Radiation from Hot Gases. 261 



The hot walls of the metal tube emit radiation of all wave- 

 lengths, and some of this is absorbed by the C0 2 — viz. that 

 of wave-length in the neighbourhood of 2*8 //,, 4*4 //,, and 

 14' 1 fji. This absorption by the gas goes primarily to excite 

 the low-frequency vibrations of its molecules corresponding 

 to radiation of these wave-lengths, and these vibrations 

 cause radiation of the same wave-lengths to be emitted in 

 all directions. Some of this will reach the bolometer. 



To make this clear imagine a molecule of C0 2 at some 

 point P within the cone of gas seen by the bolometer. 

 Radiation from all parts of the interior surface of: the hot 

 metal tube passes through P, but none of this radiation can 

 reach the bolometer at B because the aperture at A was 

 made small enough to prevent any part of the tube surface 

 being seen by the bolometer. The internal parts of the C0 2 

 molecule at P will be set in vibration owing to the absorption 

 of 2'8 fi, 4*4 fi, and 14*1 jx radiation. These intra-molecuiar 

 vibrations give rise to the emission of radiation in all direc- 

 tions *. Some of this obviously will reach the bolometer. 



Paschen also found the emission from C0 2 just as it issued 

 from a heated platinum tube, but in this case also the emission 

 by the hot 00 2 may have been due to absorption of radiation 

 from the hot solid body (the platinum tube) near it. It is 

 not, therefore, possible to decide from these experiments 

 whether the intra-molecular vibrations, which give rise to 

 the emission by the hot C0 2 , are excited by molecular col- 

 lisions or by absorption of radiation from the hot metal walls. 

 TyndalPs experiments on the emission from adiabatically 

 ■compressed C0 2 t gi ye perhaps more satisfactory information 

 •on this point. The C0 2 was compressed in a glass tube with its 

 ends closed by means of two rock-salt plates, and the emission 

 measured by means of a thermopile. But even here it is 

 •conceivable that the emission by the gas is due to its absorbing 

 radiation from the glass tube J, which^ being a bad con- 

 ductor of heat, would have its interior surface heated up 

 very quickly to a temperature nearly equal to that of the 

 gas, especially as it was in turbulent motion during 

 compression. 



* Wood has detected lateral emission from sodium vapour when a 

 beam of light from a sodium flame is sent through it. Also cf. curves 

 in fig. 14 of paper " Radiation in Explosions of Coal-gas and Air," 

 Phil. Trans. A. ccxi. p. 396. Lateral emission will explain why curve A 

 lies above the curve C, and the B above the D. 



t See Poynting and Thomson's 'Heat,' p. 236 (1908 edition). 



| Glass is a good radiator in the infra-red region. 



