Radiation from Hot Gases, 259 



investigated the infra-red emission spectrum of C0 2 and 

 water-vapour. He heated C0 2 in a metal tube with open 

 ends and found that between 150° and 500° C. the intensity 

 of the emitted radiation of wave-length 4'4 /m from a 7 cm. 

 thickness of gas is only a little below that of a black body 

 at the same temperature. He concludes from his experiments 

 that C0 2 and water-vapour have a true temperature emission. 

 These experiments will be criticized later. But if, for the 

 moment, we assume that Paschen's experiments do con- 

 clusively prove that these gases have a true temperature 

 emission, the theory sketched above leads us to assume that 

 the duration of molecular collisions when these gases are at 

 150° C. is short enough for the intra-molecular vibrations 

 giving rise to infra-red radiation to be excited during mole- 

 cular collisions, though the collisions are too soft at this 

 temperature to excite the higher-frequency vibrations which 

 give rise to luminous and ultra-violet radiation. 



When a gas is heated infra-red radiation is first emitted 

 (provided, of course, the internal parts of its molecules are 

 capable of executing low-frequency vibrations like those of 

 C0 2 and water-vapour *) and, according to the above theory, 

 if the gas could be raised to a sufficiently high temperature 

 (which would probably have to be much greater than that 

 which we can command in a laboratory) luminous and ultra- 

 violet radiation would be emitted. This is exactly what 

 happens in the case of a solid body when it is heated : at low 

 temperatures only infra-red radiation is emitted, but when 

 the temperature is raised the solid body begins to glow. 



In the case of a hot ionized gas containing C0 2 and water- 

 vapour (e. g., Bunsen flame) the radiation may be divided up 

 into three parts. First, the negative ions which exist at 

 thigh temperatures as free corpuscles t are retarded and 

 accelerated during collisions, and pulses will therefore be 

 emitted along the Faraday tubes attached to them. This 

 would give rise to a continuous spectrum in the way described 

 when dealing with the radiation from solid bodies. This 

 .part of the radiation would obey the Stefan-Boltzmann fourth 

 power law if the amount of ionization did not vary with the 

 temperature. But in all probability ionization is a function 

 of the temperature, and the radiation due to the retardation 

 and acceleration of the negative ions would then increase at 

 a greater rate than that given by the fourth power law. 



* Very few gases seem to emit thermal radiation. The most im- 

 Tportant, from an engineering point of view, are C0 2 and water vapour. 

 The thermal radiation from air seems to be very small. 



t J. J. Thomson, < Engineering,' April 3, 1908, p. 447. 



