﻿certain Gases in Geissler's Tubes. 419 



of a. Separated from this by a dark space, a green field is 

 seen, sharply defined on the left, and gradually shading off on the 

 right, among the yellowish-green lines which appear under a 

 pressure of 20 millims. 



On the right of the bright-green line at 63° 28', and sepa- 

 rated from this by a dark space, there is a splendid green field 

 consisting of several beautifully shaded bands. It has a simi- 

 larity to the violet part of the nitrogen-spectrum, inasmuch as its 

 individual parts are brightest and most sharply defined on the 

 left, while on the right they gradually shade off. The field ex- 

 tends to the third of the above-mentioned three faint green 

 lines. Then follows a beautifully shaded blue field, the left limit 

 of which is at about 64° 56', which continues in strongly shaded 

 parts into the violet. 



On continuing the rarefaction, when the pressure can no longer 

 be measured the character of the spectrum suddenly changes, 

 just in the manner described for hydrogen : the continuously 

 illuminated fields disappear ; and in their places, or near them, 

 splendid groups of lines stand out. These perfectly sharp 

 bright lines on a dark ground lie preferably in the green and 

 blue, corresponding to the bluish- green colour of the light 

 which the tube emits. 



The spectrum observed does not agree with that described by 

 Pliicker, and drawn on plate 2 in the Philosophical Transactions 

 for 1865 ; for the groups of lines are in other positions than 

 those given by Pliicker ; and where Pliicker draws entire groups 

 of lines, nothing or only individual lines are met with. The 

 spectrum shows five groups of lines, the first two of which are 

 the brightest and broadest. The first of these groups is just in 

 the middle between the positions corresponding to H a and H /3 

 of hydrogen, and extends from 63° 11' to 63° 20'. The second 

 group (the broadest of all) extends from 63° 48' to 64° 9'; it 

 thus lies about in the middle between the first group and H @. 

 The third group has a breadth of 6', its middle is at 64° 42'. 

 Then follows, separated by a feebly bright field, a narrow group 

 consisting of five lines, the right limit of which is at 65° 4'. The 

 fifth, very narrow group is at the beginning of the blue, at 65° 40'. 

 In the violet only three lines appear, at 66° 44', 67° 2', 67° 8' 30"; 

 and then a very faint line at 67° 36', which bounds the spec- 

 trum on the most refrangible side. 



10. The phenomena described are seen in the above order, if 

 the current of the small induction-apparatus is passed through 

 the spectrum-tube filled with oxygen. In that case the last- 

 mentioned spectrum is obtained when, after the appearance of the 

 continuous spectrum, it is attempted still further to exhaust the 

 tube. The line-spectrum is more easily obtained by adopting 



