In mixed gas installations (Dowson, water gas) of the older system, the way in which the gas is produced causes the whole apparatus and pipes to be under slight negative pressure, because the steam required for the process must be blown into the generator. In these works, therefore, a small special steam boiler is required and a gas receiver to store the gas. In more modern suction generator gas installations the piston is used to suck in steam and air as well as the gases arising in the generator and to draw them into the motor cylinder. Thus the whole system is kept in a condition of slight negative pressure during the process. While the suction generator gas system is working, only so much gas is produced as the motor uses for the time being, so that with this system there is no greater store of gas than is requisite. In such an installation the following rules should be borne in mind: 1. All the apparatus (gas pipes, valves, &c.) must be constructed and maintained in a completely impervious condition. Any water seals especially which may be in use must receive attention. 2. Precautions must be taken to prevent the gases from the generator passing into the coolers and purifiers when the engine is at rest. 3. Care is to be taken when the apparatus is at rest to prevent any possible subsequent escape of gas into the room where the apparatus is installed. 4. The return of explosive gas out of the gas engine into the gas pipe by failure to ignite or other accident, must be made impossible. 5. The apparatus through which the generator is charged must possess a tightly fitting double valve to prevent escape of gas into the room during charging. 6. The pipes for conducting away the unpleasantly smelling bituminous constituents in the water mixed with sulphuretted hydrogen from the scrubbers must not communicate with the workroom. 7. Precautions must be taken to minimise the danger during the cleaning of the generator (removal of ashes and slag). 8. All stop-cocks and valves are to be so arranged that their position at any time (open or shut) is clearly visible from outside. 9. Purifiers with a capacity greater than two cubic meters must be provided with appliances which make possible thorough removal of the gas before they are opened. 10. The gas washing and cleaning apparatus and pipes are to be fitted with gauges indicating the pressure existing in them at any moment. 11. When a suction gas plant is first installed and also at times when there is no gas in the pipes and plant between the generator and the engine, gas must be blown in until all air is expelled before the engine is set going. 12. During the cleaning of apparatus and pipes which, when in action, contain gas, the rooms must be thoroughly ventilated.