tlie Radiation of Heated Gases, 473 



molecules mnst be uniformly distributed throughout the 

 mass of vapour. 



The most telling argument, however, and one which, taken 

 alone, appears to me to prove beyond a doubt that the D 

 radiation under these conditions is the direct result of the 

 heating, is that derived from a comparison between the 

 emission and absorption spectra. 



To effect this comparison, the lamp and reflecting prism 

 previously described are brought into operation, and a beam 

 of white light is made to traverse one side of the heated tube, 

 so that one portion of the slit of the spectroscope is illuminated 

 by transmitted white light, whilst another contiguous portion 

 is illuminated by the D radiation alone. Under these cir- 

 cumstances the absorption and emission spectra appear side 

 by side in the field of view, and may be readily compared. 



Figs. 3 and 4 show the corresponding phases of these spectra 

 (denoted by letters of the alphabet). It will be noticed that 

 the emission-line or band in c, d, and e is represented as of 

 the same width as the absorption-band. Careful observa- 

 tion under various conditions as to density shows that, 

 excepting for the dark line in the centre, the bright D line is 

 in every respect the exact counterpart of the absorption-line, 

 whether the broad hazy band of the dense vapour is studied , 

 or the relatively narrow line seen with more attenuated vapour. 



Assuming, then, that the width of both absorption and 

 emission lines is determined by the molecular density of the 

 absorbing and emitting vapour, it follows that in the densest 

 region every molecule that is concerned in the absorption is 

 also concerned in the radiation. In other words, practically 

 every molecule in the hot part of the tube contributes its 

 share to the radiation. But it is surely impossible to sup- 

 pose that every molecule, or even a large proportion of the 

 molecules, is continually undergoing chemical change. The 

 supply of oxygen or other reacting bodies — supposing they 

 do gain access to the sodium — will never be equal to even a 

 small fraction of the demand ; also, if oxidation is proceeding, 

 one would expect to find traces of oxide forming after several 

 hours. But the heating may be continued for six hours at 

 the least without touching the apparatus, and at the end of 

 this time the D line, with its central absorption-line, is seen 

 as clearly as at the beginning, there being no trace of any 

 opaque clouds of oxide such as are always seen when traces 

 of oxygen are known to be present in the tube. 



There seems no possible alternative, therefore, to the obvious 

 and simple explanation which ascribes the radiation to heat 

 alone. If it were assumed that there exists diffused through- 

 out the sodium vapour some substance capable of setting up 



