a volcanic product found near Naples, Italy, when p,-round and mixed with equal portions of lime and sand, or five of pozzuolana to two of lime, forms one of the best of bonding materials, hardening- very rapidly even under water.
Pona'ing masonry- required for vertical enclosures the bondinp.-„ or union of individual pieces composing- the structure, is indis pensable, as it not only insures stability to the walls, but also contributes materially toward effecting a distribution of pressure throughout the build ing. The only exception is found in the case of plastic materials which cannot be bonded together in the above sense. In bonding-, particular attention must be paid to the arrangement of the joints, as a distinction must be made between those lying- between the superposed courses of the work (coursing, joints) and those between. the individual stones of a course (heading joints). The principal rule to be observed for a perfect bond is that the heading joints of the superposed courses shall never be placed either wholly or in part over one another.
Bond and illasonry to the relatively small size of bricks, the application of mortar is an important element in securing solid itv in a wall. The thickness of the layer of mortar in the joint is gener ally about rt- of an inch, and this comparatively heavy layer gives the wall a characteristic appearance. To obtain a regular bond, the brick, which has the shape of a parallelopiped, must have dimensions determined by fixed rules, the length being at least twice the breadth. This proportion is observed uniformly throughout the German empire, where the dimensions for length are 0.f. inches and for width inches, calculating the width of joint at 1-46 of an inch. Though a similar proportion between the breadth and the thickness might be desirable, it would weaken the brick, so that it would be very liable to fracture.' The bond is always to be so arranged that, with the exception of corners, whole bricks only are used for the mass of the wall; and for this reason the thickness of brick walls is specified according to the length of the bricks. The bond may, however, admit of great variation. There should be distinguished the " block bond " and " cross bond," and more rarely the "Gothic " or Polish bond.
Block and Cross the block bond 05/. 1, fig. x), the bricks in one and the same row lie either with the long side (stretcher) or the narrow side (header) forward. The difference in the two bonds is that in the block bond there is a mere alternation of the stretching and heading courses, so that the heading joints of all the headers, as well as those of the stretchers, stand over one another, while in the cross bond (fig. 2) this
applies to the headers only, and not to the stretchers, as the latter have an alternation of the heading joints among themselves. Consequently, in the block bond, as shown in Figure 1, the toothing is regular on the sides, while the racking is irregular; but in the cross bond (fig. 2) this arrange ment is reversed.
The disposition of bricks laid on the inside of a wall will depend upon its thickness (in Figures r and 2 a wall of the thickness of two bricks is supposed). In walls whose thickness is divisible only by half-bricks, as one and one-half, two and one-half, etc., if a row of stretchers be at one side, there is a row of heaclers at the other side, and the reverse in the next course, etc. In other respects the thickness of the wall will not influence the exterior appearance of the bond. To lead in the bond at the corners, as shown by the Fig-ure, thc wall is started either with a three-quarter brick in each stretching- course, or in the heading courses with a brick of one-half .width, inserted immediately behind the first header.
Gothic the Gothic or Polish bond—which according- to the above rule cannot be called perfect—stretchers and headers are alternated in the same course, as shown for cut stone in Figure 8.
Hollow hollow bricks have been extensively used; thereby the weight of the wall is diminished, though the bond is not altered. The weight is also lessened through making the wall itself hollow by spaces left between bricks, as seen in Figures 3 and 4; here the Gothic bond may be used to advantage. In Figure 3 common bricks are supposed, and it is only necessary to lay a quarter-brick before each header that there may be a corresponding hollow in the wall, which will then be one-and-a quarter bricks in thickness. In Figure 4 (.,6/. r) the bricks are supposed to be square in cross-section, with a side of about 4 inches; the cavity, with bricks ro inches in length, would then be only 2 inches and the wall one brick thick. Hollow bricks and fire-clay tiles of various forms used for interior construction, as for partitions, spanning spaces between girders of iron, etc., will be considered in the section specially devoted to the subject.