128 THE CHEMISTRY OF THE SUN. [CHAP. 



It happened, however, that in the Geissler tubes employed 

 when the molecules of the gas were most agitated so as to give 

 the phenomena of great pressure, the current might be held to 

 give us the highest temperature. It was therefore of importance 

 to eliminate electrical effects altogether. This was accomplished 

 by volatilising a piece of sodium in a glass tube in an atmo- 

 sphere of hydrogen and observing its absorption spectrum. The 

 experiments were conducted as follows : l 



(1.) Into a piece of hard glass combustion-tube, thoroughly 

 cleaned and closed at one end, a few pieces of metallic sodium, 

 clean and as free as possible from naphtha, were introduced. The 

 end of the tube was then drawn out and connected with a 

 Sprengel pump and exhausted as rapidly as possible. Hydrogen 

 was then admitted, and the tube re-exhausted, and when the 

 pressure was again reduced to a few millimetres, carefully sealed 

 up. The tube thus prepared was placed between the slit plate 

 of a spectroscope and a source of light giving a continuous 

 spectrum. 



Generally, unless the atmosphere of the laboratory was very 

 still and free from dust, the two bright D lines could be 

 distinctly seen on the background of the bright continuous 

 spectrum. 



The tube containing the sodium was then heated with a 

 Bunsen flame and the spectrum carefully watched. Soon after 

 the application of the heat, a dark line, thin and delicate as a 

 spider's thread, was observed to be slowly creeping down each 

 of the bright sodium lines and exactly occupying the centre of 

 each. Next, this thin black line was observed to thicken at the 

 top, where the spectrum of the lower denser vapours was ob- 

 served, and to advance downwards along the D line, until, arriving 

 at the bottom, they both became black throughout ; and if now 

 the heat was still applied, thus increasing the density of the 

 various layers of the sodium vapour, the lines began to broaden 



1 Phil. Trans. 1872, p. 253. 



