THE PRELIMINARY PURIFICATION OF COAL GAS 513
and whilst th.e bottom edges are perforated there are a number of narrow slots in the side of the troughs through which the bulk of the gas passes. In general, the washer forms a distinet piece of apparatus ; but, if desired, it may be fitted in the base of a serubber, as seen in Fig. 329.
In some cases the funetions of tar-extracting, washing and serubbing are merged into one vessel, known as a purifying machine. In the Walker apparatus (Fig. 318) the gas is first treated in the lower portion of the vessel, which contains a washer
similar to that shown in Fig. 317. It then travels upwards to the next compartment, wliere it comes in contact with. strong liquor distributed over devices containing wetted boards. The gas tlien travels through, a number of superimposed tiers of such boards and is taken off at the top of the machine. A vertical shaft passes through the centre of each. of the boxes containing the boards, the ends of the shaft being connected to rocking levers carried from pivots on. the top of th.e machine. The rocking levers are operated by a small engine placed at ground level, and by means of these levers the boxes are raised and lowered about every ten seeonds. The boards contained in the boxes are, in this way, frequently immersed in the liquor contained
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