THE RELATIONS OF THE SOIL TO HEALTH. 183 



it, and the ground air consequently heavier and denser, the latter is not easily 

 displaced. It is perhaps due to this fact that those infectious diseases which are 

 probably dependent upon the movements of the ground air, are less prevalent in 

 the spring and early summer than in the latter part of summer, autumn, and early 

 winter. In the autumn, the ground air being warmer than the air above ground 

 is easily displaced by the latter, and forced out into the streets and houses to be 

 inspired by men and animals. The same conditions may explain the greater 

 likelihood of infection at night, which is proved for such diseases as malarial and 

 yellow fevers. The colder outside air penetrates the interstices of the soil, and 

 forces out the impure ground air. 



The researches of Fodor have demonstrated that the proportion of carbonic 

 acid in the ground air may be taken as an approximative measure of the impurity 

 of the soil whence the air is taken. The influence of the permeability of the soil, 

 as before pointed out, must however not be overlooked in estimating the signifi- 

 cation of the carbonic acid. Fodor has shown that the proportion of carbonic 

 acid in the ground air, and consequently the amount of organic decomposition, 

 is greatest in July and least in March. That the carbonic acid is derived from 

 the decomposition of organic matter has been proved by Pettenkofer. This ob- 

 server examined specimens of air brought from the Libyan desert, and found that 

 the proportion of carbonic acid in the ground air was exactly the same as in the 

 air collected above ground. There being no vegetable growth in the desert 

 there can, of course, be no vegetable decomposition going on in the soil. 



The excess of carbonic acid in the ground air is an indication of the deficiency 

 of oxygen, as has been shown. The air at a depth of thirteen feet below the sur- 

 face was found to contain only from 7 to 10 per cent of oxygen — one-half to one- 

 third of the normal proportion. Many basements occupied by people as living 

 rooms extend from five to ten feet under ground, and hence are liable to be sup- 

 plied with an atmosphere approaching in impurity that just mentioned. It re- 

 quires no very vivid imagination to appreciate the dangers to health that dwell in 

 such habitations. 



III. The Water of the Soil, or Ground Water. — At a variable depth 

 below the surface of the ground, a stratum of earth, or rock, is found, through 

 which water passes with difficulty, if at all. Above this, there is a stratum of 

 water which moves from a higher to a lower level, and which varies in depth at 

 different times according to the amount of precipitation (rain or snow-fall) and 

 according to the level of the nearest body of water toward which it flows. This 

 stratum of water is termed the gt-ound water^ and has within the last few years 

 assumed considerable importance from its apparently close relations to the spread 

 of certain of the infectious diseases. The direction of horizontal flow of the 

 ground water is always toward the drainage area of the district. Thus, it is usually 

 toward lakes, rivers, or the sea. Rains or a rise in the river cause a rise in the 

 ground water, while long continued dry weather, or a low stage of the river, which 

 drains off the ground water, causes a fall in the latter. On the sea-coast the 

 ground water oscillations probably correspond with the tides. The writer is not 



