RADIATION IN THK SOLAR SYSTEM. 



187 



hot. T pi-cfci- tt) cull tliciii full radiiitoi-s. since llicv nidiMlc iiiot'c 

 fully than any olhcis. 



For a loiiii' time i)ast (.'xperiiiiciits ha\(' hct'ii iiiaih' to seek a law 

 coniu'ctini!,' the radiation or energy How IVoiu a black oi' fully radiat- 

 ing- surface with its temperature. P>ul it was only twenty-hve years 

 a<>-o tiiat a law was sujiiijested by Stefan which agrees at all satisfac- 

 torily with experiment. This law is that the stream of energy is 

 l)roi)ortional to the fourth power of the temperature, reckoned from 

 the absolute zero 2T:i° below freezing point on the centigrade scale. 

 This suggestion of Stefan sei-ved as the starting point of new and 

 most fertile researches, both theoretical and practical, and we are 

 o-lad to Avelcome to this meeting Professors Wien, Lummer, and 

 Rubens, who have all done most brilliant w^ork on the subject. 



Among the researches on radiation recently carried out is one by 

 Kurlbaum, in wdiich he determined the actual amount of energy 

 issuing from the black or fully radiating surface per second at 100° 

 C., and therefore at any temperature. 



Here is a table which gives the amount at xarious temperatures, 

 as determined by Kurlbaum: 



Rate ofjiow of energy from 1 vni'^ <>//»//// radiaflinj or " hldck" surface. 



Absolute temperature. 



0" 



1(X)° air boils 



300'' earth's surface. 



l,mr red heat 



3,(K)0'' arc carbon 



6,000° ... 



6,250° 



Grams of water 



heated 1° per 



secpnd. 



Calories. 

 0. 000000 



(i.(KK)iaT 



0.01(«(IO 



l(i:{. 000(100 



1,650.{KXK)00 

 L9mO(KXXK) 



As an illustration of the '• fourth power law," let us see what value 

 it w ill give us foi- the tempei-ature of the sun, assuming that lie is a 

 full radiator, or that his surface, if cooled down, would be quite 

 black. 



AVe can measure ai)])roximately the stream of energy which the 

 sun is pouring out by intercepting the beam falling on a surface 

 exposed to full sindight, lnea^ul•ing the heat given to that surface 

 per second, and then calculating what fraction the Ix'ani is of the 

 whole stream issuing from tlie sun. 



This was hrst done by Pouillet, and his method will serve to illus- 

 trate the i)rinciple of all other methods. 



In his ai)[)aratus the sunlight fell full upon a box containing 

 water, and the rate at which the water rose in temperatui'e gave the 

 energy- in the stream of solar radiation falling on the box. 



