Fig. 28 shows the second type of hopper-bottom flue adopted. It is of very light construction, of 274 sq. ft. area in the clear. The beginning of this flue being 473 ft. from the blast furnaces removes all possibility of any material floor-load, as the dust is light in weight and does not collect in large quantities. The hopper-bottom floor is formed of 4 in. concrete slabs, in panels, placed between 4 in. I-beams. Cast-iron door frames, with openings 12 × 16 in., are placed on 5 ft. centers. The concrete floor is tamped in place around the frames. The side walls and roof are built of 1 in. angles, expanded metal, and plastered to 2.5 in. thickness. At every 10 ft. distance, pilaster ribs built of 2 in. angles, latticed and plastered, form the wind-bracing and arch roof support. [Illustration: FIG. 28.—Arched form of Concrete Dust Chamber.] Fig. 29 shows the beehive construction. This chamber is of 253 sq. ft. cross-sectional area. It is built of 2 in. channels, placed 16 in. centers, tied with 1 × 0.125 in. steel strips. The object of the strips is to support the 2 in. channels during erection. No. 27 gage expanded metal lath was wired to the inside of the channels and the whole plastered to a thickness of 3 in. The inside coat was plastered first with portland cement and sand, one to three, with about 5 per cent. lime. The filling between ribs is one to four, and the outside coat one to three. The above types of dust chamber have been in use over three years at Leadville. Cement and concrete, in conjunction with steel, have been used in Utah, Montana and Arizona, in various types of cross-section. The results show clearly where not to use cement; namely, where condensing sulphur fumes come in contact with the walls, or where moisture collects, forming sulphuric acid. The reason is that portland cement and lime mortar contain calcium hydrate, which takes up sulphur from the fumes, forming calcium sulphate. In condensing chambers, this calcium sulphate takes up water, forming gypsum, which expands and peels off. [Illustration: FIG. 29.—Beehive form of Concrete Dust Chamber.] In materials of construction it is rather difficult to get something that will stand the action of sulphur fumes perfectly. The lime mortar joints in the old types of brick flues are soon eaten away. The arches become weak and fall down. I noted a sheet steel condensing system, where in one year the No. 12 steel was nearly eaten through. With a view of profiting by past experience, let us consider the acid-proof materials of construction, namely, brick, adobe mortar, fire-clay, and acid-proof paint. Also, let us consider at what place in a dust-chamber system are we to take the proper precaution in the use of these materials.