THE FUEL OF THE SUN. 23 



these experiments, I found that with the same radiant sur- 

 face presented to the thermometer, every addition to the 

 thickness of the flame produced a proportionate increase of 

 radiation. 



This important law, though hitherto unnoticed by philo- 

 sophers, is practically understood and acted upon by work- 

 men who are engaged in furnace operations. Present space 

 will not permit me to illustrate this by examples, but in 

 passing I may mention the "mill furnaces," where armor- 

 plates and other large masses of iron are raised to a weld- 

 ing temperature by radiant heat, and the ordinary puddling 

 furnace, where iron is melted by radiant heat. In both of 

 these special arrangements are made to obtain a "body" or 

 thickness of radiant flame, while intensity of combustion 

 is neglected and even carefully avoided. 



According to this there are two factors engaged in pro- 

 ducing the radiant effect from a given surface, intensity^ 

 and quantity, i. e., brilliancy and thickness in the case of 

 light, and temperature and thickness in the case of heat. 

 In the Bude light, for example, consisting of concentric 

 rings of coal-gas, we have small intensity with great quan- 

 tity, in the lime-light we have a mere surface of great bril- 

 liancy but no thickness. If I am right, the surface of the 

 moon maybe brighter than the luminous surface of the sun, 

 the peculiarities of moonlight depending upon intensity, 

 those of sunlight upon quantity <3f light. 



Thcflamo that roars from the mouth of a Bessemer con- 

 verter luis but small intrinsic brilliancy, far less than that 

 of fin ordinary gas flnme, as may be seen by observing the 

 thin waifs that sometimes project beyond the body of tho 

 flame. Nevertheless, its radiations are so effective that it 

 is i painfully dazzling object even in the midst of sunny 

 daylight; but then we have here not a holloAv flame fed only 

 by outside oxygen, but a solid body of flame several feet in 

 thickness. Even the pallid carbonic acid flame which ac- 

 companies the pouring of the spiegeleisen has marvellous, 

 illuminating power. 



The reader will now be able to understand my explana- 

 tion of the sun-spots, of their nucleus, umbra, and penum- 

 bra. From what I have stated respecting the planetary 



