THE HABITATION AND THE ATMOSPHERE. 179 



the level of the soil, but extends below it to a considerable depth. 

 The most compact soils include a considerable volume of air, as well 

 as an ever- varying quantity of moisture. When we pour water into a 

 vessel full of well-packed gravel, and displace the air which is present, 

 we find that it generally forms one third of the total volume of the 

 mass. The porosity of the earth sometimes reaches fifty per cent ; 

 and miners and well-diggers accidentally buried under cavings-in have 

 sometimes been known to live for several days by the aid of the air 

 circulating through the earth. 



Porous soil does not become impermeable to air till below the level 

 at which the subterranean water ceases to exist. Frozen ground does 

 not lose its porosity by the solidification of the water. Incessant in- 

 terchanges are taking place between the underground air and the free 

 atmosphere. It is by such means that infiltrations of lighting-gas im- 

 pregnate the soil of the street, penetrate sewers, and cause ills which 

 are wrongly attributed to typhoid affections ; and this is most liable to 

 take place in winter when the rise of gas from the soil is promoted by the 

 draught of the chimneys. Ventilation is thus partly carried on through 

 the floor, to such an extent that the atmosphere of a room sometimes 

 contains from ten to fifteen per cent of air from the ground. Hence 

 the danger from impurities absorbed by the soil. They rise, pitilessly re- 

 turning from the earth, as if to chastise us for our carelessness. The 

 air included in a garden-soil, and generally in any soil rich in organic 

 matters, always contains a strong proportion of carbonic acid. At 

 the same time the oxygen is in diminished quantity, proving that the 

 carbonic acid proceeds from slow combustions, and not from subter- 

 ranean emanations. According to the observations of Pettenkofer, 

 Fleck, and Fodor, the proportion of acid increases with the depth, and 

 at a few yards beneath the surface sometimes exceeds ten per cent. 

 This presence of carbonic acid is a sign of the activity of the life in 

 the soil. We do not know the exact manner in which the soil and 

 subsoil intervene in the etiology of endemic diseases and the appear- 

 ance of epidemics. It is a subject of active controversy. We can, 

 nevertheless, approve the teaching of the hygienists who advise us to 

 render our dwellings independent of the soil-air by making provisions 

 for aeration under the basements, or by making the floors impermeable. 



Parks and gardens are beneficial, not only because they give a de- 

 gree of shade and coolness in hot weather, but also because vegetation 

 absorbs waste matter and purifies the soil, and thus diminishes the lia- 

 bility to epidemics.* It is well, for other reasons, to increase these oases 

 in cities where the air is not directly vitiated. But the quantity of oxy- 

 gen which the plants disengage is too small to be made an object. The 

 phenomena of vegetation are extremely slow of accomplishment. Vast 

 spaces and a long time are needed to produce the grass and the wood 



* We may here take notiee of a scheme of M. Autier's for serving the citizens of Paris 

 in their houses with pure air brought through pipes from the forests. 



