220

PURIFICATION.

ammonia being formed. The fatty oils adhere to the coke, etc., and probably a porousmaterial may be preferable to a close one for this adhesion.

If the crude gas has been thoroughly condensed, the scrubbers are placed after the puri-fiers ; but if there is the least chance of tar remaining, the gas must be scrubbed beforeit is allowed to enter the purifiers, because the tar would form more or less under-coatingsto the layers of lime, and prevent their proper action. The safest arrangement is to place adry or a simple water-scrubber between the condensers and purifiers, and a second one,supplied with acidulated water, after the purifiers. It is not necessary in small works to

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have a change of these machines ; their cleansing and the renewal of the coke, etc., occupiesbut a short space of time, and a side-pipe may be arranged so as to take the gas past themduring the operation.

In works of moderate size, the capacity of the scrubbers may be increased so that the