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. No one maintains that houses have water-tight walls, and everybody 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 running 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-
