4:70 Mr. J. EversliecFs Experiments on 



of the tube. When, however, a slow current of the undried 

 gas was allowed to impinge on the sodium vapour, by partly 

 opening the stopcock A, the absorption-line vanished and an 

 intensely brilliant but fine line appeared in its place in the 

 centre of the broad but relatively faint emission-line. This 

 sharply-defined narrow line resembled the ordinary D line 

 seen in flames. It was brighter than the continuous spectrum 

 of the glowing sides of the tube, on which it appeared to be 

 superposed. 



IV. Atmospheric nitrogen was substituted for coal-gas in 

 the gas-holders, and was freed from traces of oxygen and dried 

 before entering the heating- tube by passing over heated 

 phosphorus and through a tube of CaCl 2 . The phenomena 

 observed on volatilizing the sodium in this gas were in every 

 way the same as in coal-gas. It was subsequently found that 

 the same results could be obtained with unpurified nitrogen, or 

 even with common air, the sodium itself effecting the purifica- 

 tion almost immediately on vaporizing, producing at the 

 same time a brilliant flash — in the spectroscope a brilliant 

 but sharply-defined and narrow D line — and clouds of oxide ; 

 afterwards showing the broad hazy emission-line and the 

 central black absorption-line, when all the oxygen in the tube 

 had been consumed and the oxide had subsided. 



These results appear to me to show that impurities in the 

 neutral gases used are not concerned in the production of the 

 broad hazy emission-line, for when traces of these, particularly 

 oxygen and moisture, are known to be present and are allowed 

 to impinge on the sodium vapour, a line is seen which is fine 

 and sharp, showing that the region of chemical action is only a 

 surface-layer of no great density, whilst the fainter but broad 

 and diffuse D line, always seen when the vapour is undisturbed, 

 evidently originates at a great depth where the vapour-density 

 is considerable and in a region protected from chemical action 

 by the outer relatively cool layers giving the absorption-line. 



While this central region of the vapour may be considered 

 to be well protected by the outer layers from impurities in 

 the neutral gas employed, there still remains the possibility 

 that the porcelain tube itself reacts with the sodium through- 

 out its heated part, thus furnishing a continual supply of 

 chemical energy ; and some support is given to this view T of 

 the case from the fact that the bright line cannot be maintained 

 as a wide line indefinitely without a continual addition of 

 fresh sodium, also the tube becomes much corroded, black 

 silicon being deposited inside : thus proving a reaction between 

 the silicates of the porcelain and the sodium. 



In the experiments which follow, the effect of such reactions 

 between the tube and the sodium is eliminated by the use of 



