)N GASEOUS EXPLOSIONS. 201 



characteristics of its design. Even to the Qninstructed eye the most 

 obvious features about large internal-combustion engines are the 

 arrangements for cooling, and the great size and weight for a given 

 power which is necessitated mainly by those arrangements. The diffi- 

 culties which the designer has to meet are due in the main to the stresses 

 set up by the temperature gradients which are necessary to sustain the 

 flow of heat. In the present state of the art it is probable that the most 

 important service which science could render to the gas-engine con- 

 structor would be to establish definitely the principles upon which 

 depends the heat-flow from hot gases into cold metal with which they 

 are in contact, and thus to enable him to predict the effect upon heat- 

 flow of changes in the temperature, density, or composition of the 

 charge, and in the state of the cylinder walls. 



The Committee do not propose in this report to deal with the whole 

 of this large question, but will confine their attention to one important 

 factor in heat-flow, namely radiation. The subject is a wide one, which 

 has excited much attention among physicists and chemists, and on 

 several important points agreement has not yet been reached. No 

 attempt will, therefore, be made to do more than state shortly the 

 experimental facts, and to define the issues which have been raised 

 in regard to the explanation of these facts. 



Practical Effects of Radiation. 



It is believed that the first instance in which radiation from a flame 

 was used in an industrial process, with knowledge of its importance, 

 was the re-generative glass furnace of Frederick Siemens, which he 

 described at the Iron and Steel Institute in 1884. Here the combustible 

 gas was burnt in a separate chamber and the hot products of combustion 

 were led into the furnace. The objects to be heated were placed on the 

 floor of the f urnace out of contact with the stream of flame which flowed 

 above them. They would therefore receive heat only by radiation, and 

 it was supposed that this radiation came in a large measure from the 

 flame. Siemens, however, was of opinion (in 1884) that the radiation 

 was due to incandescent particles of carbon, and that there was little 

 radiation from a non-luminous flame. 1 



In 1890 Robert von Helmholtz measured the radiation from a non- 

 luminous coal-gas flame 6 mm. diameter, and found it to be about 5 per 

 cent, of the heat of combustion. 2 The radiation from a luminous flame 

 was greater, but not very much greater — rising to a maximum of 11J 

 per cent, for an ethylene flame. Discussing the Siemens furnace in 

 the light of these results, E. von Helmholtz calculated that radiation 

 from the flame in the furnace could only account for a small fraction 

 of the actual heat transmission. He pointed out, however, that a 

 large flame would probably radiate energy at a greater rate than a 

 small one. But while admitting that for this reason gaseous radiation 



1 Captain Sankey has prepared an abstract of papers relating to the Sierrena' 

 furnaces. See Appendix C, p. 225. 



2 Die Lirht- und Wormestrahhrnrj verhrennender Gase, Robert von Helmholtz, 

 Berlin, 1890. 



