malaria, yellow fever, and a long list of other diseases. The importance of the soil with reference to the communicable diseases diminishes with our increase in knowledge. The number of infections directly. associated with the ground are very few, and the indirect influences are less than formerly supposed. Apart from the one real danger, viz., soil pollution with human excrement, the sanitarian is now inclined to belittle the influence of the soil upon health.

Dampness and cold may favor rheumatic and neuralgic conditions, and also predispose to respiratory infections. In this way association with a cold, damp soil may be prejudicial to health. Clay soils are apt to be damp; sand and gravel soils are readily drained and may be kept dry by means of simple devices. Such soils, therefore, make the best building sites for habitations. As a rule, the foundation of a house should be at least two or three feet above the level of the ground water.

The soil greatly influences the character of the water which rests upon it and which passes through it. This will be discussed in the section on water.

HOOKWORM DISEASE.-Hookworm disease is closely associated with the soil. It may fairly be considered an infection the result of soil pollution. It occurs especially in moist sandy soils rather than on clay or rocky soils. This is due to the fact that hookworm eggs, when deposited in fecal matter, soon dry up and die upon hard rocky or clay surfaces, whereas they find favorable conditions for development upon moist sand or loam. Under these conditions the larva develop as far as the second ecdysis, which have the power of penetrating the skin (see page 114).

OTHER ANIMAL PARASITES.-In a somewhat similar sense many of the animal parasites of man are deposited on the soil and reinfect man during one of the stages of their cycle of development. Most of the intestinal parasites of man are deposited on the soil, and, after a varying journey, sometimes through an intermediate host, again find lodgment in man. In the case of trichina, for instance, man pollutes the soil with feces containing the eggs. Hogs devour this infection and return the disease to man. In a somewhat similar way the tapeworms of cattle and also some ameba and flagellates pass part of their life history upon the soil.

The Ascaris lumbricoides and the Trichiuris trichiura, two very common worms inhabiting the intestinal tract of man, have thick-shelled eggs and must rest in the soil about a month before they are infective. It requires about a month for the embryo to develop. If fresh eggs of these two worms are ingested, they pass through the intestinal tract without hatching.

Dr. Stiles has prepared the following list of animal parasites which

have some relation to the soil during some part of their life history, and may, therefore, be more or less associated with soil pollution:

LIST OF ANIMAL PARASITES OF MAN WHICH MAY BE SPREAD BY SOIL POL

SECTION VI

WATER

CHAPTER I

GENERAL CONSIDERATIONS

"The greatest influence on health is exerted by those things which we most freely and frequently require for our existence, and this is especially true of water and air" (Aristotle).

While water is not technically classed as a food, it is an essential article of diet. In nature water comes in contact with many surfaces and substances and, therefore, is particularly liable to contain impurities, especially as it is the most universal solvent known. Water is also a frequent medium for the transmission of infection.

From the remotest antiquity the highest value has been placed upon an abundant and pure water supply. Centers of population sprang up in ancient times around those points where it was most readily available, and great expenditures of labor and treasure were made to carry it to places where it was not naturally plentiful.1

Water is a prime necessity of life--not only as an article of diet, but also for the proper cleanliness of person, clothing, and things.

It is interesting to note that the number of towns in this country before 1800 having a public water supply was only 16, supplying about 2.8 per cent. of the existing population at that time. In 1850 there were only 83 public water works, supplying about 10.6 per cent, of the census population. In 1897 the total number was 3,196, supplying about 41.6 per cent. of the population. Since then the number has greatly increased, but exact information is not available."

1

The date of construction of the Appian aqueduct carrying water to Rome is placed at 312 B. C. Eighteen other aqueducts were constructed at various times until 226 A. D. The one commenced by Emperor Caius and completed by Claudius, according to Pliny, cost 350,000,000 sesterces, or about $12,700,000.

2 Baker, M. N.: "Manual of American Water Works,'' 1891 and 1897.

COMPOSITION

At the close of the eighteenth century water was regarded as an elementary substance. In 1781 Cavendish discovered that, when an electric spark is passed through a mixture of 2 parts of hydrogen to 1 part of oxygen, these gases combine to form water. Since then water has been made synthetically, and separated analytically into its component constituents by various methods.

The composition of pure water (HO) is:

Pure water is a chemical curiosity; it does not exist in nature. All water in nature contains impurities, in solution and in suspension. Some of these impurities are organic and some are inorganic. They consist of various gases, fluids, and solid substances. The more im portant impurities and their sanitary significance will be considered in detail under the chemical analysis of water.

From a sanitary standpoint water is either good or bad. Commonly waters are classified as pure or impure. It is not possible, however, in the present state of our knowledge, to draw a sharp line of distinction. In the classical reports of the Massachusetts State Board of Health waters are spoken of as normal or polluted. A normal water is free from direct or indirect pollution by waste products from human life or industries. The difficulty with this classification is that normal waters may differ widely in color, taste, odor, and composition, and may, therefore, be unfit for household or manufacturing purposes.

A practical classification of waters is as follows: (1) good, (2) polluted, (3) infected. A good water may be defined as one of good sanitary quality, as determined by physical inspection, bacteriological and chemical analyses, a sanitary survey of the watershed, and, finally, by clinical experience. A polluted water is one containing organic waste of either animal or vegetable origin. A polluted water is a suspicious water. An infected water contains the specific microorganisms of human diseases.2

In Europe waters are frequently classified as potable or non-potable.

1 Sometimes spoken of as contaminated water.

Chemical poisons such as lead are not included in this classification.