3IO DEVELOPMENT OF HEAT. chap.
reactions can be realised which are characteristic of the silent electrical discharge.
The ozoniser devised by yon Babo (;^^) has the form shown in Fig. 58. Metal wires are inserted into glass tubes sealed at one end, and they are alternately connected with the poles of an induction coil. When the coil is in action, electrical oscillations arise in the capillary spaces between the glass tubes, and these ozonise the air. A current of air passed through a tube containing the wires is therefore ozonised. From a large number of experiments with ozon- isers of this type it has been found that the presence of a
���•^"' ^— — — — — ^j'^ ^^ otane Fig. 58.
small quantity of water vapour favours the production of ozone, whilst a large quantity (or carbon dioxide) has a dis- turbing effect. The air to be ozonised is, therefore, dried with some not too hygroscopic substance (sulphuric acid at the ordinary temperature, or calcium chloride at temperatures below 0°). Low temperature favours the formation of ozone because the amount formed is not then so easily decomposed as at higher temperatures. The air should be free from dust, as the ozone in oxidising this is destroyed. The yield of ozone diminishes with decreasing pressure; since ozone occupies I of the volume of the oxygen from which it is formed, increase of pressure must favour its formation (see p. 99).
Working at the pressures and temperatures given in the table, Hautefeuille and Chappuis (.?J) obtained the following percentages by weight of ozone : —
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