The outer walls of a stable, while serving their purpose as a means of protection to the horses in the building, should be durable; should be as nearly non -porous as possible, so that the walls may not become damp, and may not absorb noxious vapours and disease germs; should not be soluble in water; should not be inflammable; and by their low conducting power of heat or by their thickness should protect the interior from changes of temperature, so that it may remain cool in summer and warm in winter. These conditions are best fulfilled as a rule by thick walls of hard, tough stone. The quality of bricks, which are the next best material, depends on the nature of the brick-earth (brick-clay), from which they are made, and on the manner in which they are manufactured. To obtain the best bricks for this purpose, clay which is free from all injurious impurities, and which contains a small admixture of silica in the form of sand, should be used. The addition of silica is necessary to prevent the clay from cracking on drying. Pure clay consists of hydrated silicate of alumina. As an example of good bricks we may take those of Candy and Co., which are made from the granite clay of South Devon, and which have the following composition : -

The chief deleterious substances which are found in brick-clay are : organic matter (vegetable mould for instance), lime, magnesia, soda, and potash. As organic matter will not stand heat, its presence in the clay of which bricks are made, would evidently spoil them when they are put into the furnace. The more bricks are heated in manufacture, the less porous do they become; hence the best temperature is that which is just short of fusion. If lime, magnesia, potash or soda be present, it will form a silicate which will act as a flux in causing the clay to melt before the required temperature is attained. Another important reason for the desirability of the absence of lime or limestone (CaC03), is that if it be present, a nodule of quicklime (CaO) may be formed in the brick when the clay is heated. The chemical action with limestone would be as follows:-

CaC03 = CaO+C02 (carbonic acid gas).

If a brick wall which contained one or more nodules of quicklime became damp, the quicklime would be liable to unite with the absorbed water, and the resulting chemical action would probably be sufficiently violent to crack the brick. Here we have the following chemical equation: - CaO+H10 (water) = Ca(OH)2 (slaked lime).

Soda and potash not only form fluxes, but their silicates are to some extent soluble in water, and consequently their presence is objectionable.

In some districts the brick earth contains clay and sand in proportions which are suited to good brick making, and is practically free from undesirable constituents. "All brick clays contain iron, and the colour of a burnt brick almost entirely depends on the amount of it which is present. Thus, clay containing less than I or 1 1/2 per cent. of iron, changes in the kiln to various shades of cream colour and buff, whilst those containing more than 2 per cent. range in colour from yellowish-fawn to dark red. Blue bricks are made from the same clay as the red, by controlling in a peculiar way the supply of air in firing, and by carrying the heat somewhat further. It is asserted by some that the red is changed to the black oxide of iron in the process" (Chambers' Encyclopedia). The dark blue or purple Staffordshire bricks are a well-known example of good "blue bricks." If on breaking a blue brick, we find that the internal surfaces of the fractured parts are of comparatively light colour, we may conclude that the brick is of inferior quality.

Tiles, earthen drainage pipes, and the various varieties of paving bricks are all made in the same way. Although the respective powers of absorption for water of different kinds of bricks and tiles vary within wide limits, no kind of unglazed brick or tile is absolutely waterproof.

The quality of the bricks used, and the amount of exposure (as at the seaside) to damp and rain which the building may have to endure, will naturally have to be taken into consideration in determining the thickness and mode of construction of the walls. The best arrangement for a brick wall is to have it double; the exterior wall being 9 in. thick and the interior wall 4 1/2 in., with a 2 in. hollow space between them. This is commonly called a "14 in. wall." An ordinary brick is 9 in. long, 4 1/2 in. wide, and 3 in. thick. Or an "11 inch wall" may be constructed with the exterior and interior walls each 4 1/2 in. thick and a 2 in. hollow space between them. In both cases, iron ties connect the two walls together. The use of glazed bricks for the interior surface of the walls gives a stable a smart appearance. These bricks can be obtained of any desired colour and glazed on the side, top, or end, so as to meet the exigencies of construction. The lower rows, so as to give the look of a dado, may be of different colour to that of the upper rows; for instance, the former being a dark green or dark red, and the latter a pale green. Glazed bricks cost about five times as much as good ordinary bricks. In the construction of walls, care should be taken that only the best of mortar is used, neglect of which precaution is often shown by the presence of gaping interstices, especially in brick walls. The sand employed in making mortar should be "clean and sharp," that is to say, it should be free from organic matter and should be siliceous (composed of quartz).

Wood is in most respects objectionable. It decays quickly; harbours vermin and parasites; is very absorbent; is liable to catch fire; is apt to shrink and warp; and, when used in a thickness suitable for economic requirements, it fails to keep out heat in summer and cold in winter. The only advantages it possesses over brick and stone are its cheapness; the facility with which it can be adapted to purposes of construction; and the fact that wooden stables can be made portable.

Walls made of good clay and wheaten straw (mud walls) are inexpensive; last a long time, considering their cost; and give good protection against inclement weather and changes of atmospheric temperature. They are, however, too unsightly for general use, and harbour insects, especially wasps and bees.