Portland cement concrete if made with a non-porous aggregate is impervious to moisture, and yet at the same time, if not hydraulically compressed, will take up a sufficient quantity of moisture from the air to prevent condensation upon the surface of the walls. It not only resists the disintegrating influences of the atmosphere, but becomes even harder with the lapse of time. It may also be made in several different colors, and can be finished off to nearly a polished surface or can be left quite rough. Walls built of this material may be made so hard that a nail cannot be driven into them, or they can be made sufficiently soft to become a fixing for joinery, and, if a non-porous aggregate be used, no damp course is required. Further than this, if land be bought upon which there is sufficient gravel, or even clay that can be burnt, the greatest portion of the building material may be obtained in excavating for the cellar; and in seaside localities, if the (salt) shingle from the beach be used, sound and dry walls will be obtained. The use of concrete as a material for building will be found to meet all the defects set forth by practical people, as it may be made fire-proof, vermin-proof, and nail-proof, and in dwellings for the poor will therefore resist the destructive efforts of the "young barbarian." Nothing, therefore, can be better as a building material. The system ordinarily employed to erect structures in concrete consists of first forming casings of wood, between which the liquid concrete is deposited, and allowed to become hard, or "to set." The casings are then removed, the cavities and other imperfections are filled in, and the wall receives a thin facing of a finer concrete. If mouldings or other ornament be required, they are applied to this face by the ordinary plasterer's methods. This system finds favor in engineering construction, and also in very simple forms of architectural work, but with very complicated work the waste in casings is very great. Besides this, however, the face is found sometimes to burst off, especially if it has been applied some time after the concrete forming the body of the wall has set, and the method of applying ornament is not economical. [Illustration: 1.-18.]