pounds. This leaves the body in the form of urea, of which

the composition is CO {NH

NH2

It is quickly attacked by

Each

either the bacillus ureæ or micrococcus ureæ, or both.

of these, breaking down the urea, convert it into carbonate of ammonia thus:

"If the sewage is kept without undergoing purification: for a day or so, it undergoes putrefaction and begins to give off foul emanations; in the course of two or three days the albuminous matters begin to split up, and the sewage, particularly when the water contains sulphates, yields sulphuretted hydrogen, which is known by its characteristic odor of rotten eggs. When this gas is formed the sewage becomes black. As the above changes take place, more and more of the solid matter enters into solution, and the sewage becomes proportionately more difficult to treat, at any rate by a precipitation process." (Barwise, “Purification of Sewage.")

Vegetable refuse occasions much of the foulness of stale sewage, largely because of the sulphur it contains. Putrefaction is preceded by the combination of part of the nitrogen and carbon with all the free oxygen and with part of that contained in the nitrates.

It is evident that the form under which the nitrogen is found will depend to a considerable degree upon the amount of decomposition which the organic matter has undergone. This decomposition is facilitated by comminution of the particles in suspension, such as occurs in pumping, and increases with time, and its character is determined by the amount of oxygen contained in the sewage water. In a short time after entering the sewers sewage ordinarily contains no dissolved oxygen and no nitrogen in the form of nitrates; although when fresh it contains some free oxygen and generally nitrates and nitrites.

Sewage contains countless numbers of bacteria of many varieties, as many as 30,000,000 in a cubic centimeter having been estimated, of 200 or more varieties. One of the most common is the Bacillus coli communis, which originates in the animal intestine. Most of these bacteria are harmless; many are beneficial in breaking down complex organic compounds and assisting in the oxidation of the sewage; but a few are the cause of disease if taken into the human system. Among the last are the bacterium of cholera (Spirillum choleræ asiatica) and that of typhoid fever (Bacillus typhosus). B. coli communis and B. enteritidis sporogenes are the bacteria most easily identified as directly derived from sewage. The former is most abundant in sewage-polluted water; the latter is not so abundant, but is much more probably pathogenic, being a possible cause of acute diarrhoea. There are also present in sewage large numbers of enzymes, lifeless organic substances which exert chemical action in breaking down complicated organic molecules. Such are pepsin, pancreatin, and other digestive ferments. is not well understood.

Their mode of action

ART. 87. SEWAGE ANALYSES.

If sewage be heated in a platinum dish until evaporated, a solid residue is left, composed of mineral and organic matter. If this be weighed and then heated to a low red heat, the organic matter will be almost entirely burned up, while the mineral will be but little if at all changed. The difference in weight before and after burning will be almost exactly the amount of organic matter in the sewage. The first amount is generally called "residue on evaporation" or "total solids"; the burned part, "loss on ignition " or "organic residue" and the unburned part the "fixed residue" or "mineral residue." If a sample of the raw sewage

be filtered through fine filter-paper, that in suspension will be intercepted, and the difference between this and the total amount of solids will give the amount in solution. If each of these be heated so as to burn the organic matter, the amount of this in suspension and that in solution are ascertained.

Organic matter, as it decays, gives off carbonic acid, which in part remains in solution and in part escapes. The ammonia resulting from the decay is taken into solution. Other organic matter, about ready to decay, gives up ammonia when the sewage is boiled. The ammonia in solution, and the ammonia thus set free from the organic matter in the sewage, pass off in the steam in a short boiling; and if this steam be again condensed, the ammonia is all held in solution and its quantity can be readily determined. This is the quantity of ammonia called "free ammonia," and, being the product of decay, is the most characteristic ingredient of stale sewage. "Free ammonia" is not chemically "free," but is generally in combination with carbonic and organic acids, or even appears as chloride or sulphate of ammonia.

There is still a quantity of combined nitrogen in the remaining organic matter, called "organic nitrogen," about two-sevenths of which can be made to pass off as ammonia by putting into the sewage an alkaline solution of permanganate of potash-a strong oxidizing agent—and again boiling, the ammonia thus obtained being called “albuminoid " or "organic ammonia." Albuminoid ammonia is being constantly changed by decomposition into free ammonia, and hence the older the sewage is the greater the proportion of the latter to the former. When comparing two samples of sewage by their ammonias we must remember that free ammonia is largely the result of decomposition of that previously, but not now, existing as organic matter

In oxidation, upon which sewage purification largely depends, nitric acid is formed from the nitrogen of the

ammonia and of the organic matter and the oxygen of the air. This strong acid immediately combines with the potash, soda, lime, or other base in the sewage, forming nitrates of potash, soda, etc., which are entirely harmless in the quantities found in the strongest sewage effluent. The nitrogen contained in these salts is called "nitrogen as nitrates" or "as nitrites," or simply "nitrates" and nitrites"; the nitrites being nitrous acid salts in which the oxidation is carried less far than in the nitrates owing to lack of oxygen. (Nitrites are also formed by the combination of nitrates and unoxidized matter, the former sharing its oxygen with the latter.) This is probably the most important chemical determination made of sewage. The organic matter may vary from 3 to 100 parts of sewage. It would be unusual to find as much as .01 part of nitrogen as nitrates or nitrites in sewage; but in the effluent or purified sewage as much as 5 or 6 parts may be found.

Some analysts determine the amount of oxygen absorbed from permanganate, calling this "required oxygen." This test is rapidly and easily made, but measures carbon rather than nitrogen, and is adapted to rough comparisons only. Table No. 26 gives the analyses of the sewage of several cities.

As an illustration of the chemical effect of purification by oxidation, the Lawrence sewage is seen to lose by filtration 89% of the organic matter (“loss on ignition "). The free and albuminoid ammonia is reduced 99.1%, most of that lost appearing as nitrates in the effluent. The chlorine is practically unchanged, as it should be. The bacteria are reduced 99.97%.

In the Meriden sewage is seen the effect of dilution in the decreased chlorine. If the ground-water contained 0.2 parts of chlorine and the sewage 45.8, there would appear to be in the effluent analyzed about 45% as much ground-water as true sewage effluent. The true amount of purification would