268 



Mr. J. N. Lockyer. 



[June 19, 



described is that wliieh I have found to answer best, so far as the trials 

 have yet gone. 



D is the spectroscope. 



B is the lens used for focussing the image of the Geissler tube on 

 the slit. 



F is the spirit lamp for heating the retort. 

 H is the battery. 



K and L are the wires connected with the coil. 



In the second photograph the method of observing the spectrum of 

 the vapours close to the surface of the metal is indicated ; the same 

 letters apply, D' being however in this case a direct- vision spectro- 

 scope, which was sometimes employed for convenience. 



For determining the exact positions of the lines in the spectrum 

 of the vapour in any part of the retort, the larger spectroscope, with 

 its illuminated scale, was used in the place of the direct-vision spec- 

 troscope. 



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 capil- 

 lary 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 interference with the retort, 

 both wires being connected with the Geissler tube. If, for example, 

 the sodium 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. 



After a vacuum has been obtained the retort is heated gradually. 

 The pump almost immediately stops clicking, and in a short time be- 

 comes 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 approach to a vacuum is again ob- 

 tained. 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 spectrum con- 

 sists of hydrogen and the yellow line^of sodium, the red and green ISTa 

 line being both absent until the experiment has gone on for 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 Na lines presently make their appearance as very faint lines. 



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

 " structure " 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 



