One of the most interesting examples of ozone ventilation is that of the Central London tube electric railway. The installation consists of a separate ozonizing plant at every station, except Shepherd’s Bush, which is close to the open end of the tunnel. Fig. 42 is a diagram of the general arrangement of one of these plants, and it shows how the air is purified, ozonized, and sent into the tunnel. The generating plant is seen at the top left-hand corner of the figure. Air is drawn in as shown by the arrows, and by passing through the filter screen F it is freed from dirt and smuts, and from most of the injurious gases which always are present in town air. The filter screen is kept moist by a continual flow of water from jets above it, the waste water falling into the trough W. The ozone generator is shown at O. Continuous current at about 500 volts, from the power station, is passed through a rotary converter, which turns it into alternating current at 380 volts. This current goes to the transformer T, from which it emerges at a pressure of 5000 volts, and is supplied to the ozone generator. From the generator the strongly ozonized air is taken by way of the ozone pipe P, to the mixing chamber of the large ventilating fan M, where it is mixed with the main air current and then blown down the main air trunk. From this trunk it is distributed to various conduits, and delivered at the air outlets marked A. Altogether the various plants pump more than eighty million cubic feet of ozonized air into the tunnels every working day. In many industries pure air is very essential, especially during certain processes. This is the case in brewing, in cold storage, and in the manufacture and canning of food products; and in these industries ozone is employed as an air purifier, with excellent results. Other industries cannot be carried on without the production of very unpleasant fumes and smells, which are a nuisance to the workers and often also to the people living round about; and here again ozone is used to destroy and remove the offending odours. It is employed also in the purification of sewage and polluted water; in bleaching delicate fabrics; in drying and seasoning timber; in maturing tobacco, wines and spirits, and in many other processes too numerous to mention. CHAPTER XXVII ELECTRIC IGNITION