On the Spectrum of the more Volatile Gases of Air. 471 



time. The position of the line was almost identical with that of the 

 ii'on spark line X 4957 '8, and the conclusion 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 A. 3727. Our 

 photographs showed a rather strong line very close to the iron sp;urk 

 line A 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 probable 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 treat- 

 ment to which it has been subjected. 



Although the residual gases of the atmosphere uncondensed at tha 

 temperature of liquid hydrogen do not show the nebular lines, we 

 found that another lube gave a ray very close indeed to the principal 

 green nebular ray. This tube had been tilled with gas prepared in che 

 same way as the others, with the exception that, in passing from the 

 vessel into which the first fraction of liquid air was distilled, it was not 

 passed through a U-tube immersed in liquid hydrogen on its way to 

 the exhausted tube. In consequence it contained traces of nitrogen 

 and argon, and when sparked showed the spectra of these elements as 

 well as those of hydrogen, helium, &c. The nitrogen spectrum dis- 

 appeared after some sparking, but the tube still shows rays of argon 

 as well as those of the gases in the other tubes. On examining 

 the spectrum of the negative pole in the neighbourhood of the prin- 

 cipal nebular green ray, a weak ray was seen in addition to those given 

 by the other tubes. It was found by comparison with the nitrogen 

 rays A 5002-7 and X 5005-7 to be a little less refrangible than the 

 latter of these rays, and by measuring its distance from the nitrogen 

 rays and from the two helium rays A 4922-1 and A 5015*7 with a 

 micrometer eye-piece, the wave-length A 5007 '7 for the new ray was 

 deduced. This looks as if we might find the substance which is 

 luminous in nebula? to be really present in the earth's atmosphere, 

 and we hope shortly to be able to verify the observation of it. 



Turning to the coronal rays, our tubes emit a "weak ray at about 

 A 5304. This is not far from the wave-length A 5303-7 assigned by Sir 

 N. Lockyer to the green coronal ray. It is, however, greater than 

 that assigned by Campbell, namely, 5303'26.* Other lines observed 

 by us near the places of coronal lines are at wave-lengths about 4687, 

 4570, 4358, 4323, 4232, 4220, 3985, 3800. These are all weak lines 

 except that at A 4232, which is of tolerable strength, and that at 

 A 4220, which is rather a strong line. The wave-lengths 4323, 4232, 

 4220, and 3800 come very close to those assigned to coronal rays, but 

 * ' As.troph. J.,' vol. 10, p. 190. 



