As the vaporised zinc is emitted at the mouths of the retorts _a_ in a partially ascending current, it immediately encounters a plentiful supply of air, and thereupon takes fire (undergoes combustion or oxidation). In this condition it enters the lower and funnel-shaped end of the sheet-iron flue _e_, by which it is conveyed into the series of settling compartments _f_. While the bulk of the zinc oxide thus formed passes into the settling chambers, a portion of it is too heavy to do so; its specific gravity is such that the force of the draught is not sufficient to carry it up. This portion falls at once into a receptacle placed beneath the mouth of the flue _e_. In order to obtain the necessary draught, the conduits _h_ are open to the outside atmosphere, and introduce a supply of air just below the mouths of the retorts _a_, so that it impinges against the current of escaping zinc vapour. After passing through the settling chambers _f_, the superfluous air finds an outlet at _i_ into a sufficiently capacious flue _k_, which communicates with the chimney stack _d_. Thus the draught created by the fuel consumed in the reverberatory furnaces is made to assist the current through the settling chambers. These settling chambers _f_, are constructed of wood, and are usually about three in number, intercommunicating of course. The zinc oxide enters the first compartment through an aperture in the top of the side to which the discharge end of the iron flue _e_ is attached. After traversing the first chamber, the stream of air and such oxide as has not yet settled passes into the second chamber through the orifice _l_, near the bottom of the partition dividing the two chambers. To reach the next compartment, the stream has to ascend again, the aperture being at the top of the partition, and this alternation is carried on to the end of the series, thus checking the through draught and facilitating the settlement of the zinc oxide. The floors of all the chambers are made funnel-shaped, with a door at the lowest point, so that the discharge of their contents may be as automatic as possible. The flue _k_ contains screens hung at intervals for the purpose of hindering, as far as possible, the escape of minute particles of zinc oxide into the chimney, and thence into the outer air, whereby they would be lost.