20 THE POPULAR SCIENCE MONTHLY. 



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 re- 

 appearance 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 experi- 

 ment, but I beg to ask. What is the reason of this action of the wa> 

 ter? 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 can- 

 not 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 col- 

 ors, 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 appear- 



