RELATION OF AIR TO THE HOUSE WE LIVE IN. 199 



The eftect of wetting porous materials is quite surprising. In pro- 

 portion 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 mate- 

 rials. 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 efiects. 



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 mor- 

 tar, 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 weigh- 

 ing 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 bet- 

 ter conductors of heat than dry walls, just like wet garments, and 

 considerably raise our heat-losses by a one-sided and increased radia- 

 tion. Diseases which are known to be often caused by cold are par- 

 ticularly frequent in damp dwellings rheumatism, catarrh, chronic 

 Bright's disease, etc. 



What can we do to get rid of that immense quantity, of these 



