The

sary it is. Fish life, excepting of a few hardy kinds, has disappeared from the river, and fifteen years ago shad, which formerly frequented the stream, abandoned it. The manufacturers have reported that the acid of the sewage-laden water affected boilers so as to make its use inadvisable. use of the river for pleasure purposes, which at one time made it a delight to thousands, has become comparatively infrequent, and the attractiveness of the river may be said to have disappeared." (Report of the Passaic Valley Sewerage Commission, 1897.) While this is an extreme case, there are many others in this country almost as bad; and as the country becomes more thickly populated other streams will become similarly polluted.

The mortality due to sewage-polluted water may occur through almost any enteric disease, but the greatest is probably from typhoid fever. An illustration of the mortality from this disease due to sewage is found in the city of Lawrence, Mass., which uses the Merrimac River as a source of supply, which river receives the sewage of Lowell, nine miles above. Since August 1893 the supply has been filtered and the result is apparent in the following table.

MORTALITY FROM TYPHOID FEVER IN LAWRENCE, MASS.

(See also Art. 9 of the author's work on "Water-supply Engineering.")

Another illustration was the epidemic of typhoid fever

which, in the winter of 1898-99, visited two or three cities on the Passaic River which, for a few days when the supply of pure water ran low, pumped water from this river into their mains.

In this connection reference should be made to the danger of spreading certain diseases through the agency of oysters, and that of the destruction of fish by disposing of sewage by dilution. There seems to be little doubt that typhoid and probably other fevers have been so conveyed by oysters, as at Wesleyan University, Middletown, Conn., in 1894,* and at Brightlingsea, England, oysters from the latter place being accused on good evidence of having caused twenty-six cases of typhoid fever in 1897. These were exposed, however, to contact for hours at a time, at low tide, with sewage but little diluted. In view of this and of similar cases both in this country and abroad it would seem advisable that precautions be taken by the authorities to protect oyster-beds from sewage or to prevent the gathering of oysters from sewagecontaminated waters.

It is probable that germs of enteric diseases are conveyed on the outside rather than the inside of the body of the oyster, and that there is little danger in eating sewage-fed fish or cooked shellfish, since the organic matter is digested by them and converted into healthy tissue, and such bacteria as enter the digestive organs are either destroyed or leave at once in the excrement. A moderate amount of fresh organic matter attracts most kinds of fish which live upon it or upon the minute animal and vegetable life of which it forms the food; but the gases of putrefaction are poisonous to animal life.

ART. 12. EFFECTS OF DILUTION.'

Legal and sanitary considerations make it desirable to determine whether any amount of dilution of sewage renders * In 1906 occurred another epidemic of typhoid fever at the University under circumstances almost exactly identical with those preceding the 1894 outbreak.

it innocuous, and whether a river, lake, or body of salt water, whether with or without currents, which has once been polluted will naturally purify itself. Dead organic matter at temperatures between 35° and 120° is attacked by bacteria. which decompose it and enable its elements to unite with others to form new compounds. If oxygen is present in sufficient quantity, odorless and harmless mineral compounds are formed. Such of the elements of the decomposed organic matter as are not supplied with oxygen will, in most cases, form obnoxious and poisonous hydrogen compounds, among these being sulphuretted hydrogen and marsh-gas, which cause the floating bubbles seen on the surface of foul water. Most waters contain considerable free oxygen, and if the amount of this in any given body of water is sufficient to oxidize all the sewage reaching it, the organic matter will very shortly be decomposed without offence and lose permanently its power for evil. (See also Art. 93.)

Polluted water purifies itself not only by oxidation, but also by sedimentation, dilution, and the agency of animal and vegetable life.

Organic matter in water forms the food of filth infusoria, hydra, rotifera, entomostracan crustacea, fresh-water shrimp, and the larvæ of a number of water insects. Entomostraca seem to be the most efficient in the purification of streams, and thrive on human excrement. A sewage-polluted river may contain 25 to 50 or more per gallon; but when the pollution becomes intense they seem to disappear, probably because of lack of oxygen, but their place is taken by larvæ. Diatoms, desmids, confervoid algæ and other vegetable organisms, together with bacteria, act largely upon the dissolved impurities; although the last-named seem to attack organic matter also. These all serve as food for fish; and fish, in turn, for man; and sewage matter disposed of by dilution is therefore not wasted, although it does not serve as fertilizer for plant life.

By sedimentation, only the matter in suspension is re

moved, the proportional amount depending upon the velocity and turbulence of motion in the water, the specific gravity and size of the matters in suspension, and the time allowed. Sedimentation is most active when clay, sand, or other heavy matter is carried in the sewage. This, in sinking, carries with it other finer and lighter matter, and if there is no motion of the water a large percentage of the matter in suspension will be deposited. With this will be carried a large number of bacteria, many of which, however, will continue to thrive in the deposit on the bottom. An illustration of sedimentation of bacteria is offered by the river Spree, which above Berlin has been found to contain 2000 to 20,000 bacteria per c.c., below 50,000 to 500,000 (the increase being due to Berlin's sewage), and below Havel Lake (a lake a few miles below Berlin, seven miles long with a slow current through it) 1500 to 20,000 per c.c. If the water moves with considerable velocity, and especially if the bottom be uneven, the suspended matter is carried along and little sedimentation takes place. It is doubtful if excessive sedimentation is desirable in any body of fresh water or in shallow salt water, since the deposit, even in the deepest water, will be worked over by bacteria and give off offensive gases, also returning to the water much of the organic matter deposited, although in a more finely comminuted condition. If the depth be consid

erable, or the precipitate small in amount, the gases and organic matter may be rendered unobjectionable by oxidation or otherwise before they reach the surface.

A large part of the suspended matter does not settle under ordinary conditions, but remains on or near the surface of the water, with which it mixes. Such intermingling is often slow, and the discharge of a sewer can in many cases be traced for a long distance as a separate stream, mingling but slowly and along its edges with the purer water. For this reason discharge should be in a current where possible,

and above rather than below a rapids. The dilution effected brings additional oxygen to the organic matter and also makes it less apparent, thus decreasing the nuisance; and in salt water, or in fresh water where this only is the aim, a sufficient dilution may meet the requirements. Authorities differ as to the minimum amount of dilution necessary for this purpose, but this is usually placed between 1500 and 3500 gallons per day per person contributing to the sewage. (The proportion is sometimes stated in terms of cubic feet of sewage, but since the amount of impurity is not increased by greater per-capita consumption or waste of water, the former method seems preferable.) In the case of the river Exe, it was found, in 1895, that the addition of sewage to 40 times its volume of water made no serious alteration in the chemical or physical quality of the stream. Putrefaction of London's sewage has been arrested by adding to it 35 times its volume of pure water. In the Illinois & Michigan Canal, in 1888, sewage was discharged amounting to one-seventh the volume of water flowing. For a distance of 29 miles no additional water entered, and there was no sedimentation, the current being too swift and the bottom being constantly stirred up by boats. During May to October about 750 samples were taken at both ends of the stretch, from which it was found that the matter in suspension was reduced 46%; most of it remaining in solution, however, the ammonias being reduced but 14% and the total solids 8%.

If the water must be maintained potable, such dilution as is above referred to is not sufficient. Since typhoid-fever germs have been known to live in ice-water for twenty-five days, it is argued that the water of a river receiving sewage is dangerous for use as drinking-water for a distance below the sewer-mouth covered by the flow of the river during at least twenty-five days, or say six hundred miles. Some sanitarians. maintain that house-sewage should never, or only in very exceptional cases, be discharged into a stream or lake. The