REFRACTORIES FOR GASWORKS PURPOSES 227
from these walls to a minimum. Chief among these methods is that of building up a cavity wall (a clear 2-inch space usually being left, whilst for the sake of
stability the two walls are tied together at intervals by a brick), the cavity being left clear, or filled in with. some type of non-conducting material, such as asbestos composition. A more recent means is to fill in the cavity or to build up a separate interior wall from one of the special types of insulating bricks. These bricks are extremely light, thus indicating high porosity and low conductivity.
Another practice is to constnict the wliole of the outside wall of large hollow blocks as shown in. Fig. 126.
In the case where a screening wall of some type of insula-ting material is employed, there seems little reason why the face wall of the setting should not be built up entirely from ordinary stock brickwork. Such bricks, owing to ih.eir relatively high porosity, would ensure the retention of a furtlier amount of heat.
Fig. 126.—Special Aik-blooks fok Outside Wall of Setting.
The extremely light and porous insulating bricks which. have spræng into favour at the present day are mainly composed of diatomaceous or infusorial earth (Kie-selguhr). This material is composed of the siliceous remains of mimite aquatic
Fig. 127.—Microphotogkafhs of Diatoms (Diato_mj.tr: Company).
plants known as diatoms, which originally secreted silica in the process of building their shells. The raw product, therefore, consists almost entirely of the silica shells of diatoms, the fi.nish.ed article containing about 92 per cent, of silica. The efficiency of diatomaceous bricks depends almost solely upon the air spaces enclosed by the