Irrespective of these considerations, many building materials are invaluable from their own intrinsic merits. Granite is one of the best known, from its hardness and durability, marble from the exquisite polish it takes, and for its colour, which for internal decoration is a property that can hardly be over-estimated. Stone is valuable on account of the largeness of the blocks that can be obtained and because it easily receives a polish sufficient for external purposes. Bricks are excellent for their cheapness and the facility with which they can be used, and they may also be moulded into forms of great elegance, so that beauty may be easily attained; but sublimity is nearly impossible in brickwork, without at least such dimensions as have rarely been accomplished by man. The smallness of the material is such a manifest incongruity with largeness of the parts, that even the Romans, though they tried hard, could never quite overcome the difficulty. Plaster is another artificial material. Except in monumental erections it is superior to stone for internal purposes, and always better than brick from the uniformity and smoothness of its surface, the facility with which it is moulded, and its capability of receiving painted or other decorations to any extent. Wood should be used externally only on the smallest and least monumental class of buildings, and even internally is generally inferior to plaster. It is dark in colour, liable to warp and split, and combustible, which are all serious objections to its use, except for flooring, doors, and such purposes as it is now generally applied to. Cast iron is another material rarely brought into use, though more precious than any of those above enumerated, and possessing more strength, though probably less durability. Where lightness combined with strength is required, it is invaluable, but though it can be moulded into any form of beauty that may be designed, it has hardly yet ever been so used as to allow of its architectural qualities being appreciated. All these materials are nearly equally good when used honestly each for the purpose for which it is best adapted; they all become bad either when employed for a purpose for which they are not appropriate, or when one material is substituted in the place of or to imitate another. Grandeur and sublimity can only be reached by the more durable and more massive class of materials, but beauty and elegance are attainable in all, and the range of architectural design is so extensive that it is absurd to limit it to one class either of natural or of artificial materials, or to attempt to prescribe the use of some and to insist on that of others, for purposes to which they are manifestly inapplicable. VIII.—CONSTRUCTION.