In Parts I and II of the present series an account was given of an investigation of the ignition and slow non-self-propellant combustion of electrolytic gas in direct current discharges. In those experiments a smooth direct current discharge was maintained between cooled platinum or copper electrodes in pure electrolytic gas, confined over sulphuric acid, at pressures between 30 and 180 mm. in such a manner as to eliminate, as far as possible, any chemical combination other that that caused only by the ionisation of the gases. The proportionality between the rate of chemical change and the current was then studied. It was shown that (i) an electric discharge can be passed through electrolytic gas in such a manner that combination takes place at a rate which is determined only by the discharge; (ii) up to a certain limiting current, which depends upon (a) the nature of the cathode material, (b) the gas pressure, and (c) the gap width, combustion is confined to the cathode zone, its rate being directly proportional to the current, i. e., to the number of ions arriving at the cathode in a given time; (iii) after a certain limiting current has been exceeded, combustion commences abruply in the inter-electrode zone and is thereafter superposed upon the cathodic combustion, the two then continuing as independent simultaneous effects; (iv) this inter-electodic combustion, like the cathodic, is also proportional to the current passing; unlike the cathodic, however, it is independent of the material of the electodes, but dependent upon (a) the gas pressure, and (b) the gap width.