﻿certain Gases in Geissler's Tubes. 413 



(3) Secon d brightest line of the third group, con-l^ 90 90 

 sisting of two bright lines J 



(4) First bright line of the fourth group, consist- 1 „„ ac* 9* 

 ing of two very closely adjacent lines J 



(5) Middle line of the fifth group, consisting of "1 



three bright lines : this line is the brightest, \> 64 22 20 

 and is more than a slit in breadth , . J 



(6) Middle bright line of a group of at least sixl „, qq. ac\ 

 individual lines J 



(7) H /3 still faintly visible 64 51 10 



These groups, as is evident, corresponding to the green co- 

 lour of light, all lie in the green part of the spectrum ; in the 

 red and yellow part there is nothing to be seen. Besides these 

 measured groups, at the boundary of the green towards the yellow 

 a feebly bright part was seen, between the first and second groups 

 two feebly bright lines, and between the fourth and fifth groups 

 about three faint lines. About as far to the right of H (3 as 

 the sixth group is to the left of ft, there is a feebly bright line, 

 too obscure, however, to be measured. Then at about 65 c 20' 

 there is a faint blue field bounded on both sides by two beauti- 

 fully shaded bright bands; and behind this, after a perfectly 

 dark space about half as broad as the field just mentioned, there 

 is a faint field of considerable breadth ; at times there is in the 

 neighbourhood of H 7, at 67° 10', a faint lustre. 



This spectrum occurs whenever the gas in the tube has attained 

 the extreme degree of rarefaction attainable with a SprengePs 

 pump. The resistance in the tube is here so great that the posi- 

 tive electrode becomes quite incandescent, bends, and appears to 

 consist of a series of fused globules. That this deformation at 

 the same time seriously hinders the passage of the current, as 

 before mentioned, follows from the fact that on its entrance the 

 current no longer started from the point of the electrode of 

 aluminium wire, but from the part of it which lay against the 

 platinum wire melted into the glass of the tube, where such a 

 fusion could not be perceived. 



If the extreme rarefaction which furnishes the spectrum just 

 described is maintained for some time with closed stopcocks, the 

 light of the tube again assumes a white colour and again shows 

 the continuous spectrum, the reddish-yellow part is again seen, 

 the six groups of lines again disappear, and the green appears once 

 more. But the density of the gas in the tube is not changed ; 

 for if the stopcock be opened which connects the tube with the 

 air-pump, the position of the mercury remains quite unchanged. 

 Notwithstanding this, renewed pumping again produces the line- 

 spectrum. 



Another means of again evoking the line-spectrum is the si- 



