Volatile Gases of Atmospheric Air. 157 



probable that such rajs are the outcome of some material not 

 contained in the Bath gas. A very conspicuous pair of lines 

 appears in photographs of the spectrum of the air residue, at 

 about X 3587, which is not traceable in spectrum of Bath gas. 

 The helium line, X 3587*4, is seen in the latter spectrum, but 

 is quite obscured in the former spectrum by the great intensity 

 of the new pair. This helium ray is really a close double, 

 with the less refrangible component much the weaker of the 

 two, but the new pair are wider apart, and of nearly equal 

 intensities ; this character also distinguishes them from the 

 strong argon line at X 3588*6. They are, however, very much 

 more intense at the negative pole than in the capillary, and it 

 will require a good deal more study to determine whether 

 these rays, and many others which we have not tabulated, are 

 due to the peculiarity of the stimulus at the negative pole, or 

 to the presence of a previously unrecognized material. 



As our mixture of gases probably includes some of all such 

 gases as pervade interplanetary and interstellar space, we early 

 looked in their spectra for the prominent nebular, coronal, and 

 auroral rays. Searching the spectrum about X 5007 no indica- 

 tion of any ray of about that wave-length was visible in the 

 spectrum of any one of the three tubes which had been filled 

 as above described. Turning to the other green nebular line 

 at about X 4959 we found a weak, rather diffuse line to which 

 our first measure assigned a wave-length 4958. The correct- 

 ness of this wave-length was subsequently verified by measur- 

 ing with a micrometer eye-piece the distances of the line from 

 the helium lines X 4922*1 and X 5015*7 which were in the field 

 of view at the same time. The position of the line was almost 

 identical with that of the iron spark line X 4957*8, and the con- 

 clusion arrived at was that the wave-length was a little less 

 than 4958, and that it could not be the nebular line. There 

 remained the ultra-violet line X 3727. Our photographs showed 

 a rather strong line very close to the iron spark line X 3727*8, 

 but slightly more refrangible. As the line is a tolerably strong 

 one it could be photographed with a grating spectrograph 

 along with the iron lines. This was done, and the wave-length 

 deduced from measuring the photograph was 3727*4. This is 

 too large by an amount which considerably exceeds the prob- 

 able errors of observation, and we are forced to conclude that 

 the nebular material is either absent from our tubes, or does not 

 show itself under the treatment to which it has been subjected. 



Although the residual gases of the atmosphere, uncondensed 

 at the temperature of liquid hydrogen do not show the nebular 

 lines, we found that another tube gave a ray very close indeed 

 to the principal green nebular ray. This tube had been filled 

 with gas prepared in the same way as the others, with the 



