That the Aurora has two spectra.
1st. That the Aurora has two different spectra—the one comprising the one bright line in the yellow-green only, and the other the remaining fainter lines.
That bright line does not coincide with HC₂.
2ndly. That the bright line falls within a group of hydrocarbon lines, but does not actually coincide with any prominent line of such group, and that Dr. Vogel’s finding this line to coincide with a not well-marked band in the air-spectrum must be regarded as a case of accidental coincidence.
That moisture is nil in Aurora region.
3rdly. That moisture in the region of the Aurora must be regarded as nil, and that oxygen and hydrogen must alone there act as conductors of electricity.
Ångström’s flask-experiment described.
Professor Ångström then details the examination of an exhausted dry air-flask filled with a discharge analogous to the glow of the negative pole of a vacuum air-tube.
Flask-spectrum compared with Aurora-spectrum.
The experiment is described as follows:—“Into a flask, the bottom of which is covered with a layer of phosphoric anhydride, the platinum wires are introduced, and the air is pumped out to a tension of only a few millimetres. If the inductive current of a Ruhmkorff coil be sent through the flask, the whole flask will be filled, as it were, with a violet light, which otherwise only proceeds from the negative pole, and from both electrodes a spectrum is obtained composed chiefly of shaded violet bands.” The comparison of the spectrum of this violet glow with that of the Aurora gives, according to Ångström, the following results:—