ure.

CHAPTER XV.

SOIL.

THE nature of the soil in its relation to health is an important subject. The relation of the soil of a locality to the public health is dependent upon its intimate structSoils are composed of varying proportions of mineral, vegetable, and animal constituents. These constituents vary in size not only in different localities, but even in the same locality. The interstices of the soil are filled either with air or with water. A soil is moist or dry according to the preponderance of small- or large-sized soil-particles. The finer and more uniform the soilparticles, the greater the amount of moisture usually contained in the soil.

All soils are porous, and contain varying amounts of air and moisture. The relation of the soil to health is influenced by the amount and nature of the contained air and water. The degree of purity of the ground-air and ground-water is influenced by the amount and nature of the vegetable and animal organic matter contained in the soil, and the temperature of the locality-whether favorable or unfavorable to the decomposition of organic matter through the agency of bacteria.

Ground-air.-Ground-air is usually rich in carbon dioxid, derived from decomposing organic matter in the soil. It is also very moist, because there is usually plenty of opportunity to take up moisture. It also contains decomposition-products, such as marsh-gas, hydrogen sulphid, and ammonia. This air is, consequently, not suitable for respiratory purposes. The amount of soil-air that gains access to houses under ordinary conditions is, however, so small that its influence prob

ably is not felt. In newly made soils, in which there is considerable decaying organic matter, there is some danger of the entrance of large amounts of ground-air into houses built on such soils unless special provision is made to exclude it. In such houses there should be cemented foundation walls and cellars, and the supply of fresh air should be derived from the outside at some distance above the ground. Unless the foundation walls and cellars are cemented, the houses, when warmed, may serve as an immense chimney in extracting the air from the surrounding soil.

Ground-water.-Ground-water is rain-water that has fallen upon the soil of the locality and penetrated its surface. It differs from stored rain-water according to the nature of the soil constituents. It is richer in dissolved solids, and contains also the products of decomposition derived from decaying organic matter in the soil. It contains also numerous bacteria derived from the soil. The relation of ground-water to health is directly dependent upon the presence or absence of pathogenic bacteria in the soil, and the presence or absence of mineral constituents derived from the soil which may be injurious to health, such as salts of calcium, magnesium, or iron.

Pettenkofer's theory of the relation of soil moisture to typhoid fever and cholera. is no longer tenable. We know now that the height of the level of the groundwater has no direct influence in the production of either of these two diseases. There is evidently an indirect relation between low ground-water and the development of these diseases, because at such times the drainage area of all wells is increased, and the polluting material in or upon the soil of a corresponding greater area is conducted into the well. In the same manner, when drinkingwater is derived from streams there is greater oppo ̈tunity for the entrance of concentrated polluting matter into the stream, and it exists there in a more concentrated form than in times of flood.

The relation of a damp soil to the greater prevalence of consumption, as originally pointed out by Bowditch, cannot be regarded as a direct one. The damp soil probably predisposes to colds and diseases of the lungs, and thus paves the way for the contraction of consumption. There is thus far no evidence that the bacillus of tuberculosis is capable of multiplying in damp soils.

The relation of damp, marshy soils to malaria has received a great deal of elucidation in the last few years. It is believed at the present time that the malarial organism is contracted most frequently, if not entirely, through the sting of a particular species of mosquito-Anopheles. These mosquitoes are usually indigenous to the soil of certain marshy localities, but thus far no definite relation between the nature of the soil of these localities and the prevalence of mosquitoes has been demonstrated. The fact that many localities were formerly foci of malaria would indicate that they were infested with these same mosquitoes, and that, from some unexplained cause. they became healthful. Marshy localities, when drained so as to prevent the development of mosquitoes, also become healthy and free from malaria. Where drainage is impossible, it is believed that the application of some antiseptic, such as crude petroleum, to the marsh will prevent the development of the mosquitoes and thus eradicate malaria from those localities.1

Damp soils are likely to be productive of diarrheal diseases, though these affections are most probably brought about by certain bacteria in the soil along with the detrimental influence of the dampness itself. The amount of decaying organic matter in and upon the soil is most probably in direct relation to the prevalence of diarrheal diseases in a locality.

In the same manner the amount of moisture in the soil will influence the prevalence of other diseases, such as rheumatism, bronchitis, pneumonia, and the exanthem

1 Recent studies in Africa indicate that probably other diseases, especially elephantiasis, are conveyed by species of mosquitoes.

ata. The relation of soil moisture to these diseases is probably only an indirect one in that it favors the maintenance of the life of the morbific agents by preventing drying.

Pathogenic Bacteria in Soil.-Some of the pathogenic bacteria are apparently capable of living in the soil for a long time, and some of them may even be able to multiply in the soil. Among these, the bacillus anthracis, the bacillus tetani, and the bacillus of malignant edema are perhaps most capable of subsisting in the soil because of their faculty of passing into the spore stage. The bacillus enteritidis sporogenes of Klein may also be included in this class.

The bacillus typhosus and the cholera organism are less tenacious, and die after a time through the detrimental influence of the soil organisms. The bacillus tuberculosis can remain alive in soil for some time when protected from the influence of soil organisms, though the danger of infection through polluted soil is a remote one. The pyogenic cocci and the diphtheria bacillus cannot exist in the soil for any length of time unless. protected from the influence of the soil organisms.

Improvement of a Damp Soil.--A damp soil may be improved by opening the outflow or by laying a system of underground drains. The construction of sewers often serves to drain the soil to a considerable extent because the ground-water follows the outside of the

sewer.

Configuration of the Surface and Soil-covering.Aside from the intrinsic nature of the soil itself, and the character and amount of air and water contained in its interstices, the healthfulness of a soil is influenced also by the configuration of the surface, the condition of the surface, and the nature of the soil-covering. With regard to the configuration of the surface, it may be said that, as a rule, highlands are more healthful than lowlands. The degree of healthfulness of lowlands is influenced by the nature of the soil composing them. The

condition of the surface of the soil with regard to soil-covering which is least healthful is what is known as a desert. Here the soil is exceedingly dry and cannot be cultivated. Cultivated areas and areas covered with forests are more healthful, because the soil is shaded and thus the heating effect of the sun's rays is partly excluded. Cultivated areas that are thickly populated are less healthful because of the organic impurities which gain access to the soil. The soil of cities can be maintained in a comparatively healthful condition only by systematic drainage, so as to carry away all the organic impurities without contaminating the soil, air, or drinking-water.

21