Modern Gasworks Practice

582 MODERN GASWORKS PRACTICE This is due to too vigorous oxidation. In the case of the disulphide or sulphate of iron, revivification is impossible, and a portion of the material loses its activity in consequence. There is, however, a remedy, for if an alkali is added it will, on account of its basic properties, release the iron from its undesirable combination with the sulphur. The alkali may be added in the form of ammonia from the serubber, which may be by-passed for a short period; but preferably a small proportion of powdered slaked lime should be admixed to the oxide when it is removed from the box. So far as the diluent properties of air are concerned, mention. may first be made of the oft-quoted work of Audouin and Bérard, carried out in 1862. These investi-gators found that— 1 per cent, of air reduoes illuminating power by 6 per cent. 2 „ „ „ „ „ 11 3 „ „ „ ,, 18 4 „ „ „ „ „ 26 and if 45 per cent, of air is added the loss of candle-power amounts to 100 per cent. Later results have been given by Wurtz, who states that— 3 per cent, of air causes 15-69 per cent, loss in light. 5 » » „ 24-0 „ „ „ More recent experiments in this direction have been carried out by W. B. Davidson. The figures he gives are the folio wing:— 1 per cent, of oxygen added deoreases candle-power by 3 per cent, and oalorifio power by I per cent. 1 per cent, of nitrogen added deoreases candle-power by 2'6 per cent, and oalorifio power by 1 per cent. So far as carbon dioxide is concerned, Davidson states that 1 per cent, added to an average sample of coal gas will reduce the illuminating power by 3J per cent. Reverse Action In the ordinary way, the gas enters at the base of the purifiers and passes upwards through the material. Recently, however, it has been pointed out that reverse flow (i.e. gas entering above the material and passing downwards) possesses a number of advantages, so that the system is growing in popularity. Foremost, perhaps, is the back-pressure consideration ; and when a purifier gives trouble in this direction an efiectual remedy will be found in the reverse flow. This is largely on account of the inereased area of oxide exposed to the gas, which (owing to the grids) is nearly halved when the flow is in the usual upward direction. When excessive pressure is thrown by a box, it is usually attributable to the first (i.e. the bottom) tier, wliilst a common remedy is to draw a slide, and so by-pass the one layer. In this respect the downward travel certainly makes matters easier, for the lid may be lifted and the top material lightened (or exchanged for new) with little delay. Thus the efficiency of the box is inereased and not reduced materially, as is the case when a layer is slipped. It occasionally happens that a purifier suffers as regards activity owing to the material becoming too dry. The reverse system in such cases will usually provide