The spectra in the range 300 Å to 1400 Å, obtained from the vacuum sliding spark, indicate that, at ringing frequencies of 50 kc/s and lower when using tungsten, aluminium or steel electrodes, the insulator is the main source of ions in the discharge. High intensity vacuum ultra-violet radiation is produced if the insulator is quartz, porcelain or alumina, but if the material is easily decomposed only long wavelength radiation is produced. The effect of the electrode material can be qualitatively explained in terms of the stability and vapour pressure of the oxide, and depends to a certain extent on the sputtering rates. With aluminium, tungsten and steel the oxides are stable and non-volatile, and very low concentrations of the metallic ions are produced. With graphite carbon rapidly enters the discharge producing mainly an ionized carbon spectrum, and with brass a great deal of material enters the discharge and produces conditions which give only long wavelength radiation. At higher ringing frequencies there is some evidence for the existence of a wavelength continuum.