The observations in the different parts of the tube show plainly the dependence of the spectrum on the temperature. The aura of the negative pole gives the line near 10·07 so characteristic of the air-spectrum. This is the same line which is met with in the spectra of most of the nebulæ. The very striking groups of lines in the red and yellow in the spectrum of the narrow part of the tube disappear entirely in the wide part. If we compare the spectra with those above quoted, of oxygen and hydrogen, we find line Hβ very faint in the spectrum of the narrow part of the tube near 11·03; on the other hand, oxygen-lines appear in the broad part near 8·20, 8·94, and 14·07. Thence I would conjecture that the tube was not filled with pure nitrogen, the appearance of which is precise, but with dry rarefied air, since Wüllner’s researches have proved that dry air yields the same spectrum as nitrogen gas. Perhaps the air in the tube examined by me had not been thoroughly dried, and thus the appearance of some lines of the elements before named is to be explained. I must further mention that the electrodes of the tubes consisted of aluminium; yet a comparison of the spectra observed and the aluminium spectrum has shown no connexion between them. IV. _Atmospheric Air._ +-----------+------------+------------------------------------------+ | Screw. |Wave-length.| Remarks. | +-----------+------------+------------------------------------------+ | 5·88 | 5892 |Very bright double line (Na). | | 6·67 | 5680 |Very bright line. | | 7·20 | 5550 |Faint line. | | 9·00 | 5180 |Very bright line. | | 9·79 | 5047 |Fine faint line. | | 10·03 | 5008 |} Very bright double line. | | 10·07 | 5002 |} | | 11·43 | 4803 |Faint confused line. | | 12·69 | 4651 |} | | 12·84 | 4633 |} Faint line not sharply defined. | | 13·04 | 4612 |} | | | | | |From 14·61 | 4453 |} Confused band of light, which ends with | |to 15·88 | 4444 |} a broad washy line. | +-----------+------------+------------------------------------------+ Here follow several other lines. Rarefied air saturated with aqueous vapour.