The Home Medical Library Volume V Part 10

_Acknowledgment_

We beg to tender grateful acknowledgment to author and publisher for the use of Dr. George M. Price's valuable articles on sanitation. The following extracts are taken from Dr. Price's "Handbook on Sanitation," published by John Wiley & Son, and are covered by copyright.

CHAPTER I

=Soil and Sites=

=Definition.=--By the term "soil" we mean the superficial layer of the earth, a result of the geological disintegration of the primitive rock by the action of the elements upon it and of the decay of vegetable and animal life.

=Composition.=--Soil consists of solids, water, and air.

=Solids.=--The solid const.i.tuents of the soil are inorganic and organic in character.

The inorganic const.i.tuents are the various minerals and elements found alone, or in combination, in the earth, such as silica, aluminum, calcium, iron, carbon, sodium, chlorine, pota.s.sium, etc.

The characteristics of the soil depend upon its const.i.tuents, and upon the predominance of one or the other of its composing elements. The nature of the soil also depends upon its physical properties. When the disintegrated rock consists of quite large particles, the soil is called a _gravel soil_. A _sandy soil_ is one in which the particles are very small. _Sandstone_ is consolidated sand. _Clay_ is soil consisting princ.i.p.ally of aluminum silicate; in _chalk_, soft calcium carbonate predominates.

The organic const.i.tuents of the soil are the result of vegetable and animal growth and decomposition in the soil.

=Ground Water.=--Ground water is that continuous body or sheet of water formed by the complete filling and saturation of the soil to a certain level by rain water; it is that stratum of subterranean lakes and rivers, filled up with alluvium, which we reach at a higher or lower level when we dig wells.

The level of the ground water depends upon the underlying strata, and also upon the movements of the subterranean water bed. The relative position of the impermeable underlying strata varies in its distance from the surface soil. In marshy land the ground water is at the surface; in other places it can be reached only by deep borings. The source of the ground water is the rainfall, part of which drains into the porous soil until it reaches an impermeable stratum, where it collects.

The movements of the ground water are in two directions--horizontal and vertical. The horizontal or lateral movement is toward the seas and adjacent water courses, and is determined by hydrostatic laws and topographical relations. The vertical motion of the ground water is to and from the surface, and is due to the amount of rainfall, the pressure of tides, and water courses into which the ground water drains. The vertical variations of the ground water determine the distance of its surface level from the soil surface, and are divided into a persistently low-water level, about fifteen feet from the surface; a persistently high-water level, about five feet from the surface, and a fluctuating level, sometimes high, sometimes low.

=Ground Air.=--Except in the hardest granite rocks and in soil completely filled with water the interstices of the soil are filled with a continuation of atmospheric air, the amount depending on the degree of porosity of the soil. The nature of the ground air differs from that of the atmosphere only as it is influenced by its location.

The princ.i.p.al const.i.tuents of the air--nitrogen, oxygen, and carbonic acid--are also found in the ground air, but in the latter the relative quant.i.ties of O and CO2 are different.

AVERAGE COMPOSITION OF ATMOSPHERIC AIR IN 100 VOLUMES

Nitrogen 79.00 per cent.

Oxygen 20.96 "

Carbonic acid 0.04 "

AVERAGE COMPOSITION OF GROUND AIR

Nitrogen 79.00 per cent.

Oxygen 10.35 "

Carbonic acid 9.74 "

Of course, these quant.i.ties are not constant, but vary in different soils, and at different depths, times, etc. The greater quant.i.ty of CO2 in ground air is due to the process of oxidation and decomposition taking place in the soil. Ground air also contains a large quant.i.ty of bacterial and other organic matter found in the soil.

Ground air is in constant motion, its movements depending upon a great many factors, some among these being the winds and movements of the atmospheric air, the temperature of the soil, the surface temperature, the pressure from the ground water from below, and surface and rain water from above, etc.

=Ground Moisture.=--The interstices of the soil above the ground-water level are filled with air _only_, when the soil is absolutely dry; but as such a soil is very rare, all soils being more or less damp, soil usually contains a mixture of air and water, or what is called _ground moisture_.

Ground moisture is derived partly from the evaporation of the ground water and its capillary absorption by the surface soil, and partly by the retention of water from rains upon the surface. The power of the soil to absorb and retain moisture varies according to the physical and chemical, as well as the thermal, properties of the soil.

Loose sand may hold about 2 gallons of water per cubic foot; granite takes up about 4 per cent of moisture; chalk about 15 per cent; clay about 20 per cent; sandy loam 33 to 35 per cent; humus[10] about 40 per cent.

=Ground Temperature.=--The temperature of the soil is due to the direct rays of the sun, the physicochemical changes in its interior, and to the internal heat of the earth.

The ground temperature varies according to the annual and diurnal changes of the external temperature; also according to the character of the soil, its color, composition, depth, degree of organic oxidation, ground-water level, and degree of dampness. In hot weather the surface soil is cooler, and the subsurface soil still more so, than the surrounding air; in cold weather the opposite is the case.

The contact of the cool soil with the warm surface air on summer evenings is what produces the condensation of air moisture which we call dew.

=Bacteria.=--Quite a large number of bacteria are found in the soil, especially near the surface, where chemical and organic changes are most active. From 200,000 to 1,000,000 bacteria have been found in 1 c.c. of earth. The ground bacteria are divided into two groups--saprophytic and pathogenic. The saprophytic bacteria are the bacteria of decay, putrefaction, and fermentation. It is to their benevolent action that vegetable and animal _debris_ is decomposed, oxidized, and reduced to its elements. To these bacteria the soil owes its self-purifying capacity and the faculty of disintegrating animal and vegetable _debris_.

The pathogenic bacteria are either those formed during the process of organic decay, and which, introduced into the human system, are capable of producing various diseases, or those which become lodged in the soil through the contamination of the latter by ground water and air, and which find in the soil a favorable lodging ground, until forced out of the soil by the movements of the ground water and air.

=Contamination of the Soil.=--The natural capacity of the soil to decompose and reduce organic matter is sometimes taxed to its utmost by the introduction into the soil of extraneous matters in quant.i.ties which the soil is unable to oxidize in a given period. This is called contamination or pollution of soil, and is due: (1) to surface pollution by refuse, garbage, animal and human excreta; (2) to interment of dead bodies of beasts and men; (3) to the introduction of foreign deleterious gases, etc.[11]

_Pollution by Surface Refuse and Sewage._--This occurs where a large number of people congregate, as in cities, towns, etc., and very seriously contaminates the ground by the surcharge of the surface soil with sewage matter, saturating the ground with it, polluting the ground water from which the drinking water is derived, and increasing the putrefactive changes taking place in the soil. Here the pathogenic bacteria abound, and, by multiplying, exert a very marked influence upon the health by the possible spread of infectious diseases. Sewage pollution of the soils and of the source of water supply is a matter of grave importance, and is one of the chief factors of high mortality in cities and towns.

_Interment of Bodies._--The second cause of soil contamination is also of great importance. Owing to the intense physicochemical and organic changes taking place within the soil, all dead animal matter interred therein is easily disposed of in a certain time, being reduced to the primary const.i.tuents, viz., ammonia, nitrous acid, carbonic acid, sulphureted and carbureted hydrogen, etc. But whenever the number of interred bodies is too great, and the products of decomposition are allowed to acc.u.mulate to a very great degree, until the capacity of the soil to absorb and oxidize them is overtaxed, the soil, and the air and water therein, are polluted by the noxious poisons produced by the processes of decomposition.

_Introduction of Various Foreign Materials and Gases._--In cities and towns various pipes are laid in the ground for conducting certain substances, as illuminating gas, fuel, coal gas, etc.; the pipes at times are defective, allowing leakage therefrom, and permitting the saturation of the soil with poisonous gases which are frequently drawn up by the various currents of ground air into the open air and adjacent dwellings.

=Influence of the Soil on Health.=--The intimate relations existing between the soil upon which we live and our health, and the marked influence of the soil on the life and well-being of man, have been recognized from time immemorial.

The influence of the soil upon health is due to: (1) the physical and chemical character of the soil; (2) the ground-water level and degree of dampness; (3) the organic impurities and contamination of the soil.

The physical and chemical nature of the soil, irrespective of its water, moisture, and air, has been regarded by some authorities as having an effect on the health, growth, and const.i.tution of man. The peculiar disease called cretinism, as well as goitre, has been attributed to a predominance of certain chemicals in the soil.

The ground-water level is of great importance to the well-being of man. Professor Pettenkofer claimed that a persistently low water level (about fifteen feet from the surface) is healthy, the mortality being the lowest in such places; a persistently high ground-water level (about five feet from the surface) is unhealthy; and a fluctuating level, varying from high to low, is the most unhealthy, and is dangerous to life and health. Many authorities have sought to demonstrate the intimate relations between a high water level in the soil and various diseases.

A damp soil, viz., a soil wherein the ground moisture is very great and persistent, has been found inimical to the health of the inhabitants, predisposing them to various diseases by the direct effects of the dampness itself, and by the greater p.r.o.neness of damp ground to become contaminated with various pathogenic bacteria and organisms which may be drawn into the dwellings by the movements of the ground air. As a rule, there is very little to hinder the ground air from penetrating the dwellings of man, air being drawn in through cellars by changes in temperature, and by the artificial heating of houses.

The organic impurities and bacteria found in the soil are especially abundant in large cities, and are a cause of the evil influence of soil upon health. The impurities are allowed to drain into the ground, to pollute the ground water and the source of water supply, and to poison the ground air, loading it with bacteria and products of putrefaction, thus contaminating the air and water so necessary to life.

=Diseases Due to Soil.=--A great many diseases have been thought to be due to the influence of the soil. An aetiological relation had been sought between soil and the following diseases: malaria, paroxysmal fevers, tuberculosis, neuralgias, cholera, yellow fever, bubonic plague, typhoid, dysentery, goitre and cretinism, teta.n.u.s, anthrax, malignant Oedema, septicaemia, etc.

=Sites.=--From what we have already learned about the soil, it is evident that it is a matter of great importance as to where the site for a human habitation is selected, for upon the proper selection of the site depend the health, well-being, and longevity of the inhabitants. The requisite characteristics of a healthy site for dwellings are: a dry, porous, permeable soil; a low and nonfluctuating ground-water level, and a soil retaining very little dampness, free from organic impurities, and the ground water of which is well drained into distant water courses, while its ground air is uncontaminated by pathogenic bacteria. Exposure to sunlight, and free circulation of air, are also requisite.

According to Parkes, the soils in the order of their fitness for building purposes are as follows: (1) primitive rock; (2) gravel, with pervious soil; (3) sandstone; (4) limestone; (5) sandstone, with impervious subsoil; (6) clays and marls; (7) marshy land, and (8) made soils.

It is very seldom, however, that a soil can be secured having all the requisites of a healthy site. In smaller places, as well as in cities, commercial and other reasons frequently compel the acquisition of and building upon a site not fit for the purpose; it then becomes a sanitary problem how to remedy the defects and make the soil suitable for habitation.

=Prevention of the Bad Effects of the Soil on Health.=--The methods taught by sanitary science to improve a defective soil and to prepare a healthy site are the following:

(1) Street paving and tree planting.

(2) Proper construction of houses.

(3) Subsoil drainage.