266 Mr. W. T. David on Thermal 



(absorption), 4*3 fi at 600 c C, 4'388 ^ at 1000° C, and 4*4/* 

 at the temperature of the Bunsen flame. Coblentz has con- 

 firmed this, and finds that in the arc the maximum goes up 

 to 4*55 p. This shift is most peculiar, and Coblentz com- 

 menting on it says : " Since the absorption band of C0 2 is 

 at 4*3 fji and that of CO is at 4*6 fi, if the emission band is 

 due to a pure thermal effect, then one would expect to find 

 that with rise of temperature and the consequent dissociation 

 of C0 2 into CO the maximum of the emission band of C0 2 

 will shift towards that of CO." It is interesting to notice 

 in this connexion that the maxima of the emission bands of 

 both CO and C0 2 are at the same point, viz. 4*75 /jl, in a 

 vacuum-tube*. Coblentz believes that this is due to the 

 dissociation of C0 2 . It may be mentioned that the intensity 

 of the CO emission band from the vacuum-tube is much 

 greater than that of C0 2 for all pressures. 



R. von Helmholtz made some very interesting experiments 

 on the radiation from hydrogen, CO, marsh-gas, ethylene, 

 and coal-gas flames. He found that C0 2 produced in the 

 CO flame emitted about 2*4 times as strongly as an equal 

 volume of water- vapour produced in a hydrogen flame, and 

 shows that this ratio is preserved in the other flames whose 

 products of combustion consist of C0 2 and water-vapour. 

 The flames in these experiments were just rendered non- 

 luminous by adjusting the air supply, and the temperatures 

 of all of them were probably pretty much the same. A 

 rough analysis of the radiation from a mixture of coal-gas 

 and air after explosion made by the writer supports this 

 result. He measured the emission from identical gaseous 

 mixtures first through a fluorite window, and secondly through 

 a quartz window f. The fluorite transmits practically all the 

 radiation emitted by the hot gaseous mixture, which contained 

 about 8*5 per cent, of C0 2 and 20 per cent, of water-vapour,, 

 while the quartz cuts off practically all the radiation from 

 C0 2 and transmits about 65 per cent, of that from water- 

 vapour. The radiation from the gaseous mixture transmitted 

 by the quartz window was about 50 per cent, of that trans- 

 mitted by the fluorite, and the C0 2 therefore emitted about 

 twice as strongly as an equal volume of water-vapour at the 

 same temperature. 



Theoretical. 



Some experiments made by the writer show that the 

 intrinsic radiance from thicknesses of gas containing the 



* Coblentz, ' Investigations of Infra-red Spectra,' Part II. p. 313. 

 t Phil. Trans. A. vol. ccxi. p. 388. 



