80 SCIENCE IN SHORT CHAPTERS. 



that the 100 flames thus arranged illuminate the screen 100 

 times as intensely as the single flame. Other modifications of 

 these experiments, described in Chapter 7 of " The Fuel of the 

 Sun," establish the principle that a common hydrocarbon gas 

 flame is transparent to its own radiations, or, in other words, 

 that the amount of light radiated from such a flame, and its 

 apparent intensity of luminosity, is proportionate to its thick- 

 ness ; therefore the luminosity of the sun may be produced by 

 a photosphere having no greater intrinsic brilliancy than the 

 flame of a tallow candle, provided 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. 



A similar series of experiments upon the radiation of the 

 heat of flames through each other, indicated similar results ; 

 but my apparatus for these experiments was not so delicate and 

 reliable as in the experiments on light, and, therefore, I 

 cannot so decidedly affirm the absolute diathermancy 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 thick- 

 ness of the flame produced a proportionate increase of radia- 

 tion. 



This important law, though hitherto unnoticed by philoso- 

 phers, is practically understood and acted upon by workmen 

 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 welding temperature 

 by radiant heat, and the ordinary puddling furnace, where iron 

 is melted by radiant heat. In both of these, special arrange- 

 ments 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 produc- 

 ing the radiant effect from a given surface, intensity and quan- 

 tity i.e. brilliancy and thickness in the case of light, and tem- 

 perature 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 peculiarities of moonlight 



