Radiation from Hot Gases, 269 



Conclusion. 



In this paper the writer has briefly reviewed and criticized 

 the experimental work which has been done on the emission 

 of thermal radiation from hot gases. Probable theories of 

 the origin of radiation from gases have also been dealt with. 

 The question as to whether a hot gas can emit radiation in 

 virtue of its temperature alone has been rather fully dis- 

 cussed, and, although it is impossible to settle this question 

 definitely until further experimental work has been done, it 

 seems highly probable that the low-frequency vibrations of 

 0O 2 and water- vapour giving rise to infra-red radiation are 

 excited by molecular collisions. Paschen's experiments on 

 the emission from hot C0 2 and water-vapour have been 

 looked upon as affording conclusive evidence that molecular 

 collisions excite the low-frequency intra-molecular vibrations 

 of these gases, but the writer shows that in these experiments 

 the absorption of radiation from the hot tube walls might be 

 wholly responsible for the exciting of these vibrations. The 

 writer believes that this question could be definitely settled 

 by experiments such as the following : — 



(i.) By measuring the emission (if any) from a gas like 

 C0 2 heated by adiabatic compression during com- 

 pression and subsequent cooling. Many of the ex- 

 periments made by the writer* on the radiation 

 emitted in explosions of coal-gas and air might with 

 advantage be repeated, imitating the explosion by 

 adiabatic compression ; 



(ii.) By measuring the rise of pressure (if any) or tempe- 

 rature of C0 2 when it is absorbing radiation at a 

 known ratef. 



Some experiments by Prof. Hopkinson and the writer are 

 of interest in this connexion. Hopkinson finds a greater 

 maximum pressure and a slower rate of cooling of a gaseous 

 mixture after explosion when the walls of the explosion 

 vessel are reflecting than when they are black |; and the 

 writer finds that the radiation emitted from a certain thickness 

 of the gaseous mixture is greater when the walls of the vessel 

 are reflecting than when they are black §. The most natural 

 interpretation of these experiments is that the radiation 



* See Phil. Trans. A. vol. ccxi. pp. 375-410. 



t The air is warmed by sunlight, but air has no marked absorption 

 bauds, and in this case it is believed that the transfer of energy is direct 

 from the aether to the translational degrees of freedom owing to the 

 pressure of radiation. (See Jeans's ' Dynamical Theory of Gases,' p. 212.) 



X Proc. Rov. Soc. A. yol. lxxxiv. p. 155. 



§ Phil. Trans. A. vol. ccxi. p. 394. 



