BOS Lord Rayleigh on the Cooling of Air by Radiation 



light transmitted through half a centimetre or a few centi- 

 metres of sodium-vapour, of the same density as the densest 

 part of the sodium-vapour in the portion of the solar atmo- 

 sphere traversed by the light analysed in any particular 

 observation. The question of temperature cannot occur except 

 in so far as the density of the vapour, and the clustering in 

 groups of atoms, or non-clustering (mist or vapour of sodium), 

 are concerned. 



§ 8. A grand inference from the experimental foundation 

 of Stokes'' and KirchhofPs original idea is that the periods 

 of molecular vibration are the same to an exceedingly minute 

 degree of accuracy through the great differences of range of 

 vibration presented in the radiant molecules of an electric 

 spark, electric arc, or flame, and in the molecules of a com- 

 paratively cool vapour or gas giving dark lines in the spectrum 

 of light transmitted through it. 



§ 9. It is much to be desired that laboratory experiments be 

 made, notwithstanding their extreme difficulty, to determine 

 the density and pressure of sodium-vapour through a wide 

 range of temperature, and the relation between density, 

 pressure, and temperature of gaseous sodium. 



XXIV. On the Cooling of Air by Radiation and Conduction, 

 and on the Propagation of Sound. By Lord Rayleigh, 

 F.R.S* 



ACCORDING to Laplace's theory of the propagation of 

 Sound the expansions (and contractions) of the air 

 are supposed to take place without transfer of heat. Many 

 years ago Sir G. Stokes f discussed the question of the 

 influence of radiation from the heated air upon the propagation 

 of sound. He showed that such small radiating power as is 

 admissible would tell rather upon the intensity than upon the 

 velocity. If x be measured in the direction of propagation, 

 the factor expressing the diminution of amplitude is e~ mx , 

 where 



m=2^1jL (1) 



In (1) 7 represents the ratio of specific heats (1*41), a is the 

 velocity of sound, and q is such that e~^ represents the law of 

 cooling by radiation of a small mass of air maintained at 

 constant volume. If t denote the time required to traverse 

 the distance x } r=-xja, and (1) may be taken to assert that 

 the amplitude falls to any fraction, e. g. one-half, of its original 



* Communicated by the Author. 



t Phil. Mag. [4] i. p. 30o, 1851 : Theory of Sound, § 247. 



