In the case of the Aurora, the question naturally arises whether the oxygen of the air may be changed into ozone, perhaps also whether the nitrogen may not be modified in some similar manner. [Sidenote: Ozone destroyed by heat.] The absorption spectra of oxygen, and of the same gas in its form of ozone, may possibly differ; but this can hardly happen in the case of incandescent oxygen, for ozone is at once destroyed by heat at 300°, and slowly at 100°, and must be partially at least destroyed by the heat of the discharge. If any lines were due to ozone in such a spectrum, we should expect they would be weakened by heat and brightened by cold. [Sidenote: Ozone in a large bell-receiver not manifested in spectrum.] In the case of a continued discharge in a large exhausted bell-receiver, the presence of ozone in considerable quantities was manifested to us by its odour when the receiver was removed from the pump; but the spectrum of the stream of light did not appear to differ from that in Geissler tubes. [Sidenote: Professor Dewar demonstrates that ozone is condensed oxygen.] In a course of lectures at the Royal Institution in March 1878, on the Chemistry of the Organic World, Prof. Dewar appears to have demonstrated, by Prof. Andrews’ apparatus, that ozone is really condensed oxygen, and, further, that during this condensation heat is absorbed, which is evolved during the decomposition or re-expansion. [Sidenote: Refers to the silent discharge between the atmosphere and the earth.] He also exhibited the oxidizing power of ozone in its action on mercury, and commented on its similar action upon organic matter in forming nitrates, and on its remarkable bleaching properties, but added there was as yet no proof of its combining with free nitrogen. That peroxide of hydrogen accompanies the formation of ozone by the slow combustion of phosphorus, and that this peroxide acts with ozone in decomposing organic bodies, though in an inexplicable manner, the Professor considered to be proved. He also referred to the silent discharge probably perpetually going on between the upper and lower strata of the atmosphere, and also between these and the earth, accounting, as the Professor considered, for some of the chemical actions whereby nitrogenous compounds are formed in the soil. [Sidenote: No spectrum of ozone obtained.] As far as I am aware, no information as to a possible spectrum of ozone, or a modification of the oxygen or other spectra by its presence, has, up to the present time, been obtained[8]. [Sidenote: Suggestion to subject electric discharge to influence of cold.] It has been suggested by Mr. Procter and myself that the electric discharge in an exhausted moist tube, if subjected to a considerable degree of cold, might produce a modification of the air-spectrum, perhaps even a spectrum analogous to that of the Aurora. For some further notes on this subject see Appendix D (Aurora and Ozone). _Polarization of the Aurora Light._