Radiation in a Gaseous Explosion. 493 



to account for the large emission during this epoch. They 

 felt that much of this radiation must be due to thermal 

 causes. All were agreed, however, that part of the emission 

 recorded was due to chemical causes. 



3. In a previous paper * the writer examined this question 

 in some detail, and he came to the conclusion that it was 

 difficult to account tor the large emission during cooling 

 unless the gaseous mixture was capable of emitting radiation 

 in virtue of its temperature. He pointed out, however, that 

 there was no experimental work which could be regarded as- 

 proving conclusively that a gas may emit radiation in virtue of 

 its temperature alone. But on reviewing the matter recently 

 he has come to the conclusion that the experiments he made 

 with hydrogen and air mixtures t do afford fairly conclusive 

 evidence that water vapour has a pure temperature emission. 

 Chemists appear to be firmly of opinion that the chemical 

 reactions involved in the formation of H 2 molecules are 

 simple, and are completed at the moment of attainment of 

 maximum pressure, or very shortly afterwards, so that the 

 cooling of a hydrogen and air mixture after explosion would 

 appear to take place undisturbed by chemical or quasi- 

 chemical reactions. The large emission of radiation during 

 the cooling of an exploded hydrogen and air mixture would 

 therefore appear to be thermal in origin. 



4. The molecules of C0 2 are capable of vibrations of the 

 same order of frequency as those peculiar to H 2 molecules ; 

 and it is considered that what is true of the latter in this 

 respect is probably true of the former. There is certainly 

 nothing in the writer's experiments on radiation from exploded 

 gases to indicate that the C0 2 molecules behave differently 

 from H 2 molecules, except that the former radiate more 

 powerfully. (This, however, would be expected from a study 

 of the infra-red absorption spectra of both gases.) At any 

 given temperature the rate of emission of radiation from an 

 exploded hydrogen and air mixture containing about 30 per 

 cent, of steam (the remainder almost entirely nitrogen) is 

 only a little less than that from an exploded coal-gas and air 

 mixture which contains about 8*5 per cent, of C0 2j 20 per 

 cent, steam, and the remainder almost entirely nitrogen J. 



5. The radiation measured in the gaseous explosion expe- 

 riments was mainly of large wave-length (2 fi and over). 



* Phil. Mag. Feb. 1913, p. 256. 



* Phil. Trans. A. vol. ccxi. p. 391. 



% Phil. Trans. A. vol. cexi. p. 386 (fig. 9). 



