Page:Popular Science Monthly Volume 11.djvu/294

280

F in the two preceding lectures I have tried to draw your attention to the penetration of the air into our clothing and our dwellings, I shall try in this last lecture to do the same in reference to the air which is in the ground, and to its connection and intercourse with the air above the ground. The air in the ground has been somewhat a stranger to our minds; the terms air and earth, just like air and water, implied to our mind things contrary, and exclusive of each other. The earth seemed to have its limit where the air began. Common-sense seems inclined to believe that there can be no air in that whereon we walk and stand. If we say of the surface of the earth that it is the limit of the earth and the beginning of the atmosphere, we are not correct in reference to the latter. The air begins much below the ground, and we ought to say that where the ground, which is a mixture of earth, water, and air, ends, from there the atmosphere exists alone. It is no wonder that no particular attention was paid to the air in the soil; its presence there does not make any direct impression on anyone of our senses; we infer its presence more from other experiences and consequent conclusions. The human mind formerly looked upon the air as something unsubstantial, spiritual, although men saw the effect of hurricanes; no wonder, then, that no one thought of the air hidden in the ground, which cannot even blow the hat from our head.

We again meet here with the fact that, originally, only that calls forth ideas which impresses our senses directly. No one doubts that water penetrates the soil, and moves there according to hydrostatic laws, because we see it run, vanish into the soil, collect and run out again, or we pump it up; but hitherto not many have clearly understood that the whole surface of the earth, as far as it is porous and its pores are not filled with-water, contains air, which is also subject to aërostatic laws. And why so? One feels nothing of that air; it is always calm, it has no color, no smell, no taste, in fact we take it for nothing, I have shown you already how great an error we commit when we suppose a calm air to be motionless. This applies just as much to the air in the soil, which, if its motion were even snail-like, would still travel from a good depth to the surface in one day.

Perhaps I shall succeed in giving you a better idea of the change of the air in the ground than of that in walls. To have a correct idea