RADIATIOlSr IN THE SOLAR SYSTEM. 



187 



hot, I prefer to call thciii full radiators, sina* thev radiate inoi-c 

 fully than any others. 



For a ]()n<i' time past ex]M'riineiits hav(> been made to seek a law 

 connecting the radiation or energy flow from a black or fully radiat- 

 ing surface with its temperature. But it was only twenty-five years 

 ago that a law was suggested l)y 8tefan which agrees at all satisfac- 

 torily with experin)ent. This law is that the stream of energy is 

 proi^ortional to the foui'th power of th<» temj)erature, reckoned from 

 the absolute zero 27)'° below freezing ])()int on the centigrade scale. 

 This suggestion of Stefan served as the starting })oint of new and 

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

 glad to welcome to this meeting Profc^ssors Wien, Lunnner, and 

 Rubens, who have all done most brilliant work on the subject. ■ 



Among the researches on radiation recently carried out is one by 

 Kurlbaum, in which he determined the actual amount of energy 

 issuing from the black or fully radiating sui'face \)vy second at 100° 

 C, and therefore at any temi)eratur('. 



Here is a table which gives the amount at \arious tvnn)eratures, 

 as determined bv Kui'lbaum: 



Rate ofjioir of energy front, 1 cm- of full if radintinxf or '' hhtek " snrfdce. 



Al)si)lute tf^mpei'ature. 



0" 



UK)" air boils 



I^H)" earth's surfar 

 l,a)()" red heat .-. 

 3,(KK)'' arc carbon. 



6,0I1()" 



6,250° 



Grams of water 



heated 1"^ per 



second. 



Ciihirics. 



(I.IHKIKK) 

 D.IKIUL'iT 

 0.01(18(10 

 1.2T0(HIO 



103. ooonoo 



l,(i!'>l).(KXKJOU 

 l,'.tH().0;K)l)O0 



As an illustration of the '" fourth ])owe]' law," let us see what value 

 it will give us for the temperature of the sun, assuming that he is a 

 iidl radiator, oi' that his surface, if cooled down, would be (juite 

 black. 



A\'e can nieasui'e appro.ximately the stream of energ\- which the 

 sun is ])ouring out by intercepting the beam falling on a suid'ace 

 exi)osed to full sunlight, measui'ing the heat gixcn to that surface 

 per second, and then calcidating what fraction the beam is ol' the 

 whole stream issuing from the sun. 



This was first done by Poiiillel, and his method w ill sei've to illus- 

 trate the pi'iiicij)le of all other methods. 



In his api)aratus the smdight fell full upon a box containing 

 water, and tlie rate at which the water rose in temperature ga\-e the 

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



