The effect of wetting porous materials is quite surprising. In proportion as the pores fill with water, they become impervious to air. The adhesion of water to stone and mortar is greater than that of air, by as much as water is heavier than air. It is not difficult to blow great volumes of air through dry mortar and dry bricks, but it requires a great exertion to drive a few drops of water through the same materials. You know this cylinder of mortar (see above). Instead of blowing air through it into water, I will suck the air out of it: you see now the water rise in the tub and wet the surface of the mortar. Now I'll try to blow again air through the mortar; I cannot, with all my exertions, because the pores of the mortar are filled with water.
This simple experiment lays bare the great hygienic disadvantage of wet walls; they are air-tight, not to speak of other injurious effects.
We all know that new houses are dreaded on account of their humidity. In some countries there are regulations by law, and new houses must be approved with respect to their dryness before they may be let. But the notions about the causes of their humidity, and the means of removing it, are very different and discordant. Allow me, therefore, to explain how water gets into the new house, and how it is to be got out of it.
I need not call to your mind the first steps in a building operation, and how soon a connection is made with some abundant source of water, and that a great deal of water is required for making the mortar, etc. Let us now try to come to an estimate of this quantity of water.
Suppose that 100,000 bricks were used for a building, each weighing ten pounds. A good brick can suck up more than ten per cent, of its weight in water, but we will put down at five per cent, what gets into it by the manipulations of the bricklayer. We will assume that the same amount of water is contained in the mortar, a quantity certainly much understated, although the mortar forms only about one-fifth of the walls: we have thus 100,000 pounds of water, equal to 10,000 gallons, which must have left the walls of the house before it becomes habitable.
The two principal ways in which wet or damp walls are injurious are: 1. By impeding ventilation and diffusion of gases, through their pores being closed up or narrowed by water; 2. By disturbing the heat-economy of our bodies. Damp Avails act as bodies abstracting heat in one direction; they absorb heat by their evaporation, and act like rooms which have not been warmed thoroughly; they are better conductors of heat than dry walls, just like wet garments, and considerably raise our heat-losses by a one-sided and increased radiation. Diseases which are known to be often caused by cold are particularly frequent in damp dwellings—rheumatism, catarrh, chronic Bright's disease, etc.
What can we do to get rid of that immense quantity, of these
