448 The Fuel of the Sun, [October, 



sun may be produced by a photosphere having no greater 

 intrinsic brilliancy than the flame of a tallow candle, pro- 

 vided the flame is of sufficient depth or thickness. I see 

 good reasons for inferring that its intrinsic brilliancy is less 

 than that of a candle — somewhere between that and a 

 Bunsen's burner. 



I made a similar series of experiments upon the radiation 

 of the heat of flames through each other, and arrived at 

 similar results ; but my apparatus in these experiments was 

 not so delicate and reliable as in the experiments On light, 

 and, therefore, I cannot so decidedly affirm the absolute 

 diathermacy of flame to its own radiations. Within the 

 limits of error of these experiments, I found that* with the 

 same radiant surface presented to the thermometer, every 

 addition to the thickness of the flame produced a pro- 

 portionate increase of radiation. 



This important law, though hitherto unnoticed by philoso- 

 phers, 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 armour 

 plates and other large masses of iron are raised to a welding 

 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 quantity, in the lime 

 light we have a mere surface of great brilliancy but no 

 thickness. If I am right the surface of the moon may be 

 brighter than the luminous surface of the sun, the pecu- 

 liarities of moonlight depending upon intensity, those of 

 sunlight upon quantity of light. 



The flame that roars from the mouth of a Bessemer 

 converter has but small intrinsic brilliancy, far less than 

 that of an ordinary gas flame, as may be seen by observing 

 the thin waifs that sometimes project beyond the body of 

 the flame. Nevertheless, its radiations are so effective that 

 it is a painfully dazzling object even in the midst of sunny 

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

 by outside oxygen, but s solid body of flame several feet 



