298 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



vapor is very much less through mold than through saud or dust. 

 Iu hot climates, such as India and Italy, on bare sandy ground and in 

 valleys it seems probable that the differences in temperature between 

 soil and surface air may amount at night to between 30° and 40°, and 

 in malarious places the flow of impure vapor toward the surface may 

 be equal to the evaporation from a marsh. These facts have a very 

 distinct bearing on the generation and prevalence of malaria, diarrhea, 

 dysentery, and other diseases. 



Herr Singer, at Munich, found that the maximum temperature of the 

 soil (59.3) at 4 feet 3 inches, was reached on August 24, and Fodor's 

 results gave a maximum temperature at depths between half a meter 

 and 1 meter in August. Liebenberg observed that sand is warmed 

 throughout more rapidly than clay and that the richer a soil in organic 

 matter the greater its power of absorbing heat. Pettenkofer's obser- 

 vations show that a very large amount of air is contained even in firm 

 soils and that effluvia from decomposing organic matter may pass for 

 a long distance through very loose soils. Permeable soils are sand- 

 stones, loose sands, and chalk, and are generally healthy unless they 

 contain much organic matter or are superposed upon a clay or other 

 impervious stratum which holds up the water near the surface. Move- 

 ment of subsoil water of course greatly affects the quantity of earth 

 vapor given off during certain periods. The dried beds of water 

 courses are well adapted for the evolution of malaria, for the super- 

 ficial layer is usually permeable, the soil contains much organic matter, 

 the water level is not far from the surface, cold air collects over the 

 valley and is often moist and stagnant. In the dry regions of Aus- 

 tralia it is well known that water may be found at a little depth below 

 the dry channels of rivers. 



Vegetable mold near the surface of the earth is very rich in sapro- 

 phytic bacteria, and Flugge states that infusions made from manured 

 fields and garden earth contain thousands of bacteria in every drop, 

 though diluted one hundred times. But the observations of the pres- 

 ent writer tend to prove that the retention of heat and moisture by 

 this kind of earth is much greater than that of other soils, and that 

 much less emission of vapor takes place from it into the air, so that 

 the organisms which might be expected to invade in excess the air over 

 cultivated ground may in reality be scarcely capable of entering it. 



GROUND AIR. 



The amount of air in the upper layers of the earth is very consider- 

 able, but varies greatly with the nature of the soil. Gravel and sand 

 contain a large quantity of air, which has been estimated at one-third 

 of its bulk. A bird has been experimentally inclosed in a glass cylinder 

 with a solid bed of gravel below and above it, and was not affected, the 

 air which passed through the earth being sufficient to maintain life. 

 The proportion of carbonic acid, however, in some soils, especially 



