Page:Popular Science Monthly Volume 11.djvu/212

200 10,000 gallons of water, before we remove into the new house? All this water—we cannot make it run off, we cannot squeeze it out, we cannot boil it away—it must take its leave in one way, a very safe but rather long one, that of spontaneous evaporation into and by the air.

The capacity of the air for receiving water depends on the different tension of the vapor at different temperatures, on the quantity of water already contained in the air flowing over a moist body, and finally on the velocity of that air. For the first two moments let us assume the average temperature of the year to be about 50° Fahr., and the average hygrometric condition of the air to be seventy-five per cent, of its full saturation. Under these conditions, one cubic foot of air can take up four grains of water, in the shape of vapor, but as it contains already seventy-five per cent, of these four grains, which amounts to three grains, it can only take up one additional grain. As often, then, as one grain is contained in the 10,000 gallons of water mentioned above, as many cubic feet of air must come in contact with the new walls, and become saturated with the water contained in them; or about 700,000,000 cubic feet of air are required to dry the building in question.

I will at once pass on to the consideration of a subject which some of you may be acquainted with already by experience—I mean the reappearance of damp in new buildings, which had seemed quite dry, after they had become inhabited. There appear damp spots on walls and in corners, the panes in the windows sweat, and the air becomes musty and oppressive. How does this water return, after the house has been declared and considered dry? Most people, because they see it only then, suppose that there is a new formation of water in the wall, or that it was set free by the presence of the new dwellers. Here, again, our sensuous perceptions mislead our judgment, and give us no clew as to the circumstances under which moisture in the walls becomes visible to our eyes and humidity produces a damp spot. I produce here a piece of brownish-yellow paper of somewhat indistinct tint. Where I wet it with water, the color appears more intense, darker even, just as if the water had been colored. Now the paper is, getting dry again, and the former appearance comes back.

Some of you may laugh at me for making such a trivial experiment, but I beg to ask, What is the reason of this action of the water? It takes place only on porous colors or colored surfaces which are porous, only on aquarelles or frescoes, scarcely on porcelain or glass paintings. If the water cannot penetrate into the color it cannot alter its appearance any more than that of a colorless, transparent glass.

Oil-paintings, when they are new or lately varnished, are in this respect like glass-paintings, but when they get older, some of the colors, by the longer action of the air, get dull, and then water has the same effect on them as on this paper. It gives them a fresher