472 Mr. J. Ever shed's Experiments on 



consideration that in the iron the temperature-gradient on each 

 side of the central red-hot part of the tube is much less steep 

 than is the case when porcelain or glass is used, and con- 

 sequently there is a much greater thickness of relatively cool 

 vapour through which the emission-line is seen. 



Under the conditions of these experiments, therefore, the 

 bright D line appears to be quite uninfluenced either by the 

 nature of the neutral gases used and the impurities they may 

 contain, or by the material of which the heating-tube is 

 composed: iron giving exactly the same results as porcelain. 

 It would hardly be safe, however, at this stage of the inquiry 

 to infer that chemical reactions are not concerned in the pro- 

 duction of the light ; for it would be argued that, as iron 

 becomes slightly porous at a red beat, oxygen, or at any rate 

 some of the gaseous constituents of the Bun sen flame, might 

 find their way into the tube by diffusion from outside, and in 

 this way maintain a continual reaction with the sodium vapour. 



In order to diminish the possibility of this diffusion inwards 

 affecting the results, a constant pressure of a few millimetres 

 above atmospheric pressure is maintained within the tube, 

 and if gases diffuse in at all it must be in opposition to the 

 outward diffusing hydrogen. It has been pointed out to me, 

 however, by Prof. Smithells that in dealing with the D line 

 we are dealing with a reaction that is sensitive to 180 Q q o0O 

 of a grain of sodium. It is only necessary to suppose, therefore, 

 an equivalent amount of oxygen or other reacting body to be 

 continually present in the tube to determine the D radiation. 



While I am not prepared to deny the possibility of such 

 minute traces of oxygen or other bodies constantly finding 

 their way into the middle of the sodium vapour, I consider 

 that any reactions so caused could under no circumstances 

 produce the broad ill- defined line actually observed. At the 

 most a fine double D line would be seen similar to that of a 

 flame tinted with a salt of sodium, where the density of 

 the reacting molecules is not great. Moreover, if reacting 

 bodies were diffusing in from outside the tube — the absorbing 

 layer of the sodium vapour itself forming an effectual barrier 

 in other directions — the action would be greatest in an annular 

 region in contact with the sides of the tube where the in- 

 coming molecules first encountered the sodium. This should 

 cause a brightening or widening of the D line at each end *. 

 But there is no such inequality seen : the line is quite uniform 

 in width and brightness throughout its length ; showing that 

 if chemical reactions are producing the light, the reacting 



* The D line with the optical arrangement employed represents a 

 section of the space inside the hot part of the tube. 



