Page:Popular Science Monthly Volume 24.djvu/191

Rh 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 interchanges are taking place between the underground air and the free atmosphere. It is by such means that infiltrations of lighting-gas impregnate 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 returning 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 subterranean 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 appearance 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 degree of shade and coolness in hot weather, but also because vegetation absorbs waste matter and purifies the soil, and thus diminishes the liability 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 oxygen 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