ance till it has dried away. This is because all oil-colors become porous in the course of time.
For water to penetrate thus into the colors, we are entitled to assume that there must be free spaces within them to receive it, pores and interstices. These cannot have been vacua before, but must have contained air. This air in the painted surface is displaced by the water, and hence the difference in the optical effect. Air and water have different optical properties. In the first instance our colors—dry and dulled—are mixed with air; in the second instance with water. Water refracts, disperses, and reflects light quite differently from air; therefore it must have quite a different effect on colors when it gets admixed with them instead of air. The whole question has been more fully treated by me in a little treatise on oil-colors and the preservation of galleries; it may suffice here, and for the present, to know that damp spots on a wall can appear only when the pores are filled with water or some other transparent liquid. Our sensations have rightly taught us to associate the words dry and airy, damp and confined.
If we have moved into a new building too soon, we may be deceived by its appearance. It is quite possible that the walls have become sufficiently free from water and full of air for the colors of the papers and walls to appear mixed with air and free from all interstitial water; still, we are not entitled to suppose that all water has left the walls. A good deal may remain unnoticed, provided some air is lodged in the pores of the surface sufficient to produce the optical effect of real dryness.
How does it, then, happen that, on receiving their complement of inhabitants, the pores of the new walls become obstructed, partly or locally, by water? The ordinary explanation is completely erroneous, although it sounds quite scientific and rational, and has its place in books and lectures on chemistry. It is stated that it is the effect of carbonic acid on the hydrate of lime which remained in the mortar. Mortar is a very interesting object, and I regret that I cannot enter more fully into its nature and process of hardening. I'll tell you so much, that the burned and slaked lime used for its preparation is a compound of lime (oxidized calcium) and water, the above-mentioned hydrate of lime. This, by the action of the air, is changed into carbonate of lime. This change takes place at first very rapidly, and to the extent of about one-half, but then slower and slower, so that in very old masonry one finds frequently some of the original hydrate of lime. This is a perfectly dry substance, which yields none of its water to air which is dry and free from carbonic acid. When changed into carbonate of lime, the water, which as a hydrate it contained, chemically combined, is set free, while the lime and the carbonic acid combine. This water is commonly considered to produce the damp spots which appear here and there in new buildings. It has been im-
