air from a hot-air furnace is perhaps drier than that furnished by any other system of heating or ventilation. Thus, an out-of-door air in winter at a temperature of 0° F., with a relative humidity of 50 per cent., when heated to 70° F., will have a relative humidity of only 3 per cent. This is drier than the air of the driest climate known, which is seldom less than 30 per cent. It is not unusual for the excessively dry air of a furnace-heated house to cause the woodwork to shrink and fall apart, the bindings of books to crack, etc. Living in such an atmosphere is not normal and must be harmful.

Hot-water and Steam Pipes.-This is a very simple and effective system of heating buildings. The hot-water system is especially applicable to small buildings and steam pipes to large buildings. The hot water is more readily controllable than steam, which has a tendency to overheat. Special furnaces are found on the market to heat the water or to generate the steam, which then circulates through pipes to the rooms where wanted. If the hot-water radiators or steam coils are exposed directly in the room, the system is known as the "direct." In the direct-indirect system the hot-water pipes or steam coils are placed in a special box where the air from the outside is heated, and this heated air flows by thermal circulation through ducts into the rooms where wanted. In the direct system the air of the room is simply heated and reheated over again, while in the direct-indirect system the fresh warmed air is constantly pumped into the building and it is, therefore, an efficient method of ventilation. In both these systems the air is abnormally dried, just as it is in the hot-air furnace, though not to the same degree.

Electric Heating.—Electric heating is clean, easily regulated, but expensive. It has the disadvantage of being insufficient as a ventilating device, unless special inlets and outlets are provided. Electric heaters consist simply of resistance coils which heat the room mainly through radiation and convection.

The Cooling of Rooms.-Much attention has been given, through necessity, to the heating of rooms in winter time, but heretofore little attention has been given to the cooling of rooms in the hot season. It is quite as practicable to cool rooms as it is to heat them, and sometimes quite as important to health.

The principle of practically all cooling devices depends upon the fact that when a fluid evaporates to its gaseous state it absorbs a considerable amount of heat-latent heat. This heat is taken from the surrounding objects which, therefore, become correspondingly cold. Cold may also be produced by the expansion of air. This was pointed out in 1845 by Joulé. Thus, if a jet of air at 60° F. were blown into a room under a pressure of 10 inches of mercury above the ordinary barometric pressure, the sudden expansion of this compressed air would

reduce it to a theoretical temperature of 13.3° F. below freezing. This principle of dynamic cooling has been applied to refrigerators.

Ammonia gas is now almost universally employed in freezing machines. This gas is readily condensed into a liquid. The compressed gas is allowed to expand into tubes, and the cold thus produced utilized directly; more frequently an indirect method is used by which the expanding gas first cools a freezing mixture consisting of a saturated solution of calcium chlorid; this chilled brine is then pumped through a series of pipes to the refrigerator or apartment where it is desired.

A simple method of cooling a room is by the rapid evaporation of water. Dr. Manning was able satisfactorily to cool a large room in the Government Printing Office at Washington by blowing air by means of an electric fan over a moist sheet. This sheet, about a yard wide, was hung near the ceiling, and constantly wetted by a stream of water flowing over it.1

Many of the facts in this chapter upon "Ventilation and Heating" are taken from Donald C. Macfie's book on "Air and Health," 1909, published by E. P. Dutton & Co.

SECTION V

SOIL

CHAPTER I

GENERAL CONSIDERATIONS

The upper layer of the earth's crust, known as the soil, is derived from the disintegration of rocks and the decay of animal and vegetable matter of all kinds. It varies from a few inches in depth to several feet. The sub-soil also varies from a few feet to hundreds of feet in depth, to hard pan or an impermeable stratum.

From a sanitary standpoint the soil must be regarded as our friend rather than our enemy. Enormous quantities of organic matter and infections of all kinds find their final resting place in the soil and are there disposed of and rendered harmless by nature's beneficent processes. In fact, a closer study of the functions of the superficial layer of the soil shows that it is not only the organ of digestion and respiration of the earth, but, like the liver, it is the great organ in which toxic substances of all kinds are neutralized or destroyed.

The sanitarian does not look upon the soil as dead and inert, but rather as a living being, for it presents many of the vital phenomena that characterize life: digestion, metabolism, assimilation, growth, respiration, motion, and even reproduction. The soil breathes, it absorbs oxygen and exhales carbon dioxid; it is capable of digesting and assimilating vast amounts of organic matter by a complex process of metabolism; the waste products are excreted. If these wastes are retained the soil may be choked or killed by an accumulation of its own poison a sort of autointoxication. The soil, like all living things, demands water, but it may be drowned by an excess. A water-logged soil dies in very much the same sense that an individual dies who has suppression of urine. Sedgwick speaks of the "living earth" in the sense that it is teeming with life; bacteria, molds, amebæ, and many of the primitive forms of the animal kingdom, as well as worms, insects, snakes, birds, rodents, and many other animals, make their temporary or permanent homes in the upper layers of the earth. Earth worms by their

plowing action, so beautifully shown by Darwin in 1881, constantly turn over the upper layers of the earth. The soil, therefore, is in constant peristalsis, which helps its digestive functions. The rise and fall of the ground water is analogous to the movements of the diaphragm and assists the respiratory functions of the soil.

Classification of Soils.-Soils are variously classified, depending upon the amount of sand, gravel, clay, loam, humus, peat, muck, rock, alkali, etc., which they contain. The difference between a sandy and gravelly soil depends mainly upon the size of the particles. These soils interest the sanitarians because hookworms live and flourish in them better than they do upon clay or rock formations. "Clay exists in particles of the smallest possible size. It is very cohesive, possesses a high degree of plasticity, and plays a very important part in determining the fertility of soils, their texture, and their capacity for holding water. Its plasticity is due to the presence of a small proportion of hydrated. silicate, and is modified very greatly by the addition of less than a hundredth part of caustic lime. It is exceedingly impermeable to water, and when wet dries with great slowness" (Harrington). Loam consists of a mixture of sand, clay, and humus. If the sand predominates the soil is said to be light; if the clay predominates, heavy. A rich soil contains an abundance of humus.

By humus is meant the products of vegetable decomposition in their various intermediate stages of decay. It is the essential element of vegetable mold, and is necessarily of most complex composition. It is composed of a great number of closely related definite chemical compounds, chief among which are ulmin and ulmic acid, which are supposed to characterize brown humus; humin and humic acid, which dominate dark or black humus; and crenic and apocrenic acids. Humus contains a high percentage of nitrogen, especially marked in some of our prairie soils and in the "black soil" found in the provinces of the Ural Mountains, which, according to von Hensen, contains as much as from 5 to 12 per cent. of organic matter.

Surface Configuration.-Geodesy, or surface configuration, has an important relation to health. Low and swampy ground is a breeding place for the malarial mosquito. Highlands are apt to be drier and more healthful than lowlands. A slope affords better drainage than flat lands, and thus diminishes the dangers from soil pollution, but increases the risk of infection being washed down from those living above. In narrow valleys the air stagnates, the moisture is excessive in both the soil and the air, and there is an unpleasant blanket of cold layers of air at night. Mountain sides are notoriously windy. High plateaux suffer from extremes of temperature. Thus, at Mexico City (about 8,000 feet above sea level) there is a sharp contrast between the temperature during the day and night, and even during the daytime be

tween the sunshine and the shade. At Quito, which is 9,350 feet above the sea level, the daily variation of temperature at some periods of the year is no less than 34° F. Northern exposures do not get enough sunshine, and southern exposures sometimes too much.

The relation of the surface configuration of the land to health is intimately interwoven with the whole question of climate, and must take into consideration temperature, air movements, humidity, sunshine, barometric pressure, precipitation, and the seasons with their endless varieties from tropical to arctic.

Composition of the Soil.-Much attention was formerly given to the hygienic importance of the chemical constituents of the soil. The presence of organic substances was regarded not only with suspicion, but as a serious menace to health. It was claimed that organic pollution of the soil made a good culture medium for the germs of infectious diseases. The gaseous products of decomposing organic matter in the soil have long been looked upon as particularly injurious. These gases, with other ill-defined but unknown volatile substances, are spoken of as miasma or effluvia.

We now know that very few, if any, of the bacteria pathogenic for man grow and multiply in the soil under natural conditions. The spores of tetanus, malignant edema, and anthrax may live in garden earth for many years, but it is doubtful whether these microorganisms, especially the anaerobes, ever find conditions favorable for growth and multiplication in the soil. Ordinarily typhoid, dysentery, and cholera bacilli do not flourish in the soil; on the contrary, they soon die there. It has been shown that cities built upon polluted soils have sometimes suffered relatively less from typhoid and cholera than cities built upon rocky or virgin soil. In some cities (as Budapest) it has been pointed out that the greatest morbidity and mortality rate was in that part of the city built upon made ground filled in with trash and much organic waste. These instances have been largely coincidences, for, as a rule, the low-lying, polluted soil happened to be the poor, crowded tenement district. A sanitarian does not recommend polluted soils for building sites, but it seems that their influence upon health has been overstated, especially where cellars are properly constructed. While a polluted soil may not be hazardous in the ways just indicated, it may be dangerous so far as hookworms and other parasites are concerned, or indirectly it may lead to contamination of drinking water, food, etc. See "Pollution of the Soil," page 682.

MINERAL MATTERS IN THE SOIL.-By far the most abundant element in the soil is oxygen. According to various estimates, from 33 to 50 per cent. of the solid crust of the earth consists of oxygen. The other elements found in abundance in the soil are: silicon, carbon, sulphur, hydrogen, chlorin, phosphorus, fluorin, aluminium, calcium, magnesium,