When the induction spark is taken in oxygen, a faint line is seen nearly in the position of the nitrogen line 1718. Since the lines of oxygen have a diminished intensity when the spark passes in air, this line would be too faint to be distinctly observed in the air spectrum, in which it occurs in a position of close proximity to brighter lines of nitrogen.

LECTURE IV.

Mode of obtaining the Spectra of Gases and other Non-metallic Bodies. Plücker and Hittorf.-Huggins.-Influence of Change of Density and of Temperature. - Kirchhoff. - Frankland and Lockyer. Variation of the Spectra of certain Metals with Temperature. Spectra of Compounds.-Double Spectra.-Ångström's Conclusions. Spectrum of the Bessemer Flame. Selective Absorption. Blood Bands.-Detection of Colouring Matters.— Phosphorescence.-Fluorescence.

APPENDIX A.-Description of the Spectra of the Gases and Nonmetallic Elements.-Can an Element possess more than one Spectrum?-On the Spectrum of Nitrogen.

APPENDIX B.-On the Effect of increased Temperature upon the Nature of the Light emitted by the Vapour of certain Metals or Metallic Compounds.

APPENDIX C.-Kirchhoff on the Variation of the Spectra of certain Elements.

APPENDIX D.-Ignited Gases under certain circumstances give continuous Spectra.-Combustion of Hydrogen in Oxygen under great pressure.

APPENDIX E.-On the Spectrum of the Bessemer Flame.
APPENDIX F.-On the Spectra of Erbium and Didymium Compounds.
APPENDIX G.-Description of the Micro-Spectroscope.

WE saw in the last lecture that the light of the electric spark consists of rays emitted from the incandescent materials, first of the poles from which the discharge passes, and in the second place of the air or gas surrounding those poles; and I would remind you that Ångström was the first who pointed out that the ordinary spark thus yields a double spectrum. If the density of the