338 THE POPULAR SCIENCE MONTHLY. 



of the soil on which we dwell. Heavy, towering buildings often stand 

 on a soil which is filled to the extent of a third of its volume with air. 

 The investigation of ground-air has just begun, but it has already sur- 

 prised us with some unexpected revelations. Ground - air is distin- 

 guished from the air that passes over the surface by the higher propor- 

 tion of carbonic acid it contains, which increases, as a rule, with the 

 distance from the surface, and to which our springs owe their charges 

 of that gas. This carbonic acid is chiefly derived from organic matters 

 and organic life in the ground, with which it increases and diminishes. 

 Air brought by Zittel from the dead dry soil of the Libyan Desert, 

 sealed in glass tubes, showed no larger proportion of carbonic acid 

 than the free superficial air, but the ground-air from a palm-garden in 

 the oasis of Farafreh yielded much carbonic acid. That this gas is 

 mostly derived from organic changes is shown from the investigations 

 of Fleck, Fodor, Wolff hiigel, Moller, Wallny, and others, who found 

 that the proportion of oxygen in ground-air was lower, while that of 

 carbonic acid was higher, than in free air. , 



That the air in the soil does not become stagnant, but is always in 

 motion, though sluggish, not only follows from physical laws, but may 

 be easily proved by experiments and observations. Our houses are 

 aired or ventilated in no small degree by the ground-air. Renk has 

 been inquiring, with the aid of Recknagel's differential manometer, 

 whether the air flows from the ground into the house or from the 

 house into the ground, and has found that through most of the year 

 the draft is from the ground into the house. He has also found that 

 the ground-air, which is sucked into the house, brings, dust with it, and 

 other observers have shown that the same air also carries germs sus- 

 ceptible of development in suitable solutions. 



It is thus easy to see how the soil affects our health without our 

 having to eat it ; the ground-air plays the part of an always ready 

 intermediate agent, so far as concerns the molds. In this light it is 

 easily seen why some houses sometimes have to suffer so badly from 

 certain conditions of the soil, especially when they are badly ventilated. 

 The movement of the air in a close house is many thousand times less 

 active than where the circulation is free ; and the air entering into the 

 house suffers correspondingly less dilution than that passing into the 

 free atmosphere, and leaves in it much more of what it brings up from 

 the ground. While the house is heated during the cold season, and at 

 night in the summer, while the air within-doors is warmer than the 

 surrounding out-door air, the houses act as draught-flues, and suck air 

 out from the ground as if they were cupping-glasses set over it. Ex- 

 perience has long taught us that it is most dangerous to sleep that 

 is, to pass the night in such noted fever regions as the Pontine 

 Marshes. 



.Many persons believe that the ground-air is an object whose exist- 

 ence is still pre-eminently theoretical, and that its practical influence 



