262 Mr. W. T. David on Thermal 



Discussion of further Work on the Emission of Infra-red 

 Radiation from C0 2 and H 2 0. 



Most chemists seem to hold the view that a gas can only 

 emit radiation (infra-red as well as luminous) when it is 

 undergoing chemical or a quasi-chemical reaction* ; dis- 

 ruption of molecules or violent combination are necessary 

 according to them to excite the intra-molecular vibrations. 

 R. von Helmholtz held the view that the radiation from a 

 flame came mainly from molecules which had just been 

 formed, and which are in a state of vigorous vibration. Ac- 

 cording to this view the infra-red radiation emitted by flame, 

 like the radiation of higher frequency which gives rise to 

 luminosity, is due to violent chemical combination, and not 

 to purely thermal causes. In support of his view R. von 

 Helmholtz brings forward the experimental fact discovered 

 by him that the radiation from a flame is diminished when 

 the gas and air are heated before entering the burner, although 

 the temperature of the flame must be higher. This he ex- 

 plains as being due to the quicker approach to the state of 

 equilibrium owing to the more frequent collisions of the 

 freshly-formed compound molecules with their neighbours. 



Some experiments made by Prof. Hopkinson and the 

 writer may be mentioned here f . They measured the radia- 

 tion emitted during explosion and subsequent cooling of a 

 mixture of coal-gas and air, and found that considerable 

 radiation was emitted as the gaseous mixture cooled. The 

 radiation emitted by the hot gaseous mixture was measured 

 by means of a bolometer placed behind a fluorite window let 

 into the walls of the explosion vessel. The result of an 

 experiment made by the writer on a 10 per cent, mixture of 

 Cambridge coal-gas and air exploded in a cylindrical cast- 

 iron vessel 30 cm. in length and 30 cm. in diameter is shown 

 in fig. 2. Curve P gives the mean gas temperature deduced 

 from the pressure record (taken on a revolving photographic 

 film by means of a Hopkinson optical indicator) by means 

 of the gas equation pv = Hd after making allowance for the 

 small contraction of volume which takes place during com- 

 bustion. Curve B shows the heat received by radiation by 

 the walls of the vessel (which were painted black) per sq. cm. 

 of wall surface ; this curve was traced by a galvanometer 

 whose deflexions were proportional to the rise of temperature 

 of the bolometer. Both curves are to a time base, and were 

 simultaneously recorded on the same photographic film. 



* In which case it is ionized. 



t Hopkinson, Proc. Roy. Soc. A. vol. lxxxiv. p. 155 ; David, Phil. 

 Trans. A. vol. ccxi. p. 375. 



