GROUND-AIR IN ITS HYGIENIC RELATIONS. 283 



freezing of their wiater. It would be just as incorrect to deny the 

 IDermeability 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 distinguislied 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 tliis air and of 

 its change, and I shall try to do this in a roundabout way, as I can- 

 not do it by direct impression on your senses. 



There are things of whose existence we become only aware Avhcn 

 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, wlien 

 we place them in an atmosphere wliich 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 dif- 

 ferent animals ; birds want a good deal proportionately, A canary- 

 bird takes about one and a third cubic inch of oxygen fi-om 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, w^as bur- 

 ied for several days under the ruins of his master's house, which had 

 fallen in. 



I expect "Fraunhofei-'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. 



