xxi.] , RESULTS WITH SODIUM. 299 



lines being both absent until the experiment has gone on foi 

 some time. 



- As the distillation goes on, the yellow glow increases in 

 brilliancy, and extends to a greater distance above the pole, and 

 the red and green lines presently make their appearance as very 

 faint lines. * .' 



The upper boundary of the yellow is quite sharp, the lines 

 and fluted spectrum of hydrogen appearing above it. 



After the yellow glow-giving vapour (which does not attack 

 the glass) has been visible for some time, the pump is stopped 

 and the metal heated more strongly. On passing the current a 

 little while afterwards, a very brilliant leaf-green vapour is seen 

 underlying the yellow one, and connected with it by a sap-green 

 vapour. The spectra then visible in the tube at the same time 

 are 



Leaf -green ...Green and red lines of sodium and c of hydrogen ; 



D absent. 

 Sap-green ...Green, red, and yellow sodium lines of equal 



brilliancy, and c of hydrogen. 

 Yellow ...p alone and c. 



Bluish-green. ...c and F and hydrogen structure. 



To observe the green sodium line alone it is necessary to 

 point the direct-vision spectroscope just above the surface of 

 the metal where the green is strongest. It is also necessary to 

 guard against internal reflections from the glass, as this may 

 sometimes cause the D line to be seen by reflection from the 

 surface. 



This method of inquiry has been tried also with potassium, 

 calcium, and some other metals, and with metallic salts. 



With potassium and calcium we get the same inversion of 

 phenomena ; the yello \v-green lines of potassium being seen 

 without the red; while in the case of calcium the blue line 

 alone was seen. We get, as before mentioned, vapours which 



