﻿certain Gases in Geissler's Tubes. 423 



Using the same induction-apparatus as in the previous expe- 

 riments, the current just began to pass through the tube filled 

 with air when the pressure was 94 millims; yet the light was 

 not continuous. A continuous passage of the current only oc- 

 curred when the pressure was diminished to 64 millinis, though 

 the luminous intensity was so small that a prismatic investiga- 

 tion of the light was not possible. On a further diminution of 

 the pressure, the brightness of the light gradually increases ; and 

 under a pressure of 46 millims. the luminous intensity is ade- 

 quate for spectrum-investigation. The less refrangible parts of 

 the spectrum in the red and yellow are barely visible ; only from 

 the green is the spectrum distinctly present ; most beautiful are 

 the violet parts, which are so characteristic of the nitrogen- 

 spectrum. 



The red and yellow parts occur first under a pressure of 30 

 millims. ; but they are so faint, that the shaded bands which 

 Plucker has described in the nitrogen-spectrum of the first order 

 are at most scarcely perceptible. The green part with its rich 

 shading stands out more ; but the blue and the violet are the 

 most beautiful ; in them the individual flutings are completely 

 developed. 



Under a. further diminution of pressure by 5 millims., red and 

 yellow come out more, and the beautiful shaded bands of the 

 complete nitrogen-spectrum are visible. Under a pressure of 

 18 millims. the spectrum is completely developed ; it quite 

 corresponds to the description which Plucker has given of it*, 

 and to what a spectrum-tube filled with pure nitrogen exhibits. 

 The brightness and beauty of the spectrum increases as the 

 pressure diminishes ; under a pressure of about 5 millims. it 

 is developed most brilliantly, and remains so until the pres- 

 sure of the gas is less than 1 millim. Only when the pres- 

 sure is so far diminished that it can scarcely be measured by 

 SprengePs pump does the brightness become less, the darker 

 parts being first extinguished, and finally only the brightest 

 parts visible. In its appearance the spectrum approximates to 

 one of the second order, without, however, changing into one, 

 for no new bright lines appear. 



With a simple RuhmkorfPs apparatus, then, only one spec- 

 trum can be exhibited in a tube filled with nitrogen ; a differ- 

 ence in density is only of influence so far, that the spectrum is 

 more or less complete and appears of greater or less brightness. 



Using, too, a Holtz's machine without superposed condenser, 

 the nitrogen-spectrum of the first order was seen as with an in- 

 duction-apparatus. Using the condenser or a small Leyden jar, 

 the spectrum of the second order described by Plucker occurred. 

 * Pliicker and Hittorf, Philosophical Transactions for 1865. 

 2F2 



