474 Mr. J. Evershed's Experiments on 



chemical reactions, it might indeed be imagined that a kind 

 of cyclical process of alternate combination and dissociation 

 takes place, the energy supplying the radiation being derived 

 indirectly from the heated walls of the tube : these alternate 

 changes being determined by differences of temperature in 

 the reacting molecules, the cooler combining and the hotter 

 dissociating, or vice versa. But it is practically certain that 

 under the conditions of these experiments there must always 

 be a large excess of sodium molecules over any others likely 

 to produce such reactions, unless, indeed, hydrogen or nitrogen 

 were to behave in this way towards sodium, and the spectro- 

 scopic evidence, as just explained, implies that all the free 

 sodium molecules are concerned in the radiation. 



Or perhaps it may be further argued in support of the 

 u chemical " origin of the radiation, that the sodium molecule 

 is itself undergoing alternate dissociation and recombination. 

 The reasons already given against this view in the case of 

 iodine apply even more forcibly in this instance. Thus, if the 

 radiation were due to such action, the intensity should follow 

 the curve representing the change of relative vapour-density 

 with temperature, rising to a maximum at the turning-point 

 in this curve — indicating the greatest interaction between the 

 atoms and the molecules — but falling away to zero when the 

 point of complete dissociation is reached. Now the most 

 reliable recent determinations of the vapour-density of sodium 

 indicate that at about the temperature of melting cast-iron 

 the vapour is entirely monatomic * : therefore, if we assume 

 that at the lowest temperature of my experiments the 

 atoms are more or less aggregated, the relative intensity of 

 the D line, compared with the continuous spectrum of the 

 glowing sides of the tube, should change as the temperature 

 is increased, — it should get brighter or fainter according 

 as the actual temperature of dissociation is above or below 

 the initial temperature of my experiments ; and it should 

 cease altogether if the temperature be raised to the point 

 where the vapour becomes entirely monatomic. 



But as a matter of fact there is no such change of relative 

 intensity. As the temperature is increased from the point 

 where the radiation begins to be seen, the D line follows 

 strictly the continuous spectrum of the glowing tube : from 

 the lowest to the highest temperature (a range of some 300 0. 

 degrees), the gaseous radiation increases in intensity exactly 

 in correspondence with the radiation from the solid, always 

 keeping the same intensity (so far as the eye can judge) as 

 the spectrum of the glowing tube. 



* A. Scott, Proc. Roy. Soc. Edinb. xiv. y. 410. 



