odors from putrefying organic matter, the trouble may be corrected by lowering the pump to near the bottom so as to prevent stagnation, or by filling up all unnecessary space with clean gravel and sand.

COLOR

Pure water, when viewed in small quantities, appears to be perfectly colorless, but, when viewed in bulk, as in the white-tiled baths at Buxton, and in certain Swiss lakes, it is seen to possess a beautiful greenishblue tint. A very small amount of suspended or dissolved impurity is sufficient to obscure this color.

Impure waters almost invariably exhibit a color varying from green to yellow and brown, when examined through a depth of two feet in suitable tubes. It does not, however, follow that a colored water is, therefore, polluted or infected.

Color in surface water is usually of vegetable origin; animal matter contributes but little color. The coloring matter is extracted largely from dead leaves, bark, and roots, from soil, and from peat. It seems to be the same material as the coloring matter of tea, and it is certainly harmless, but it makes the water less pleasing in appearance, and great efforts have rightly been made to prevent it and to remove it. Water from swamps is usually highly colored, the degree of color depending upon the length of exposure.

Ground waters are usually colorless. If the water contains iron it will be perfectly clear on coming from the ground, but will soon turn a rusty yellow color. This is caused by the oxidation of the soluble ferrous salts to insoluble ferric salts.

Color in water should be distinguished from turbidity. True color is due to dissolved impurities, turbidity to substances in suspension. The "apparent color" is the color of the original sample, due to both dissolved and suspended matter.

The prevention of color in surface waters consists in draining swamps. Thus, in the catchment areas of the various reservoirs supplying Boston thousands of acres of swampy land have been drained for the purpose of reducing the color of the supplies, and with good results.

A colored water may be bleached by exposure to sunlight and air, but the bleaching of the water in reservoirs requires great storage capacity, and the drainage of swamps is likewise very expensive. Ozone applied in large amounts also destroys color, and the only objection to its use is the cost. Color may be removed to a considerable extent by simple filtration through sand. If the coloring matter is first rendered insoluble by the use of coagulants (sulphate of alumina), it is readily removed by filtration. Color is thus successfully removed from the

waters used by Norfolk, Va.; Charleston, S. C., and Watertown, N. Y. Sulphate of iron is less satisfactory as a coagulant than sulphate of alumina for the removal of color.

Method for Estimating Color.-Turbid waters should always be filtered before the color observations are made. The intensity of color may be determined by comparing with a standard platinum-cobalt solution; the tint or shade may be determined by comparison with the standard color disks of a Lovibond tintometer.

PLATINUM-COBALT STANDARD.-The standard solution, which has a color of 500, is prepared as follows:

Dissolve 1.246 grams of potassium platinic chlorid (PtCl, 2KC1) containing 0.5 gram platinum, and one gram crystallized cobalt chlorid (CoCl2 6H2O) containing 0.25 gram of cobalt in water, with 100 c. c. concentrated hydrochloric acid, and make up to one liter with distilled water.

By diluting this solution with distilled water to the 100-c. c. graduation mark on the Nessler tubes, standards are prepared having colors of 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, and 70. These should be kept in Nessler tubes of such diameter that the 100-c. c. graduation mark is between 20 and 25 cm. above the bottom, and is uniform for all tubes. They should be protected from dust when not in use.

Procedure. The color of a sample is observed by filling a standard Nessler tube to the graduation mark with the water to be examined, to a depth equal to that of the standards, and by comparing it with the standards. The observation should be made by looking vertically downward through the tubes upon a white surface placed at such an angle that light is reflected upward through the column of liquid.

Waters that have a color darker than 70 should be diluted before making the comparison, in order that no difficulties may be encountered in matching hues.

TURBIDITY

Practically turbidity is synonymous with muddiness. The turbidity of surface waters is usually due to clay or silt, also to finely divided organic matter, microscopic organisms, and a great variety of objects. Turbidity represents the amount of foreign substances in suspension; it is frequently, though incorrectly, spoken of as color. In a general way turbid waters exist in those regions where color is not found; the former represents the washings of a readily eroded drainage basin, the latter is mostly extracted from the decaying vegetation of swamps.

Pure water is clear and sparkling, in proportion to the amount of dissolved oxygen and carbonic acid. While brilliancy and clearness do not mean purity, on the other hand turbid waters are not necessarily

dangerous. A community for years may drink and seem satisfied with a turbid water that is little less than liquid mud. This was the case with Washington and the Potomac water, St. Louis and the Mississippi, and many other cities. When, however, such a city once appreciates the beautiful appearance of a clean water, they complain if the turbidity reaches the point of a faint opalescence. The turbidity question is practically limited to river waters. Ground waters should never be turbid, and, if so, should at once excite suspicion. Some ground waters become more or less turbid through the precipitation of iron.

All river waters are more or less turbid, but the differences are very great indeed. The amount of turbidity depends largely upon the character of the catchment areas. In general, rivers draining the large areas of our North and East, covered with glacial drift of a sandy character, are but little subject to turbidity. Thus, on an average, the Merrimac and Connecticut Rivers do not carry more than 10 parts per million of suspended matter. In that part of our country which is not glaciated, and this includes the lower Susquehanna basin, much of the Ohio basin, and the Missouri basin, and all to the south of them, turbidity is often present in large amounts, and consists largely of clay in extremely fine particles. The water often runs turbid in these streams continuously for weeks and even months at a time. The Missouri River carries the largest amount of sediment of any of our rivers largely used for water supply. The annual average runs as high as 1,200 or 1,500 parts of sandy matter per million. In winter it falls to 200 parts or less, while in midsummer it rises for weeks and even months to 5,000 parts or more.

If the turbidity is sufficiently coarse-grained it may be removed by sand filtration without previous chemical treatment. Very turbid waters can be cleared, in part, in settling basins; this lightens the work of the filters and reduces the cost. Scrubbers, which are preliminary rough filters, may also be used to protect the sand filters. In many instances the individual particles of clay which make up the turbidity are much smaller than the bacteria. They will not settle out, even after prolonged storage, and they cannot always be removed by filtration alone. There is only one known way of removing such turbidity, and that is by coagulation or chemical precipitation. The substances most commonly used for this purpose are: aluminium sulphate, alum, or sulphate of iron (see page 794).

With reference to the influence of the suspended matter upon health we find some conflict of opinion. Kober states that water containing 50 parts per 100,000 or 30 grains of solid matter per gallon is unfit for drinking purposes, on account of its irritating effects upon the gastrointestinal tract. Apart from this, turbidity appears to have no special sanitary significance.

Methods for Estimating Turbidity.-There are three methods by which the degree of turbidity may be determined: (1) the platinum wire method, which consists of determining the depth of water through which a platinum wire of standard diameter may be seen; (2) comparison with waters of standard turbidity, made by adding 1 gram of finely powdered diatomaceous earth to 1 liter of distilled water; this is known as the silica standard; and (3) the amount of suspended particles in water may be determined in special instruments known as turbidimeters or diaphanometers. These instruments consist of a graduated glass tube with a flat polished bottom, inclosed in a metal case. This is held over an English standard candle, and so arranged that one may look vertically down through the tube and see the image of the candle. The observation is made by pouring the sample of water into the tube until the image of the candle just disappears from view. The graduations on the tube correspond to turbidities produced in distilled water by certain numbers of parts per million of the silica standard.

The standard of turbidity adopted by the United States Geological Survey consists of a water which contains 100 parts of silica per million, in such a state of fineness that a bright platinum wire 1 millimeter in diameter can just be seen when the center of the wire is 100 millimeters below the surface of the water and the eye of the observer is 1.2 meters above the wire, the observations being made in the middle. of the day in the open air, but not in sunlight, and in a vessel so large that the sides do not shut out the light so as to influence the results. The turbidity of such water is taken as 100, and all turbidity readings, by no matter what method used, should conform with this method.

50

REACTION

The alkaline reaction of natural waters ordinarily depends upon the carbonate and bicarbonate of calcium and magnesium. In some waters in the West it also includes the carbonate of sodium and of potassium. The alkalinity of water is determined by titrating 100 c. c. of the sample with sulphuric acid, using 0.5 c. c. of a solution of lacmoid as an indicator. The lacmoid solution consists of 2 grams in one liter of 50 per cent. alcohol. The last cubic centimeter or two of acid must be added while the sample is almost at the boiling temperature, and the end reaction is not read until a drop of acid, striking the surface of the liquid, sinks to the bottom of the dish without producing a change in the uniform reddish or purplish color of the solution. Erythrosin may be used as an indicator when it is desired not to use heat. The number of cubic centimeters of sulphuric acid used, when multiplied 50

by ten, gives the number of parts per million of alkalinity in terms of calcium carbonate.

Under certain circumstances rain water, water from peat bogs, and water from coal mines, tanneries, etc., have an acid reaction. In mining regions waters are frequently acid from high quantities not only of CO2, but also of sulphuric acid and various sulphates-those of iron and aluminium giving an acid reaction. When these are present, the total acidity is determined by titrating the water in the cold with a standard sodium carbonate solution, using phenolphthalein as an indicator.

Mine water is that which is constantly flowing from the coal and surrounding strata. It is collected in ditches at one side of the gangways and tunnels, and is allowed to flow to the lowest point in the mine or to the foot of the shaft, from which it is pumped to the surface. Large quantities of this and other water are used to wash the coal. This water is acid, and it is now well known, from the researches of Dixon, Matson, and others in the anthracite coal regions of Pennsylvania, that such water has a destructive effect upon typhoid, colon, and other bacteria. The acidity of the streams in Pennsylvania is a large factor in neutralizing the pollution of the water supplies of Philadelphia, Pittsburgh, Harrisburg, and other cities. The spent tan liquors from tanneries are also acid, and are known to exert a somewhat similar influence on sewage organisms.

Rain water collected in the vicinity of towns has usually a slight acid reaction and acts upon lead. The free acid in rain water is apparently sulphuric, no doubt derived from the sulphur in the coal used.

Water from marshes, swamps, and especially from peat bogs may have a markedly acid reaction, especially in dry weather, when the flow will be comparatively small. Heavy storms wash out the water which has long been in contact with the decaying vegetation. The acidity in this case is due to organic acids.

When the collection of an acid water cannot be avoided, arrangements should be made for filtering through some material capable of completely neutralizing the acid, as without some such arrangement the consumers of the water run the risk of lead poisoning, provided lead service pipes are used. A river water suddenly turning acid in reaction plays havoc with a slow sand filter. This has occurred in the Pittsburgh filter.

TOTAL SOLIDS

The total solids or residue on evaporation is obtained by evaporating a given quantity of water to dryness, when a grayish-white residue, composed of mineral and some organic matter which has been held by