170 THE POPULAR SCIENCE MONTHLY. 



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 com- 

 pact 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 build- 

 ing 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 ab- 

 sorbed by the walls, to be evaporated from the outside, under the ac- 

 tion of the sun and wind. For this reason it is well for building mate- 

 rials to be porous and permeable, and for them to interpose no obstacle 

 to the circulation of the air which is depended upon to promote evap- 

 oration. 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 atmos- 

 phere, 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 im- 

 pregnated 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- 



