The stone chosen should be durable, and able to resist the action of the sulphuric, sulphurous, and carbonic acids absorbed from the atmosphere, and brought in contact with it by means of rain. The stone of which a considerable part of the Houses of Parliament consists is dolomite, a double carbonate of lime and magnesia. The acid fumes in the air produce on its surface sulphate of magnesium, which is washed away in successive layers. If the stone presents any stratification, it should be laid in the wall in the same position as that in which it was originally deposited in the quarry. Thus, any planes of stratification will be horizontal, and the scaling off by the action of frost and rain is minimised. Comparatively homogeneous stones, such as granite and millstone-grit, can be laid in any position. In testing the character of any stone, the least porous, densest, and most resistent to crushing, will as a rule be the most durable. The chief difficulty in the use of stone for the walls of houses, is that of keeping out the wet. To obviate this, stone-houses are often built of great thickness, and are consequently cooler in summer and warmer in winter. In and near large towns brick is chiefly used for walls of houses, and stone employed only for window-sills, columns, steps, etc. It is even more important in these cases to carefully select the stone, as the parts where it is placed are those most exposed to the weather. If a soft, friable freestone is used, after a sharp frost large scales are seen falling off in flakes, owing to the freezing and subsequent thawing of the moisture in the stone. _Portland stone_ is the best-wearing stone to be had in the neighbourhood of London. _Bath stone_ is also considerably used, but it varies greatly in quality, and should be very carefully selected. For landing-steps and paving, _Yorkshire stone_ is extensively used, but artificial cement pavings are replacing it to some extent. Most kinds of stone can only be economically used near the quarries from which they are derived. The _Slate_ used for roofs is an altered form of clay, possessing a laminated structure. The ease with which it splits along the planes, renders it peculiarly suitable for this purpose. The Welsh slates are considered the best. =Terra-cotta= is made from certain kinds of clay, mixed with glass, pottery or sand; then ground up, strained, and kneaded; and lastly thrown into moulds and baked in a kiln. =Iron and Wood= have occasionally been employed alone in building houses. The former, owing to its good conducting powers for heat, is cold in winter and hot in summer; while the latter becomes rotten from exposure to wet, and is also very combustible. Corrugated iron buildings lined with wood are also employed, but are not very satisfactory.