The kilns are closed at the top, and the gases are drawn out by powerful air-pumps, washers being interposed be,, veen the kilns and the pumps for the purpose of purifying and cooling the gas.
Every winter the supply is exhausted by the washers, and every summer it is renewed by the river.
Of the deaths in any place the only check obtainable is from the public body-washers, but many corpses areburied without the aid of the public body-washers; and the population of the place not being accurately known, the number of deaths, however correct, is useless for statistical purposes.
The solubility of naphthalene by various oils has led some engineers to put in naphthalene washers, in which gas is brought into contact with a heavy tar oil or certain fractions distilled from it, the latter being previously mixed with some volatile hydrocarbon to replace in the gas those illuminating vapours which the oil dissolves out; and by fractional distillation of the washing oil the naphthalene and volatile hydrocarbons are afterwards recovered.
Two or three of these washers are connected in series according to the quantity of gas to be dealt with.