ON GASEOUS EXPLOSIONS. 203 



apart from the disturbing influence of speed of ignition, which in this 

 case was not very important, that the percentage of heat-loss would 

 rather diminish with increase of charge, because the temperature with 

 the stronger mixture should be relatively less on account of the increase 

 of volumetric heat. The increased temperature of piston and valves 

 would work in the same direction. The existence of radiation, however, 

 which increases more rapidly in proportion to the temperature, would 

 account for the increased heat-flow. The practical importance of ques- 

 tions of this kind is also illustrated by these figures, from which it 

 appears that the piston is 50 per cent, hotter, though the charge of gas 

 is only increased 30 per cent. 



More direct evidence of the importance of radiation is furnished 

 by experiments on the effect of the surface of the walls. In the second 

 report of the Committee reference was made to the belief which is 

 widely spread among those who are concerned with the practical design 

 and operation of gas-engines that polishing the interior of the combus- 

 tion chamber tends to increase efficiency. Some experiments were also 

 quoted in which it was found that lining an explosion vessel with bright 

 tinfoil perceptibly retarded the cooling of the products. More recently 

 an explosion vessel has been plated with silver on the inner surface, 

 and the results have been compared after exploding identical mix- 

 tures, first when the lining was polished, and second when it was 

 blackened over with lamp-black. It was found that by highly polishing 

 the interior of the vessel the maximum pressure reached could be 

 increased 3 per cent, and the subsequent rate of cooling during its earlier 

 stages reduced by about one-third. These experiments leave no doubt 

 of the reality and of the practical importance of radiation as a factor 

 in determining the heat-loss in the gas-engine. 1 



Eeference may also be made to the part played by radiation in 

 determining the heat-flow in a boiler. Attention was drawn to this by 

 Dalby in a recent report to the Institution of Mechanical Engineers. 2 

 The circumstances in this case are widely different from those usually 

 obtaining in the gas-engine, but the instance serves to emphasize the 

 importance to the engineer of the questions which will be discussed 

 in this report. 



Amount of the Radiation from Flame. 



E. von Helmholtz appears to have been the first to attempt the 

 accurate measurement of the radiation emitted by a flame. He found 

 that a ' solid ' flame 6 mm. diameter, burning coal-gas, radiated 

 about 5 per cent, of the total heat of combustion. A carbon monoxide 

 flame radiated about 8 per cent., and a hydrogen flame about 3 per cent. 

 On account of the smallness of the flame his experiments have not 

 ' much application to the problem of the gas-engine. The size of the 

 flame affects the matter in two ways. In the first place, a large flame 

 radiates more per unit of area than a small one, because a flame is 

 to a great extent transparent even to its own radiation, so that radiation 

 is received not only from molecules at the surface of the flame, but also 



1 Hopkinson, Proc. Boy. Soc. A., vol. lxxxiv. (1910), p. 155, 



2 Proc. Inst. Mech. Eng., October 1909. 



