1915] on Gassous Explosions 29D 



with the addition of air from the atmosphere, the flame had oxygen 

 more than sufficient for complete combustion, and, consequently, 

 it was short. The radiation here was 10*5 per cent of the total heat 

 of combustion. The radiation attained a maximum at 2*5 air to one 

 volume gas of about 16 per cent. This was exceeded, however, 

 as the total radiation without air was 17 per cent. For a wide range 

 of mixture the intrinsic radiance or the radiation emitted per 

 unit area of surface varied but little. Intrinsic radiance varies with 

 the temperature of the flame and the depth of it. It is usually 

 assumed to vary with temperature according to the fourth power of 

 the temperature, but Professor Callendar states that this has been 

 criticized, and the law of increase may be less. Callendar made 

 further experiments with six similar Meker burners, and found a 

 great increase in radiation due to the depth of flame from W'hich it 

 proceeded. The radiation observed, for instance, with one flame 

 deep was 68 units per square centimetre. With six flames deep it 

 rose to 282. The flames were quite similar and almost homogeneous. 

 Callendar says that " it will be observed that the flame is surprisingly 

 transparent to its own radiation. It is very commonly assumed that 

 because a flame absorbs precisely those radiations which it emits, and 

 absorbs them in the same proportion as it emits them, the flame would 

 therefore be practically opaque to its own radiation, so that the 

 radiation proceeding from the interior of a mass of homogeneous 

 flame might be neglected, and the total radiation assumed propor- 

 tional to the surface. The best observations show that this is very 

 far from being the case, owing to the relatively wide separation of 

 the radiating and absorbing molecules." From these experiments 

 Callendar finds that an infinite thickness of radiating gas under 

 the same conditions as the Meker flame described would give an 

 intrinsic radiation of 473 units per square centimetre ; and the 

 experiments show that with a thickness of 100 centimetres (about 

 40 inches) of such flame the intrinsic radiance has reached within 

 less than half per cent of this limit. One hundred centimetres at 

 atmospheric pressure corresponds to about five centimetres, or two 

 inches, thick at twenty atmospheres ; so that at the pressures attained 

 in gas engine cylinders, the limit of radiation is nearly reached in a 

 sphere or combustion space of two inches diameter. From this it 

 follows, as Callendar clearly shows, that in combustion chambers of 

 all practical dimensions, using the same temperature and nature of 

 gases, the radiation loss per unit surface is equal. In reasoning from 

 these experiments Professor Callendar comes to interesting con- 

 clusions as to the distribution of heat loss in time in the combustion 

 chambers of engines. 



Hopkinson prepared a cylindrical cast-iron explosion vessel of 

 30 cm. diameter by 30 cm. long (11*8 in. by 11*8 in.), the whole of 

 the interior surface of which was plated with silver ; and he compared 

 the result of exploding a mixture containing 15 per cent of Cam- 



