to the sewage. If the consumption is not 100 gallons, multiply by 100 and divide by the consumption.*

The total organic and other matters from the average household will probably be .00005 to .0001 times the above, and will constitute most of the pollution found in the sewage, excepting such as may come from tanneries, breweries, slaughter-houses, and markets. The principal constituents of organic matter are carbon, oxygen, nitrogen, and hydrogen. All contain carbon, but all do not contain nitrogen. Those containing nitrogen are in general the more liable to putrefy, and are regarded as the more objectionable. For this reason the quantity of nitrogen and its compounds in sewage is that most carefully determined as an indication of the quantity of harmful organic matter present.

The pollution from manufacturing establishments may consist of almost any acids, alkalis, or organic matters. A carpet, blanket, and cloth mill on the Schuylkill used daily, a few years ago, 48,700 pounds of organic matter, including 18 different substances, 2520 pounds of 21 different acids, and 950 pounds of 6 different alkalis. Brass-works discharge considerable sulphate of copper, cyanide of potash, and oils; the chief waste from iron-works is sulphate of iron; from *See also page 430d.

paper-mills come filaments of jute, cotton, and other organic matters, caustic soda, chloride of lime, and sulphite; in woollen-factories the washing of the wool produces large amounts of organic wastes, and there are also discharged soda alkalis, logwood, fustic, madder, copperas, potash, alum, blue vitriol, muriate of tin, and other dye-wastes; from cotton-factories come sulphuric, nitric, and muriatic acids, chloride of lime, soda, potash, alum, copperas, blue vitriol, lime, pearl-ash, stannate of soda, sugar of lead, indigo, cutch, sumac, alkali, soda, and various aniline dyes; from silk-factories, sericine, or silk gum, soda, and a small amount of dyestuffs. Many of the acids and alkalis from factories neutralize each other, and the principal objection to these in sewage is that they may form insoluble compounds or foul gases, or that the acidity of the sewage may interfere with the later treatment. In some instances acids discharged from brass-works and iron-mills are sufficient in quantity to kill the fish in a river, and of course to render it unfit for drinking-water.

The water itself before pollution generally contains little organic but some mineral matter. Lime, chlorine, and iron are the minerals most commonly found in solution. Sand and clay are generally found in suspension in varying quantities. Copper, zinc, lead, and other metals are sometimes found in small quantities. Lime causes the "hardness" of water, which is classified as either "permanent " or " temporary.' The former is caused by calcium sulphate and other soluble salts of calcium and magnesium, not carbonates, held in solution; such water cannot be materially softened by boiling. Temporary hardness is due to carbonates of calcium and magnesium; by boiling such water the carbonic acid is expelled and the salts become insoluble.

Chlorine is found in most waters, being washed from the soil, or from the air where it has been carried by ocean

vapors. It is unobjectionable in the quantities ordinarily found, but is significant in sewage for two reasons: first, if more than normal in quantity, it is an almost sure indication of sewage contamination, and if not more than normal, that there has been no sewage contamination; second, it cannot be removed from solution and hence remains constant through all filtration and other purification processes, thus serving as an index of the strength of domestic sewage, whether purified or not. The amount of chlorine in a sample of purified effluent and in the sewage from which it was derived must be practically the same.* To determine pollution from the amount of chlorine present it is necessary to know the normal amount in the district in question. This ordinarily varies with the distance from the ocean, being least in those localities which the ocean winds must travel farthest to reach; excepting, of course, those places where the ground-waters are rich in salt, as in west-central New York. Plate XIII shows the distribution of chlorine in the normal waters of Massachusetts and Connecticut. It is seen to reach a maximum of 2.42 parts per 100,000 on Cape Cod.

Iron is to be found in small quantities in most waters, but this and other metallic substances have no significance in sewage except as they may affect purification.

The organic and mineral matter in suspension and solution in the water before the addition of sewage matters will of course be included in that found in the resultant sewage, and it is desirable to learn what this amount is. The Naugatuck River at Union City, Conn., contained, as extremes, in Sept. 1897, 6.05 parts per 100,000 of mineral and 2.20 of organic matter, 2.00 parts being lime and .42 chlorine; and in April 1896, but 1.60 of mineral and 1.55 of organic matter; these

* For some reason not understood the chlorine in effluents from purification processes is generally a very little lower than that in the crude

sewage.

being fairly average results for New England in a thickly populated district.

The above illustrates in a general way the constitution of sewage; but to understand the methods and processes which sewage undergoes during purification it is necessary to study the chemical conditions and forms in which these matters exist in sewage, as well as those in which they generally appear in chemical analyses. Average American sewage contains about 40 to 60 parts per 100,000 of solids when the water consumption is 60 to 70 gallons per capita. Of these about 10 to 20 will be in suspension and the remainder in solution. The older the sewage and the more it has been agitated the greater will be the proportion of solid matter in solution. Of those in suspension 3 to 5 parts are mineral and 7 to 15 are organic; of those in solution 25 or 30 are mineral, 5 to 10 are organic. Owing to causes already mentioned, as well as to the great variations in per capita water consumption in different places, any individual sewage may vary greatly from the above figures; but they serve to give a general idea of the relative proportions.

The proportions of the various constituents are stated by some chemists in parts per hundred thousand; by others in parts per million, or, which is practically the same thing, in milligrams per liter; others in grains per U. S. gallon; and by many English chemists in grains per Imperial gallon. The last can be reduced to parts per 100,000 by dividing by 7 and multiplying by 10; grains per U. S. gallon by dividing by 5.8335 and multiplying by 10. In this work parts per 100,000 will be used unless otherwise stated, this being the more common practice in this country and England.

About 40 ounces per day of human urine is excreted per capita, on an average, and 30 ounces of wet fæces (see page 362). Of the urine about 0.337 grains are common salt, 0.2 being chlorine. In the excrements occurs the great bulk of the nitrogen found in sewage, mostly as albuminous com