204 REPORTS ON THE STATE OF SCtENCfi. 



from those at a depth within it. This matter will be further dealt with 

 in another section of the report. The second point is that the cooling 

 of the gas is slower in a large flame than in a small one. The 

 radiation originates in the vibration of the C0 2 and steam molecules, 

 and the life of one of these molecules as a radiating body extends 

 from the moment of its formation to the time when its vibrational energy 

 has been destroyed by radiation and by collision with colder molecules, 

 such as those of the air surrounding the flame. The smaller the flame 

 the more rapid will be the extinction of the vibrations, and the less, 

 therefore, the total amount of radiation per molecule. The products 

 of explosion in a closed vessel or in a gas-engine differ considerably in 

 this respect from any open flame, however large, which it is possible 

 to produce, for they are not subject to cooling by mixture with the 

 outside air. Moreover, the density of the gas is very much greater. 



Callendar has repeated some of Helmholtz 's experiments on a larger 

 scale, and has found that the radiation in a non-luminous coal-gas flame 

 30 mm. in diameter may amount to 15 per cent, of the whole heat of 

 combustion. Further reference will be made to Calendar's work under 

 the heading of ' transparency.' 



Hopkinson has recently made measurements of the radiation emitted 

 in the course of an explosion in a closed vessel and in the subsequent 

 cooling. A bolometer made of blackened platinum strip was placed out- 

 side a window of fluorite in the walls of the explosion vessel. The elec- 

 trical resistance of this bolometer was recorded by means of a reflecting 

 galvanometer throwing a spot of light on a revolving drum, and an 

 optical indicator traced simultaneously a record of the pressure on the 

 same drum. He found that the total heat radiated during and after 

 an explosion of a 15 per cent, mixture of coal-gas and air amounted 

 to over 22 per cent, of the whole heat of combustion. The radia- 

 tion which had been received at the moment of maximum pressure 

 amounted to 3 per cent., and it continued, though at a diminishing 

 rate, for a long period. Eadiation was still perceptible half a second 

 after maximum pressure, when the gas-temperature had fallen 

 to 1000° C. 1 



Nature and Origin of the Radiation from Flames. 



In the gas-engine cylinder and in explosion experiments we are 

 usually concerned with flames in which there is some excess of air. 

 A mixture of similar composition burnt at atmospheric pressure would 

 give an almost non-luminous flame; in the gas-engine there is more 

 luminosity on account of the greater density. There is, however, no 

 reason to suppose that the radiation in the gas-engine cylinder differs 

 materially as regards its quality or origin from that emitted by an open 

 flame. 



A very complete analysis of the radiation from different kinds of 

 flamewas made by Julius, and his experiments leave no doubt that the 

 radiation is almost wholly due to the C0 2 and steam molecules. He 



1 Proc. 7?oy. Soc. A., vol. lxxxiv. (1910), p. 155. See also Appendix B to this 



R port (p. 221). ___ .. 



