Many of these remarks will apply to the choice of bricks for building purposes ; for their relative values will depend upon their powers of resisting the transmission of water, their non-conducting powers, and their durability under the ordinary action of the atmos phere, when exposed for a long time to its effects. In many cases walls have been, and still are, executed of unburnt clay, either rammed into moulds which enclose the space to be occupied by the wall itself (in the manner used in the western counties of England, where it is known by the name of Cobb walling), or in the form of unburnt bricks (as iu the walls of Assyria and of parts of Egypt), and when due precautions are taken to isolate the lower parts of such walls from the ground, and to protect them from heavy rains, they certainly make very warm and comfortable even though temporary structures. The total dis appearance of the domestic structures of the nations of antiquity which adopted the use of unburnt clay, whilst the structures erected at the same peried of the more durable burnt bricks have survived to our times, prove (if such proof were needed) that the latter alone should bo employed in buildings which are likely to be long exposed to distinctly marked atmospheric influences. In fact, no mere mechanical preparation of the silicates of alumina (the base of all clays) can give those materials the-property of a permanent cohesion ; and no stable combination between the various elements can be effected, without the intervention of fire ; or, in other words. the pyrogenous combination of silica and alumina are the ouly stable ones. A small proportion of the carbonate of lime in clays is a desirable condition ; for it facilitates the fusion of the other ingredients ; but a large pro portion of carbonate of lime entails inconveniences which have already been alluded to. [Bates.] It may be suspected that much of the value of brickwork depends upon the reaction of the bricks themselves upon the mortars, or upon the cementing materials employed; and it would therefore seem that, for the purposes of hydraulic engineering, or of budding in water, it is desirable to use bricks which, even after calcination, retain a certain proportion of soluble silica, in a state to allow of its being taken up by the lime around it. The remarkable preservation of the underburnt Roman bricks, or of the soft kiln-burnt bricks of the middle ages, when used in positions where running waters could not affect them, may be explained by the fact of the existence of some statical chemical balance of this description. For general purposes however, it may be sufficient here to state that the hardest, and most thoroughly burnt, bricks only ought to be used in the exposed faces of walls ; that only the best hydraulic limes [LIME ; MORTAR] should be used in conjunction with them ; and that, without entering into any abstract calculations of stability, no external walls of houses in e.rposal positions, should ever be constructed of less thick ness than of one and a half brick ; so as in fact to intercept the passage of water from the exterior to the interior by a vertical joint of mortar. The combination of brickwork and of timber sometimes admits of the attainment of the latter condition, when the total thick ness does not exceed that of one brick ; but in this case the bond of the work cannot be theoretically satisfactory.
In many of our colonies, and in many well wooded countries of a more ancient date, the use of timber for external walls takes place to a great extent; nor, provided that the passage of moisture from the ground, by capillary action, be intercepted, that the rain be prevented from lodging on the upper surfaces, and that there be no danger from the communication of fire, can there be any rational objection to this mode of construction. Where wood is very abundant, as in the United States, Norway, &c., it is used simply in logs ; and those logs are laid
upon one another without being squared, but are only halved together at the angles ; the interstices are then filled in with clay, or with lime and hair; and the exterior and interior surfaces of the walls thus formed, are then carefully plastered. In countries where weed is more valuable, the style of construction becomes more elaborate, and the framing is executed with squared timber, either filled in with brickwork-panels, called technically " brick-negging," or with quarter ings, which are subsequently either covered by "shingles," (or small thin pieces of wood laid like slates,) or are boarded, or lathed and plastered. The half timbered houses in our own country, and in Nor mandy, may be referred to as illustrations of all these last named varieties of wooden construction ; and of the remarkably picturesque eflects they were able to produce. The Swiss cli4lets of the present day are equally worthy of admiration ; and it may be worth while to add that when the exposed timbers of such half timbered houses are cohered by slates, as is very frequently the case in our southern provinces, they are but little exposed to the danger of decay, or of fire; but no such palliation of the defects inherent to wood structures can ever guarantee them against the attacks of insects or of vermin.
The principal use of wood is, however, for internal works and fittings ; and the selection of the particular description to be used in any position must be regulated by so many local considerations that it is preferable to treat of them in two separate articles. [WOODS, BUILDINO ; and WOODS, FURNITURE.] The general conditions for their selection and use are, that the woods should be as light and as easily worked as possible, consistently with the requisite degree of strength ; that when intended to be used as girders, or for framed carpentry, they should be of a straight, tough, fibrous nature; that the trees should be felled in the winter, and that, under all circumstances, the wood should not be fixed in a building until the sap, or other moisture it might contain, had been allowed to escape; that a free circulation of air should be maintained around the woodwork of every description ; and that especially all contact with a damp, confined, atmosphere should be avoided. As might naturally be expected, woods transmit moisture more easily iu the directions of their vesicles, than trans versally to the latter ; and attention must be paid to this law when ever it may be necessary to employ this material partially in and out of the ground. The attacks of insects, and of the marine boring warms, together with the conditions of decay under the action of wet or of dry rot, must not be overlooked in the inquiry into the value of woods ; they aro discussed under PRESERVATION Or STONE AND WOOD. A I very essential property in woods used for decorative works is that they should not be exposed in any serious manner to absorb atmospheric, or in fact any external, moisture ; and that they should not warp under changes of temperature. To a certain extent, all woods are exposed to this inconvenience ; but the superior descriptions of mahogany, oak, and fir, do not warp in any serious or dangerous manner under ordinary changes, provided that due care has been observed in their seasoning before placing them in the work. Some woods again have a distinct action upon metals, such as the oak produces upon iron, or even occasionally upon lead; but the circumstances under which this action takes place are not sufficiently known to warrant any decided general observations with respect to it. Contact with other building materials does not seem to affect the durability of wood, provided the circulation of air around the latter be not prevented.