Energv is jjouring forth from Avails, 

 (eiling, flooi-, rushing about with the speed of light, striking against 

 the opposite sui'faces, and being reflected, scattered, and absorbed. 

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

 importance in kee|)ing us Avarm. Could it be stopped, Ave should 

 soon l)e driven out by the intense cohl, or remain to be frozen to 

 death. 



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

 lX)urs out increases in ([uantity. liiit it also changes in quality. 

 Probably the surface ahvays sends out Avaves of all lengths from 

 the lono-est to the shortest, l)ut at first, Avhen it is cold, the long AvaA^es 

 alone are appreciable. As it gets hotter, though all the Avaves become 

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

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

 and then Ave say that the bod}" is red or Avhite hot. 



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

 some surfaces sending out more than oth(>rs at the same temperature. 

 But the sti'eam is the gi'eatest from a surface which is, Avhen cold, 

 (juitc black. Its l)hickness means that it entirely absorbs Avhatever 

 racHation falls upon it, and sucli a surface Avhen heated seiuls out 

 radiation of every kind, and for a given tem]:)erature each kind of 

 i-adinlion is })i'esent to tlie full extent — that is. no sui'face sends out 

 more of a given wsnc h'ligth than a bhu'k surface at a giv(Mi tempera- 

 ture. 



A A^ery simpU* experiment sliows that a bhick surface is a better 

 radiator, oi' pours out moi-e energv Avhen hot, than a surface Avhich 

 does not ahsorb fully, but i-eflects nuich of the radiation Avhicli falls 

 u|)()n it. If a phUininn foil with some black marks on it be heated 

 to redni'ss, the marks, black when cold, ai'e much brighter than the 

 surrounding metal when hot; they are. in fact, iK)uring out much 

 more visible radiation than the mt'tal. 



It is Avith these black sui'faci's that 1 am concerned to-day. But, 

 inasmuch as it seems absin-cl to call them black Avhen thej^ are white 



