Rarefied air saturated with aqueous vapour.
| Screw. | Wave-length. | Remarks. | ||
|---|---|---|---|---|
| 3·97 | 6562 | Moderately bright line. | ||
| (5·88) | 5892 | Bright double line (Na). | ||
| (6·25) | 5789 | Bright line (H). | ||
| From | 7·03 | 5591 | Broad dull band of light; near 7·03 a somewhat brighter line. | |
| to | 7·55 | 5470 | ||
| 7·59 | 5461 | Bright line (H). | ||
| 8·72 | 5231 | Dull stripe. | ||
| 8·96 | 5187 | Broad misty stripe. | On a dull steady ground. | |
| 10·07 | 5002 | Faint line. | ||
| 11·05 | 4859 | Very bright line. | ||
| 12·21 | 4709 | Moderately bright line. | On dimly lighted ground, becoming fainter towards the violet. | |
| 12·75 | 4644 | Line fainter than the preceding. | ||
| 13·28 | 5585 | Very faint line (H). | ||
| (15·71) | 4358 | Very bright line (H). | ||
| 15·90 | 4341 | ” ” | ||
Here follow several more lines.
In the first observations, the electric spark, about 1 centim. in length, was allowed to pass between platinum points in ordinary air.
The sodium-line near 5·88 appeared continually. The bright double line at 10·03 and 10·07, with a weaker current or longer spark, was no longer to be recognized as a double line, but appeared as a broad somewhat confused line, of which the brightest part was near 10·05. No lines belonging to the platinum spectrum appeared. Ordinary rarefied air, under a pressure of 25 to 30 millims., and which was enclosed by mercury in a tube 8 millims. wide, showed exactly the same lines as Plücker’s nitrogen-tube (b), except that some lines belonging to the spectrum of mercury also appeared.
This observation may be regarded as a confirmation of the conjecture above expressed as to the condition of Plücker’s tube III. (nitrogen). In the observations described under b, the air saturated with aqueous vapour was under a pressure of 22 millims. Besides the sodium-lines, lines of the mercury-spectrum appeared at 6·25, 7·59, and 15·71. The spectrum of rarefied air under similar pressure was found to accord completely with the spectrum of the light in the broad part of Plücker’s tube.
III. (Nitrogen b.)—A comparison of the spectrum of rarefied air saturated with aqueous vapour with the former shows the striking alterations in the spectrum which are brought about by the presence of the aqueous vapour.
3. Comparison of the Aurora-Spectrum with the Spectra of Atmospheric Gases and of Inorganic Substances.
In the next place, I turn to the comparison of the observed spectra of different gases and of the air with the spectrum of the Aurora. The first band of light in the red part of the Aurora-spectrum most probably coincides with the first system of lines in the spectrum of nitrogen (a). Probably only the bright part of this group of lines is perceptible, on account of the extreme faintness of the Aurora; and as in nitrogen the increase of the brilliancy of the spectrum takes place towards the violet end, the absence of the intermediate spectrum towards this direction would find its explanation. The most intense line of the Aurora-spectrum at 7·12 is to be also found in the spectrum of nitrogen (a)—as a very faint line, however. That this line appears in the Aurora by itself, and with intensity relatively great, need not appear strange, considering the great alteration of the gas-spectra under different conditions of pressure and temperature. The third line of the Aurora-spectrum, very vaguely defined on account of its great faintness, coincides in the same way with a nitrogen-line.