freezing of their water. It would be just as incorrect to deny the permeability to air of the frozen soil as that of the Maltese rock. Still, the most erroneous views have been formed on this subject, even by men distinguished in other branches of science.
Having given you an idea of the quantity of air in a porous soil, I have to give you a correct idea of the mobility of this air and of its change, and I shall try to do this in a roundabout way, as I cannot do it by direct impression on your senses.
There are things of whose existence we become only aware when they are absent. Probably the fish is as little aware of the water he lives in as we of the air, till he finds himself on the dry land. The creatures living in the air know nothing of its oxygen, but, when we place them in an atmosphere which has none or too little of it, or too much carbonic acid, they will feel and behave like the fish out of water. There is a difference in the want of oxygen between different animals; birds want a good deal proportionately, A canary-bird takes about one and a third cubic inch of oxygen from the air in one hour. In one litre of air there are about thirteen cubic inches of oxygen, which the bird would have consumed in ten hours. But he would be dead long before, as he could not live in an air deprived of one-half of its oxygen. The bird in this glass cylinder has been shut up between gravel for the last ten hours, and you see he is quite well. The cylinder is shut at its lower end by a wire netting, on which a stratum of gravel rests. The bird stands on the gravel, and above him there is another wire netting, which supports a stratum of gravel. The free space for the bird contains about one litre of air.
This bird is shut up in the same way as workmen sometimes are, digging at a well or at some kind of shaft. If accident does not kill them at once, they seldom die from want of air, even if it takes some days to dig them out, although man's consumption of oxygen is about a thousand times as great as that of the canary-bird. Some years ago, in Saxony, two men who were shut up in the shaft of a well for ten days kept alive, and were not much the worse for it when they came out again. I may mention here that the celebrated Fraunhofer, when still apprenticed to a glazier at Munich, was buried for several days under the ruins of his master's house, which had fallen in.
I expect Fraunhofer's luck will be shared by this bird, whom I intend removing to-morrow to his old cage.
You cannot longer have any doubt about the motion of the air through gravel: but I want to convince your senses; I want you to see the motion of the air, and to see that motion taking place through a much thicker stratum of gravel than the strata shutting in the bird.
You see this high glass cylinder (Fig. 2), with a smaller glass tube inside, open at both ends. The cylinder is filled with gravel, and the glass tube connected with a manometer by some India-rubber tubing.
