298 THE CHEMISTRY OF THE SUN. [CHAP. 



H is the battery. 



K and L are the wires connected with the coil. 



In the second form (Fig. 100) the method of observing the 

 spectrum of the vapours close to the surface of the metal is 

 indicated. 



For determining the exact positions of the lines in the spec- 

 trum of the vapour in any part of the retort, a larger spectro- 

 scope, with its illuminated scale, was used in the place of the 

 direct-vision spectroscope. 



The secondary wires of the coil were connected, one with the 

 pole in the upper bulb at B, and the other with the platinum 

 at A. 



B is an ordinary Geissler tube with two bulbs separated by a 

 capillary tube. The great advantage of this arrangement is 

 that this capillary portion can be used for ascertaining what 

 gases or vapours are carried over by the pump without any in- 

 terference with the retort, both wires being connected with the 

 Geissler tube. If, for example, we are working with sodium 

 which contains an impurity of hydrocarbon, the moment at 

 which it begins or ceases to come off can be found by examining 

 the spectrum of this capillary tube. 



I now give an account of the phenomena observed when we 

 were working with sodium, in order to indicate the changes 

 observed. 



After a vacuum has been obtained the retort is heated gradu- 

 ally. The pump almost immediately stops clicking, and in a 

 short time becomes nearly full of hydrogen. The spectrum of 

 the capillary then shows the hydrogen lines intensely bright. 

 After some time the gas comes off far less freely, and an ap- 

 proach to a vacuum is again obtained. Another phenomenon 

 now begins to show itself: on passing the current a yellow glow 

 is seen, which gradually fills the whole space between the pole 

 in the retort and the metal ; its sjpectrum consists of the lines 

 of hydrogen and the yellow line of sodium, the red and green 



