Page:Popular Science Monthly Volume 11.djvu/214

202 that the respiration of the new inmates increases the amount of carbonic acid in the air, and accelerates the process, setting free the water, which renders the wall damp and chokes up its pores.

This explanation is not based on any single direct observation made on the wall itself; it is nothing but a specious conclusion. Although hydrate of lime exposed to air which contains carbonic acid changes into carbonate of lime, no one has ever found it becoming moist. The liberation of the water, however much it may be accelerated in the indicated way, is unable to refill the pores of the wall, which were supposed to be already filled with air. To do this it would be necessary that the water in the hydrate should not have occupied any space, or that, when set free, it underwent such an expansion as water becoming a gaseous substance. All scientific analogies and observations protest against this. Changes of solid into liquid bodies take place without any considerable increase of volume; it is different with the transition of liquids into gases when the increase is very considerable.

It is only by the complete choking up of the pores by water, and the complete expulsion of the air from the surface of the wall, that the damp spots can be formed; and the freed water of the hydrate, which cannot fill a space which it had not filled while in its former combination, cannot do this. So the absorption of the carbonic acid is unable to produce the required increase of volume.

The fresh spots in new buildings can only arise from the precipitation of water from the air on the walls.

The inhabitants of a house give rise to a great amount of watery vapor, not only by the functions of their lungs and skin, but also by the numerous manipulations of the household, such as cooking, washing, cleaning, etc. If the air in the house is already saturated with water in proportion to its temperature, a small degree of cold in the wall is sufficient to produce a dew, a precipitation of water from the vapor, just as one sees it on window-panes. But the porous wall can imbibe a good deal, and in old buildings we may see the windows sweating profusely while the walls seem to remain dry. It may last a long time before a well-constructed wall or partition gives any sign. They go on condensing water till their pores are filled and all the air expelled—then, not slowly and gradually, but all at once, numerous damp spots make their appearance.

It is, therefore, clear why those youngsters of houses are so much more subject to damp spots than their brethren of more mature age. Their walls have lost just enough of the building-water to allow the air to occupy part of the pores; optically, they seem dry, but still very little water is required to choke up the pores here and there anew, and wherever this takes place the spots break out. The effect of a fire is very instructive; nothing produces damp spots so easily in a fresh building as the first fire, when doors and windows are well