The great difference between the atmospheric conditions before and after a thunderstorm must have been noticed by everybody. Before the storm the air feels lifeless. It does not satisfy us as we draw it into our lungs, and however deeply we breathe, we feel that something is lacking. After the storm the air is delightful to inhale, and it refreshes us with every breath. This remarkable transformation is brought about to a very large extent by ozone produced by the lightning discharges.
As far back as 1785 it was noticed that oxygen became changed in some way when an electric spark was passed through it, and that it acquired a peculiar odour. No particular attention was paid to the matter however until about 1840, when Schönbein, a famous German chemist, and the discoverer of gun-cotton and collodion, became interested in it. He gave this strange smelling substance the name of “ozone,” and he published the results of his experiments with it in a treatise entitled, “On the Generation of Ozone.” Schönbein showed that ozone could be produced by various methods, chemical as well as electrical. For instance, if a piece of phosphorus is suspended in a jar of air containing also a little water, in such a manner that it is partly in the water and partly out of it, the air acquires the characteristic smell of ozone, and it is found to have gained increased chemical energy, so that it is a more powerful oxidizing agent. For a long time the exact chemical nature of ozone could not be determined, mainly because it was impossible to obtain the substance in quantities sufficiently large for extensive experimental research, but also on account of its extremely energetic properties, which made it very troublesome to examine. These difficulties were so great that investigators were in doubt as to whether ozone was an element or a compound of two or more elements; but finally it was proved that it was simply oxygen in a condensed or concentrated state.
Apparently ozone is formed by the contraction of oxygen, so that from three volumes of oxygen two volumes of ozone are produced. In other words, ozone has one and a half times the density of oxygen. Ozone has far greater oxidizing power than oxygen itself; in fact it is probably the most powerful of all oxidizing agents, and herein lies its great value. It acts as nature’s disinfectant or sterilizer, and plays a very important part in keeping the air pure, by destroying injurious organic matter. Bacteria apparently have a most decided objection to dying; at any rate they take an extraordinary amount of killing. Ozone is more than a match for them however, and under its influence they have a short life and probably not a merry one.
Ozone exists naturally in the atmosphere in the open country, and more especially at the seaside. It is produced by lightning discharges, by silent electrical discharges in the atmosphere, by the evaporation of water, particularly salt water, by the action of sunlight, and also by the action of certain vegetable products upon the air. The quantity of ozone in the air is always small, and even pure country or sea air contains only one volume of ozone in about 700,000 volumes of air. No ozone can be detected in the air of large towns, or over unhealthy swamps or marshes. The exhilarating effects of country and sea air, and the depressing effects of town air, are due to a very large extent to the presence or absence of ozone.
A great proportion of our common ailments are caused directly or indirectly by a sort of slow poisoning, produced by the impure air in which we live and work. It is popularly supposed that the tainting of the air of rooms in which large numbers of people are crowded together is due to an excessive amount of carbonic acid gas. This is a mistake, for besides being tasteless and odourless, carbonic acid gas is practically harmless, except in quantities far greater than ever exist even in the worst ventilated rooms. The real source of the tainted air is the great amount of animal matter thrown off as waste products from the skin and lungs, and this tainting is further intensified by the absence of motion in the air. Even in an over-crowded room the conditions are made much more bearable if the air is kept in motion, and in a close room ladies obtain relief by the use of their fans. What we require, therefore, in order to maintain an agreeable atmosphere under all conditions, is some means of keeping the air in gentle motion, and at the same time destroying as much as possible of the animal matter contained in it. Perhaps the most interesting and at the same time the most scientific method of doing this is by ozone ventilation.
In the well-known “Ozonair” system of ventilation, ozone is generated by high-tension current. Low-tension current is taken from the public mains or from accumulators, and raised to a very high voltage by passing it through a step-up transformer. The secondary terminals of the transformer are connected to a special form of condenser, consisting of layers of fine metal gauze separated by an insulating substance called “micanite.” The high tension between the gauze layers produces a silent electrical discharge or glow. A small fan worked by an electric motor draws the air over the condenser plates, and so a certain proportion of the oxygen is ozonized, and is driven out of the other side of the apparatus into the room. The amount of ozone generated and the amount of air drawn over the condenser are regulated carefully, so that the ozonized air contains rather less than one volume of ozone in one million volumes of air, experiment having shown that this is the most suitable strength for breathing. Ozone diluted to this degree has a slight odour which is very refreshing, and besides diminishing the number of organic germs in the air, it neutralizes unpleasant smells, such as arise from cooking or stale tobacco smoke. Ozone ventilation is now employed successfully in many hotels, steamships, theatres and other places of entertainment, municipal and public buildings, and factories.
By permission of]
[Ozonair, Ltd.
Fig. 42.—Diagram of Ozonizing Plant, Central London Tube Electric Railway.
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.