walls may be partly combated by a careful selection of a non-absorbent brick with a hard face and by struck joints.  But it is most effectually kept at bay by the expedient of building the wall hollow; that is to say, making the external wall of the house to consist of two perfectly distinct walls, standing about 2 in. apart, and held together by ties of earthenware or iron.  The result is that the moisture blowing through the outer skin does not pass the cavity, but trickles down on the inner face of the outer wall, while the inner wall remains dry.  The ties are constructed of shapes to prevent their conducting water themselves from without to the inner wall.  In addition to this, a series of slates forming an intermediate protection is sometimes introduced, and forms an additional and most valuable screen against weather.  Sometimes, the two skins of the wall are closer together—­say 3/4 in.—­and the space is filled with a bituminous material.

A substance of a bituminous nature, called hygeian rock, has been of late years introduced, and is being extensively used for this purpose; it is melted and poured into the open space hot, and quickly hardens.  The use of such a material is open to the objection that no air can pass through it.  The rooms of our houses are receiving air constantly through the walls, and much of the constant current up our chimneys is supplied, to our great advantage, in this very imperceptible manner.  The house breathes, so to speak, through the pores of its brickwork.  When this is rendered impossible, it seems clear that fiercer draughts will enter through the chinks and crevices, and that there will be a greater demand upon flues not in use, occasioning down draught in the chimneys.

Another mode of keeping out weather is to cement the face of the brickwork.  But this hides up the work, and so tends to promote bad work, besides being often very unsightly.

Among other peculiarities of brickwork are the facilities for introducing different colors and different textures of surface which it presents, the ease with which openings and arches can be formed in it, the possibility of executing ornament and even carving, and the ease with which brickwork will combine with other building materials.  It cannot be well made use of for columns, though it may readily enough be turned into piers or pilasters.  It cannot, generally speaking, with advantage be made use of for any large domes, though the inner dome of St. Paul’s and the intermediate cone are of brick, and stand well.  But it is an excellent material for vaulting arcades and all purposes involving the turning of arches.