tube, he was able to drive the air through the cylinder with force enough to extinguish a candle at the other extremity. Similar results may be obtained with wood and such varieties of stones as allow air to pass through them without difficulty; while other stones, like compact limestones, are hardly permeable.
All materials become impermeable to the air when they are wet. The experiment with the cylinder of mortar will not be successful if the mortar is moistened. It has also been found less easy to drive moisture through bricks and mortar than to make air pass through them; only a few drops of the liquid can be made to appear on the free surface. Water is therefore not easy to dislodge from the pores it has occupied, and is at most removed very slowly by evaporation. But, when water stops the pores, it prevents the air from circulating through them—a mischievous effect upon the permeability of building materials, which is more perceptible in proportion as their grain is finer and more compact.
In ordinary weather, and when they are dry, walls perspire. They are continually traversed by feeble atmospheric currents, which renew the air of closed rooms and rid it of the moisture with which it is loaded. The atmosphere of a house is saturated with moisture by the respiration and perspiration of its inmates, and by the water daily used in housekeeping, even if we do not take account of the dew that is deposited whenever some air from without gets into cold rooms. This moisture, which is always undergoing renewal, ought to be absorbed by the walls, to be evaporated from the outside, under the action of the sun and wind. For this reason it is well for building materials to be porous and permeable, and for them to interpose no obstacle to the circulation of the air which is depended upon to promote evaporation. This remark is especially applicable in the North, where the windows can not always be wide open; it is perhaps of less importance in the South.
The moisture which the walls receive from the exterior atmosphere, from fogs and rain, generally disappears quickly enough under the operation of the winds that constantly lick the surface of the house. But the moisture that comes from within, which is deposited on the walls of poorly ventilated rooms, passes away with difficulty when the walls are not porous. Even the heating apparatus only causes it to change its place, by leaving the surfaces that become warmed and settling farther away where the heat has not yet reached. Inconveniences from interior moisture are especially sensible in newly built houses, where the mortar still contains a large proportion of water, and in ground-floors built on a damp soil, which become impregnated by capillarity. The water stops up the invisible channels through which the air should circulate, and the wall remains damp notwithstanding the evaporation that takes place at the surface, to the great harm of the inmates. Like wet clothes, damp walls are un-
