Page:Popular Science Monthly Volume 11.djvu/215

Rh closed. The heat from the fire begins by heating the places nearest to it, and a good deal of water evaporates, so that the air in the room must come nearer its point of saturation. But at a distance from the fire, the walls being colder than the air, dew falls, and, if the pores still contain great quantities of the building-water, they soon begin to overflow.

Another proof that the water chemically combined with the hydrate of lime is not able to fill the pores when it becomes liquid lies in its proportionately small quantity. A house built, let us say, with 100,000 bricks, contains, at most, about 33,000 pounds of burned lime. This cannot combine with more than about 10,000 pounds of water in becoming a hydrate. By the time the mortar is hard and set, and the building becomes inhabited, probably one-half of the lime has become a carbonate, and there remain only 5,000 pounds of water in the remaining hydrate, which is five per cent, of the whole mass of 100,000 pounds of water which got into the new building during its erection. If, then, the other ninety-five per cent, of the building-water were gone, the five per cent., or even ten per cent., remaining, or formed by the change of hydrate into carbonate of lime, would not produce the optical phenomenon of dampness.

I have dwelt somewhat longer on this subject because it is indispensable for a correct view of the function of the wall: the removal into the open air of a great part of that watery vapor which develops itself in every human household. Our walls have to swallow a good deal of that vapor as water, and to pass it on through their body that it may evaporate on their outer surface. That is the reason why localities looking to the north, or shaded from the sun, are so much damper. This appears most clearly in unheated places, chiefly at the transition from winter to spring, when it is warmer outside than inside. We are glad to have once more the windows open to let the tepid spring air gladden the cold interior; but a good deal of water will soon be seen deposited on the walls and the objects within the rooms, which has to evaporate just as from a new building.

You are now well aware of the usefulness of porous building-materials; they alone can make dry dwellings. I cannot help thinking badly of all the substitutes for wood, brick, and mortar, which have been proposed, as zinc, iron, putty, etc. Perhaps the natural functions of the mortar-wall may one day be efficiently exercised by something else; for the present it has not been done, and will not be done as easily as many so-called practical people suppose.

Let me just relate to you a case, which shows that, without a correct view of the functions of walls, an apparently excellent plan may just produce the reverse of what was intended. In the neighborhood of iron-smelting works, the slag is often used for building-purposes. This material, associated with other stones, does very well. As it exists only in very irregular shapes, it requires large masses of mortar.