The many unexplained phenomena attending the passage of electricity through gases will probably for some time to come occupy the attention of experimental physicists. It is desirable that the subject should be approached from as many different sides as possible. One of our most powerful instruments of research is the spectroscope; but before it can be applied to the study in question, we have to settle the chemical origin of the different spectra which we observe in tubes, and to discuss in what way such spectra are liable to change under different circumstances. A special investigation has to be made for each gas; we have to study the effect of various impurities, the influence of the electrodes and that of the glass, which in the tubes generally used is considerably heated up by the spark. To make the investigation complete we have to vary as much as possible the pressure, the bore of the vacuum tube, and the strength of the spark. I have chosen Oxygen as a first subject of investigation. Though Plücker and Wüllner have, as far as their experiments went, accurately described the phenomena seen in oxygen tubes, the following paper contains much that is new, and will put some of the older facts on a firmer basis. When I first began to work, it was my intention to take the gases in groups, and to study their mixture; but as the following investigation has taken me a year’s nearly continuous work, and is complete in itself, I trust it will not be found unworthy of publication. I must, of course, at present confine myself to the purely spectroscopic point of view. As several of the observations which I shall have to record bear directly on the general theory of double spectra, I must briefly refer to our knowledge on that point.