THE PRELIMINARY PURIFICATION OF COAL GAS 525
tiers is 5 feet, at least 1 foot of clearance should be allowed, and when 4 feet, 6 to 9 inches is advisable.
Except in small works it is common to find serubbers erected in pairs, whilst in the largest works there may be batteries of several. Whatever the number, it should always be arranged for the gas to come in contact with clean water before going forward to the purifiers. When two or more serubbers are in use the last should be supplied with clean water alone, this running out at the base as a weak liquor, which is then pumped up to the pre-ceding serubber, and so on. When only one serubber is avail-able it should be so operated that the upper portion is supplied with clean water, whilst the lower tiers are treated with liquor from an intermediate spray.
An effeetive scheine has been introduced by Thos. Glover, and is illustrated in Fig. 330. The serubbers are fitted with boards and are supplied with liquor which is previously passed through a strainer. The second serubber is supplied with plate-glass windows placed opposite each other, so that if the vessel is carrying out its work effectively it is possible to see clearly through it from side to side. Half-way from the top weak liquor is sprayed on to the boards, whilst the upper half is supplied with clean water alone. The liquor flowing from the serubbers passes through a