CHAPTER X WATER IN THE BODY AND IN FOODS 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.) Composition of Water. Physical and Chemical Properties: Color, odor, taste. Substances in Suspension and Solution: Organic Matter-Sewage Pollu- The uses of water in the body: Eliminative Function; Function as Food; General Considerations.-Absolutely pure water, composed wholly of hydrogen and oxygen, as represented by the symbol H2O, is never found in nature and is never seen except in small amounts as a laboratory curiosity. Broadly speaking, pure water signifies wholesomeness and suitability for drinking purposes and for use in the preparation of food. The term also implies freedom from pathogenic bacteria and other harmful ingredients. In nature, all water contains more or less gases and solid substances in solution and suspension; provided that these are not present in such amounts as to be injurious, and that bacteria dangerous to health are absent, the term "pure water" is appropriate. Although water is essentially an article of diet, yet it cannot be technically classed as a food. From the remotest antiquity the highest value has been placed upon an abundant supply of pure water. In ancient times, places where good water was abundant became the centers of population. Great expenditures of labor, time and treasure were made to carry it to places where it was not naturally plentiful. The Appian aqueduct carrying water to Rome is said to have been constructed about 312 B.C. Eighteen other Roman aqueducts were built at various times until as late as the year 226 A.D. The great aqueduct commenced by Emperor Caius and completed by Claudius, according to Pliny, cost 350,000,000 sesterces, or translated into American currency, $12,700,000. COMPOSITION OF WATER As late as the beginning of the nineteenth century water was regarded as an elementary substance. Cavendish in 1781 discovered that when an electric current was passed through a mixture of two parts of hydrogen and one part of oxygen, these gases combined to form water. Since that time water has been made in the laboratory synthetically and separated into its elements by analytical methods. From a sanitary viewpoint, water is either good or bad. Water is commonly referred to as pure or impure-wholesome or unwholesome. However, in the present state of our knowledge(1), it is not possible to draw any sharp line of distinction. A normal water is free from any direct or indirect pollution by waste products from human life or industries. Under this classification water may differ widely in color, taste, odor and composition, and may be unsuited for household consumption. Physical and Chemical Properties of Water.-Water is a clear, transparent, tasteless and odorless fluid, colorless in small quantities, pale blue through a deep column. At a standard barometrie pressure of 76° mm., or 29,922 inches, water boils at 100° C. or 212° F., and freezes at 0° C. It is practically incompressible, having the greatest density at 4° C. When water freezes it expands almost 9 per cent of its volume, and therefore acquires a specific gravity less than that of unfrozen water in which it floats. Water is the most remarkable solvent known. Few substances are not acted upon by it to some extent. It takes up all known gases, its solvent power increasing in proportion to the fall in temperature and the rise in pressure. APPEARANCE.-Pure water is clear, bright and sparkling in proportion to the air and CO, dissolved in it. However, brilliancy of appearCO2 ance is not conclusive evidence of purity as some badly contaminated waters show remarkable brightness. Turbidity in water signifies that organic and mineral matters are in suspension; the former may be dead vegetable or animal substances or microscopic living plants and animals. Turbidity of a milky or opalescent nature is commonly due to infinitesimal particles of clay which remain in suspension for a long time. Sewage in solution may also present this appearance. COLOR.-Water in its pure state is colorless. Surface waters may derive color from contact with grasses, leaves and woody matters in general. The reaction of water is slightly acid, but most potable waters are faintly. alkaline to delicate indicators owing to minute amounts of alkaline carbonates in suspension. ODOR.-As previously stated, pure water is odorless, but good surface waters containing coloring matters have more or less odor, which is especially brought out by heating. If due to vegetable matter, it may remain after long boiling, but if due to dissolved gases, it rapidly disappears on being heated. Water sometimes contains sulphuretted hydrogen from reduction of sulphates by bacterial action, and sometimes mixtures of products of organic decomposition which suggests the gas. Odors in water do not always indicate danger to health. As in the case of color, absence of odor must not be accepted as an indication of purity, since dangerous waters may often be inodorous. TASTE.-Pure water has no taste. Whatever taste may be detected is due to dissolved gases. This is most evident when aërated water is heated to the boiling point, allowed to cool and the taste compared with the unboiled water. Saline matters impart no distinct taste unless present in large amounts. The only substance which imparts taste when it is present in small quantities is iron. Organic matter in solution, unless present in large quantities, imparts little taste to the water. Water containing only slight coloring matter may seem to have a distinct taste, but it should be mentioned that the senses of taste and smell are often unconsciously influenced by the sense of sight. Often water with both odor and taste may give no impression of either if drunk in the dark. Substances in Suspension and Solution-NITROGEN AND OXYGEN. Substances normally found in water are (a) gases in solution, (b) organic matters in solution and in suspension, (c) mineral matter in suspension. The gases of first importance are those found in the air. Water contains no air as such; only its constituents, oxygen and nitrogen, are dissolved by water, not, however, in the same proportions in which they exist in the atmosphere. In salt water the variations in the proportions are less restricted. The dissolved oxygen is, however, the important element. One hundred volumes of water at 15° C. will dissolve nearly three volumes of oxygen (2.99), and at 20° C., 2.80 volumes, but boiling increases its solvent properties. The quantity of oxygen in solution is fairly constant in waters of uniform composition freely exposed to the atmosphere; but when exposed to sewage, contamination and other oxidizable matters, the oxygen content diminishes in proportion to the per centage of the sewage content. This progressive diminution is due to the constant access of organic matter, which undergoes oxidation at the expense of the dissolved oxygen. Water from deep wells is very commonly free from dissolved oxygen, because it is abstracted by compounds of iron or manganese, organic matters and other substances. Spring water contains more dissolved oxygen than water from any other source (see directions for making tea or coffee by aërating hydrant water with the atmosphere, Volume I, Chapter XVI). The presence in water of considerable dissolved oxygen, derived from the atmosphere, leads to beneficial changes in the organic matter present. On the other hand, a diminished oxygen content permits the development of low forms of vegetable life which frequently give rise to unpleasant tastes and odors. Their growth is inhibited by a large degree of aëration, and these disagreeable effects are thereby prevented. CARBON DIOXID.-The carbon dioxid contained in water is derived largely from the atmosphere, and from the soil where it is present in abundance. The percentage of CO2 in water depends upon the character of the soil and the extent of oxidation of organic matter occurring in the interstices; the amount is also regulated to some extent by the percentage carried in by rain and dust. It is present in greatest amounts at great depths, and may constitute almost the entire amount of dissolved gases. It is alleged that sea water contains about 10 times as much CO2 as the entire atmosphere (2). ORGANIC MATTER. The organic matters in water are of both animal and vegetable origin, and consist of organisms and products of organic life resulting from disintegration and decomposition. The animal matters include living and dead organisms and dissolved and suspended products of animal life and decay, such as albuminous substances, urea and tissues. The vegetable organisms include species of numerous microscopic plants, which under certain conditions act beneficially by absorbing the products of organic decomposition for their growth, but which may, under favorable conditions, cause much annoyance by over-abundant growth, disintegration and decay. The organic matters-animal and vegetable--which are of prime importance to the physician consist of carbon, hydrogen, oxygen, nitrogen and, in many cases, small amounts of phosphorus and sulphur. The inception, progress and completion of the process of decomposition is due to bacterial activity, wherein the carbon combines with oxygen to form carbon dioxid, and the hydrogen unites, in part, with nitrogen, liberating ammonia. The presence of the latter in water always indicates that the process of decomposition is under way. In its turn the ammonia is converted eventually into nitric acid, which unites with bases to form nitrates. AMMONIA.--Ammonia in water is of importance from the standpoint of sanitation. It most often occurs in the form of hydrate, but occasionally as a chlorid or carbonate. If, on boiling, the salts decompose and the ammonia escapes in the steam, it is referred to as free ammonia. The most direct source of ammonia in water is rain water. The showers bring it down from the atmosphere in varying amounts, according to location. Under ordinary conditions the ammonia in surface waters after conversion to nitrates is absorbed very rapidly by growing vegetation, and the more active the conversion and growth, the greater the appropriation on the part of growing vegetation. If free ammonia exists in water to any marked extent, it may be measured directly by using the Nessler reagent. If the amount is comparatively small, as is usually the case, the ammonia must first be concentrated by distillation and condensation. Ammonia is always characteristic of sewage pollution, the oxidation of which yields it in abundance under conditions which do not permit it to be rapidly oxidized into nitric acid. Ammonia in drinking water produces, per se, no harmful effects. The amount present, however, is of more or less importance. In clean and properly stored rain water it is of little moment; in other waters it is usually evidence of the decomposition of organic matter. Its content. in potable water is not large, and on account of oxidation and absorption by growing vegetation, it does not accumulate, even in sewage-polluted waters. NITRITES.-Nitrites in water are regarded as a special danger signal, the presence of which indicates that active putrefaction of nitrogenous organic matter is under way as a result of bacterial activity, and their presence, therefore, at once suggests organic pollution. Small amounts of nitrites in water may be derived from the air by absorption, or by the cleansing action of rain showers. In some instances they may be due to water coming in contact with metallic surfaces, brick work and new masonry with nitrites in solution. But in the largest majority of instances the presence of nitrites may be traced to sewage contamination. Nitrites are present only in small amounts, since they are rapidly oxidized to the higher and more stable nitrates, which represent the final stage of complete oxidation. Nitrates do not undergo further change and, being permanent in character, accumulate in the water unless withdrawn by vegetable life or reduced. |