NITRATES.-Nitrates in water, like ammonia, are not in any way harmful in the amounts usually present in potable waters. They only represent what was once organic nitrogen, but now rendered harmless by being completely mineralized. The presence in water of nitrites is of far greater importance than that of nitrates; the presence of the former means that fermentative changes are in active progress, and that oxidization is not complete.

CHLORINS AND CHLORIDS.-Sodium chlorid is a normal constituent of all waters. Rain water takes it up from the air in small traces, particularly near the sea coast. The amount of chlorin normally present in water depends principally on location and other conditions. An increase over the normal is always indicative of pollution, mostly due to the presence of urine. Chlorin increases directly with the population; the amount present is also influenced very greatly by proximity to the ocean, since the air above the sea necessarily contains more chlorin than the air inland. For instance, a polluted water may have its organic nitrogen converted into nitrates, and these in turn may be absorbed by vegetable growth; it may be clear, colorless, odorless and palatable, free from pathogenic bacteria, and in every way suitable for drinking purposes, but, nevertheless, the chlorin remains as a mute witness of remote or passed pollution. The percolation of sea water into wells near the ocean renders the water hard, salty and undesirable for domestic use. Magnesium chlorid also renders water unsuitable for use in boilers. Wells driven near the sea frequently become mixed with sea water, particularly if the quantity drawn is sufficient to cause suction. When this happens it may be a difficult problem to operate the well without drawing in sea water. This sea water question has been more scientifically studied in Holland than elsewhere.

BACTERIA. Bacteria in water may be of the harmless and beneficial kinds, depending upon dead organic matter for sustenance, thereby bringing about its complete conversion into simple chemical substances. Practically all natural waters contain bacteria-rain water, surface water, water of rivers and lakes and ocean water. The number and variety of the bacteria vary greatly in different places and under different conditions. Bacteria are washed into the water from the soil and from almost every conceivable source. The excretions from animals pollute waters with enormous numbers of microorganisms, but infection from certain species of bacteria found in the intestinal tract of man makes water most dangerous for consumption. The bacterial content of surface waters varies with conditions which favor or retard growth and accessions. Sun

shine, influx of food material or of substances inimical to bacterial life, sedimentation and growth of higher organisms act for or against increase. Matters suspended in the current of a flowing stream carry down with them the bacteria that have gathered upon them or have been entangled by contact. The diminution of their number by this means is more marked in sluggish streams and still waters than in rapidly flowing brooks, rivulets and large water courses. The growth of algae and other aquatic plants causes diminution to a greater or less degree by appropriating the nutrient materials upon which they subsist, and possibly through other influences yet unknown. Certain bacteria whose natural habitat is the intestinal canal of man and animals find their way into surface waters and pollute them. These forms are pathogenic. Probably they do not increase in water, but retain their virulence in undiminished degree for definite periods of time, and then tend to become modified in this respect and rapidly to disappear. Cholera germs have been found in the Seine river in an active state after seven days and in ordinary drinking water for as long as twenty days. Typhoid bacilli remain virulent for longer or shorter periods; they have been found in very pure water after more than seven weeks, while in badly polluted water their life is very short.

Bushner (3) has shown that the rays of the sun kill cultures of typhoid bacillus at a depth of five feet in about four and one-half hours, while at double that depth their effects are practically nil. It is true that this organism survives longer in cold than in hot weather. A probable explanation is that in warm weather the conditions may be more unfavorable to the destruction of the organism than in cold weather.

Concerning the Bacillus coli communis, Kruse (4) in 1894 asserted that this organism is so ubiquitous that it cannot be regarded as characteristic of sewage, and in this position he has received the support of a number of other investigators who have succeeded in isolating the organism from waters examined. In spite of the fact that numerous investigators have found the organism where it could not be traced directly to fecal pollution, the Committee of the American Public Health Associa tion (5), on carefully examining this subject in all its phases, reports that the preponderance of evidence suggests that their mere presence rather than their number should be the criterion of recent sewage pollution. Clark and Goge(6), as a result of the examination of some 16,000 samples of water and more than 2,000 specimens of shellfish, sea water, ice, milk, dust and excrement, conclude that the colon bacillus occurs more frequently in sewage than all other bacteria.

Sources of Water. We can safely assert that all water comes to us from the aqueous vapor condensed in the form of rain or snow. Of this a certain percentage returns to the atmosphere by evaporation (7); the balance collects upon the surface of the earth or soaks into the ground. Some of it flows off in the direction of lakes, ponds or rivers. The sources of water may, therefore, be said to be (a) rain water, (b) surface water, (c) ground water, including springs and wells. This classification is a convenient one, but there is no sharp line of demarcation between rain, surface and ground water. Rain water, once it falls, becomes surface water, and surface water quickly passes into the earth, becoming ground water. Ground water in time becomes spring and well water.

ICE AND SNOW WATER. In freezing water becomes purer, losing a large portion of its saline content. Even the calcium carbonate and sulphate content is materially lessened after freezing. Thus ice water may be tolerably pure, but the air being expelled it is heavy and non-aërated. Snow water contains the salts of rain water; rather less ammonia is present, however, and the percentage of carbonic acid and air is infinitesimally small.

Upland surface water most nearly approaches rain water in all its characteristics. The dissolved matters are present in greater volume, the percentage depending somewhat upon the soil over which the water flows, and therefore it is customary to subdivide this class of water according to the geological character of the ground from which the upland water is obtained. Upland water does not contain any considerable amount of dissolved matters unless they be derived from calcareous strata. The organic substances present are chiefly vegetable. The chlorin content is low and the water soft.

SPRING AND WELL WATER.-Rain falling on the earth is partly absorbed and partly evaporated into the atmosphere, the relative amounts varying with the configuration and density of the ground, and with the circumstances impeding or favoring evaporation, such as temperature, movements of the air, etc. Springs are merely local outcroppings of the water table and are subject to variations in the volume of outflow. During a drouth the flow of water from a spring may cease altogether because of a fall in the level of the ground water; this may also happen in the case of springs located at the foot of hills or mountains. The laity look upon spring water as the purest from nature's water fountain, but they share with other waters the same chances of becoming polluted. Wells may be classed as dug, driven and bored; again, they are sometimes

classified according to depth. A well 50 feet or less is termed shallow, and 100 feet or more, deep.

RIVER WATER.-River water comes from a variety of sources, and is even more complex in its constitution than spring water. The Schuylkill River rises in the anthracite coal regions of Pennsylvania and, receiving much refuse mine matter, becomes impregnated with iron salts and free mineral acids, which render it quite unsuitable for domestic or manufacturing purposes. In its course of about 100 miles it traverses an extensive limestone district and receives several large streams heavily charged with calcium carbonate. River water is also influenced by seasons and by circumstances connected with seasons, such as the melting of snow, freshets and floods. Even samples of water taken on opposite banks of a river may vary in composition.

Drinking Water-DISTILLATION.-Distillation is now largely used, and affords an easy way of securing safe, potable water; besides, boiling effectively frees the water from its impurities. On board ships distillation of sea water is resorted to in order to free it from salts and to render it fit for drinking purposes (8). Although the water thus obtained is pure, yet all the gases have been driven off from it by the boiling; it is of course unpalatable, and by some supposed to be indigestible. It may be aerated by allowing it to slowly trickle down through a long column of wood charcoal, or by filtration through animal charcoal or other porous substances.

CHEMICAL AND BACTERIOLOGICAL ANALYSIS.-The chemical and bacteriological analyses of drinking water are of great value. Chemical analysis will reveal the presence of organic and mineral impurity, such as accompanies infectious matters from the intestines and bladder. Though it cannot give grounds for a positive assertion that the use of water thus polluted will inevitably cause disease, it can and does point out possible danger. Bacteriological analysis differentiates between pathogenic and non-pathogenic contamination. Only rarely, however, does it serve to point out danger in advance.

According to Professor Frankland, who has had a large experience in the (9) detection of specific pathogenic bacteria in drinking water:

Their detection is now known to be practically impossible, and the search for such bacteria is, in general, only carried on in deference to the special request of the layman, the uninitiated, or the hopelessly ignorant, since it cannot be repeated often enough; so that any feeling of security which may be gathered from an unsuccessful search for pathogenic bacteria is wholly illusory, and in the highest degree dangerous. By far the most important service which has been rendered by bac

teriology is the means which it affords of controlling the efficiency of filtration and other purification processes. The slightest irregularity or defect in the process of filtration is at once laid bare. Bacteriological purity of well water can also be satisfactorily controlled.

Horrocks (10), who has had a large experience in this particular field,

says:

If a considerable time has elapsed since the occurrence of pollution, the bacteriological detection of same, especially when waters of great original purity are concerned, becomes more and more difficult . . . it is, therefore, evident that a bacteriological examination has its limits of usefulness, and a slavish adherence to it under all conditions, combined with neglect of the hints to be obtained by chemical means, may lead to a perfectly erroneous judgment. Still, there is one branch of hygienic study in which bacteriology must always reign supreme; it is now acknowledged on all sides that the working of sand filters for public water supplies cannot be properly kept under control except by appealing to bacteriological methods of examination.

Mayer (11) claims to have isolated typhoid bacilli from well water, and Jackson found, by the use of lactose bile media, that he was able to isolate the typhoid bacillus (a) from Grass River, a source of water supply for Canton, New York; (b) from a pond or stream used as a private water supply at Hastings-on-the-Hudson; and (c) from two points on the Hudson River. Nevertheless, it is very rare that the isolation of the typhoid organisms from suspected water supplies is ever accomplished.

With this data before us, it may be safely said that neither chemical nor bacteriological analysis is infallible. Each method has its uses, and each may be helped by the other. The value of either lies largely in the skill of the investigator in correctly interpreting the results of his findings. This requires as much knowledge as the minutiae of the examination itself.

Below we append a table of qualitative tests for the chemical examination of water:

TABLE SHOWING REAGENTS NECESSARY FOR QUALITATIVE EXAMINATION OF WATER1