RELATION OF AIR TO THE HOUSE WE LIVE IN. 197 



she can only recognize the laws of Nature. But science changes the 

 notions of man, and often reverses them. Ideas and notions based on 

 science enrich us, partly directly, partly indirectly, with new means 

 of making use of natural laws. It was not till astronomy had found 

 and determined celestial mechanics that the human mind was enabled 

 to begin that development of the mechanical element which is the 

 pride and the power of our period as compared with former times. 



If, then, we are hopefully satisfied with endeavoring to increase 

 our insight, our science of the things that are, the benefit will not fail 

 to come, and everything is beneficial of which man learns to make 

 use. This requires time often a very long time as old experience 

 teaches. 



The task of science is to lay hold of everything perceptible, and 

 to penetrate it the small as well as the great. The insect and its 

 life is just as interesting to science as the elephant, and therefore I 

 believe that I may occupy myself with that air which flows through 

 our walls, although its motion is not recognized by our sensations. 



We may conclude from many facts that walls are permeable to 

 air. N"o one maintains that houses have water-tight walls, and every- 

 body knows that masonry is easily penetrated by water. Wherever 

 a wall is in perpetual contact with water, it becomes so soaked that 

 at last water comes out in drops on the other side. Certainly, where 

 water can pass, air must pass much more easily, because air is seven 

 hundred and seventy times more light and movable than water. It 

 is very easy to construct water-tight apparatuses and vessels, but 

 very difficult to make them air-tight. Still people are surprised when 

 they hear of a change of the air through a wall ; they see, of course, 

 and feel the water in the wall, but of the air in it their senses have no 

 direct perception. 



But we have means to demonstrate to our senses the passage of 

 air through our building-materials ; we have only to lead the air 

 which comes against some large surface of wall into and through a 

 narrow tube. I will prove this to you by experiments ; but you have 

 often seen the same thing before, when you were looking at some 

 piece of water which had some small in- and outflow. These may be 

 in lively motion and driving mills, but on the whole surface the water 

 seems to be completely at rest. But, if we do not see any water run- 

 ning in and out, we declare the whole to be stagnant, and we may be 

 very much mistaken. 



I have here a cylindrical piece of mortar, half lime, half sand, five 

 inches by one and two-thirds. The cylinder has been covered all over 

 with melted wax, which is inpermeable to air, with the exception of its 

 two circular ends. You see this glass funnel, with a tube. I fix it on 

 one circular end, where the mortar lies free, and make an air-tight 

 connection by wax with the waxen coat of the cylinder. If I blow 

 through the tube, the air must appear on the free mortar-end, pro- 



