AIR AND LIFE. 141 



may be uuable to affect the olfactory membrane ; but this is not the case 

 with the atmosphere generally. The air that surrounds us is full of 

 scents and odors, but we are so accustomed to them that we take no 

 notice thereof. But after we have spent some time in an atmosphere 

 where most ordinary odors can not conveniently gain access, and then 

 return to our ordinary surroundings, the case is altered, and we perceive 

 a very powerful odor. This has been noticed by different observers 

 after a considerable sojourn in deep caves, such as the Mammoth Cave 

 in Kentucky. The air in these caves is nearly odorless, and when, 

 after a few hours spent in this scentless environment, the visitor 

 emerges again into the open, the atmosphere seems powerfully, even 

 violently, scented or odoriferous, and some persons may even be tempo- 

 rarily affected by the intensity of the sensation. During the sojourn 

 in the unscented air the olfactory cells have rested, but the renewal 

 of their activity, generally unconscious, is accompanied by a very strong 

 sensation which however soon fades. 



The atmosphere does not stop at the surface of the seas, nor does it 

 cease at the surface of the soil. It penetrates both, the former espe- 

 cially. In the latter the access of air is very soon arrested by the 

 compactness of the rocks or strata, and, generally speaking, the pro- 

 portion of air in the soil is very small in all cases where there are no 

 clefts, fissures, or deep underground galleries. In the superficial layer, 

 however, the case is different; air is always present in appreciable pro- 

 portions in the less compact parts where plants push their roots and 

 seek their nutriment ; and in the deepest shafts, caverns, caves, and 

 other natural or artificial excavations of the soil, air exists. It should 

 not be expected to find there as pure a gas as that which surrounds the 

 exterior of the planet. In the soil many slow but continuous chemical 

 reactions are going on between the air and the solid constituents, and 

 the result is an alteration of both sets of elements ; some chemicals of 

 the earth and rocks are transformed, and while the air loses some part 

 of its constituents new elements are added to it, and thus its normal 

 composition is soon altered. This is the reason why great care should 

 always be exercised to ascertain the condition of air in all deep cavi- 

 ties, and even in normal excavations if they are rather secluded. The 

 air may have been so much altered in its composition as to have be- 

 come unfit for the maintenance of life, and cases are on record where 

 it consisted almost entirely of carbonic acid. Among the investigators 

 who have specially concerned themselves with the chemical composi- 

 tion of "ground" air, Boussingault has obtained interesting results, 

 showing that while 1 cubic meter of normal atmosphere contains 

 about 4 deciliters (or 0.21G gram) of carbon, 1 cubic meter of ground 

 air contains 9 liters (or nearly 5 grams), which is twenty-two or twenty- 

 three times more. In recently manured soil the proportion is much 

 more considerable, and the amount of carbonic acid may be twenty- 

 four times as great as in atmospheric air. This considerable amount 



