Page:Popular Science Monthly Volume 29.djvu/95

Rh house that is even moderately damp, he can hardly have failed to notice that the boards behind framed engravings swell and bulge out, which is the result of an increase in the bulk and area of the boards exactly proportionate to the quantity of water they have absorbed. When there is a sufficient supply of water certain fungoid growths will begin on the surface of the print under the glass, exactly like the growth of plants from the damp earth in a garden or conservatory. If there is iron in the paper here and there (which often happens), there will be spots of oxide of iron, or what we call rust, to give a pleasing variety of color, and if one of them happens to occur on a face, it must of course add greatly to its charm. Wooden backings are safer; and I have seen a room where the engravings with millboard behind them were all more or less spoiled by damp, while a large engraving with a thick wooden backing was entirely uninjured. Nevertheless, I would rather not trust to deal boards, as it is well known that deal is very absorbent of moisture. I remember having a heavy block of deal dead-wood removed from the hull of a boat, and when it was sawed through the water oozed freely out of every fiber. Had it been submitted to a powerful pressure, such as that from a hydraulic press, there can be no doubt that it would have been like squeezing a wetted sponge.

The necessity for careful precaution about the backing of framed engravings is not simply due to the permeability of walls that let the damp come through them; it may be also due to mere condensation on the inner surface of the wall even when it is well built and impermeable. This is best seen on a painted wall, as papers can absorb a great deal of water without letting it be immediately visible. In a very cold winter the external walls of a house become chilled throughout their mass, and when they are painted on the inside a sudden rise in temperature will produce visible condensation from the damp air, because the wall has not yet had time to raise its own temperature to that of the atmosphere. If there are engravings against the wall, they will suffer as much as if the wall itself were damp throughout its substance; for if the backings are absorbent they will drink in a quantity of moisture from the streaming wall-surface, which they will afterward slowly give off to the engraving for the encouragement of fungi and rust-spots. If oil-pictures are hung against a wall of this kind, the canvas will absorb moisture (unless certain precautions are taken, of which we may give an account presently), and then the increase in its bulk and area will cause it to hang loosely on the stretching frames. The only way to combat condensation is by heating the air sufficiently to warm the walls themselves, when, of course, it must cease. Nature herself puts an end to it ultimately in the same way if the mild weather continues, but more slowly, as it takes some time to raise the temperature of a mass of stone by a gentle increase of heat. A thin inner wall, or wainscot separated by a little space from the