RELATION OF AIR TO THE HOUSE WE LIVE IN. 201 



ance till it has dried away. This is because all oil-colors become po- 

 rous in the course of time. 



For water to penetrate thus into the colors, we are entitled to as- 

 sume 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 wa- 

 ter, 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. Wa- 

 ter 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 pres- 

 ervation 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 de- 

 ceived by its appearance. It is quite possible that the walls have be- 

 come 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 completoent 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 car- 

 bonate 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 wa- 

 ter 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- 



