RELATION OF AIR TO THE HOUSE WE LIVE IN. 203 



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 hy- 

 drate 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 re- 

 maining 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 indis- 

 pensable 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 in- 

 side. 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-ma- 

 terials ; 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 cor- 

 rect 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. 



