THE RELATIONS OF THE SOIL TO HEALTH. 181 



turned under by plow or spade, and mixed with the sand or clay-base, it consti- 

 tutes the ordinary agricultural soil. 



II. The Atmosphere of the Soil, or Ground Air. — The interstices of 

 the soil are occupied by air or water, or by both together. The soil's atmosphere 

 is continuous with and resembles in physical and chemical properties that which 

 envelops the earth. Its proportion to the mass of the soil depends upon the 

 degree of porosity of the soil and upon the amount of moisture present. In a 

 very porous soil, such as for example a coarse sand, gravelly loam, or coarse- 

 grained sandstone, the amount of air is much greater than in a clayey soil, gran- 

 ite, or marble. So, hkewise, when the soil contains a large proportion of water, 

 the air is to this extent excluded. The porosity of various soils, as evidenced by 

 the amount of air contained in them, is much greater than would at first thought, 

 be supposed. Thus it has been found that porous sandstone may contain as 

 much as one-third of its bulk of air, while the proportion of air contained in 

 sand, or loose soil may amount to from thirty to fifty per cent. 



The ground-air is simply the atmospheric air which had penetrated into the 

 interstices of the soil and taken part in the various chemical decompositions going 

 on there. In consequence of these chemical changes, the relative proportions of 

 the oxygen and carbonic acid in the air are changed, oxygen disappearing and 

 giving place to carbonic acid. It is well known that, during the decay of vege- 

 table matter in the air, carbonic acid is formed ; one constituent of this compound 

 — the carbon — being derived from the vegetable matter, while the oxygen is taken 

 from the air. Hence, if this action takes place where there is not a very free cir- 

 culation of air, as in the soil, the air there present soon loses its normal propor- 

 tion of oxygen, which enters into combination with the carbon of the vegetable 

 matter to form carbonic acid. 



Thirty years ago, MM. Boussingault and Levy, two distinguished French 

 chemists, examined the air contained in ordinary agricultural soil, and found that 

 the oxygen was diminished to about one-half of the proportion nominally present 

 in atmospheric air, while the carbonic acid was enormously increased. The exact 

 results obtained by Boussingault and Levy were as follows : 



In one hundred volumes of ground air there were 10-35 volumes of oxygen, 

 79-91 volumes of nitrogen, 9-74 volumes of carbonic acid. In atmospheric air, 

 on the other hand, there are in one hundred volumes 20-9 volumes of oxygen, 

 79-1 volumes of nitrogen, 0-04 volumes of carbonic acid, or about one twenty^ 

 fifth of one per cent of carbonic acid. 



In spite of the striking results obtained by these two chemists, very little 

 attention was paid to them by sanitarians, as very few seemed to have any clear 

 notion of the relations existing between the motions of the air above ground and 

 that under ground. 



In 1 87 1, however, Prof. Von Pettenkofer, of Munich, whose authority in 

 sanitary matters is second to none, published the results of his own examinations 

 into the constitution and physical conditions of the ground air, and the relations 

 of the latter to the propagation of epidemic diseases. These researches, which 



