or the edible portions of any plant. In some cases, generally where grass of some kind is grown, the sewage flows slowly all over the land in a thin layer. Where corn or vegetables are grown they are usually planted on the narrow ridges between plowed furrows into which the sewage flows, and where it stands, soaking downward and sideways into the soil. The roots of vegetation and the vegetable mould which forms on the surface of the ground prevent the rapid absorption of the sewage and unless the subsurface soil be clayey or quite non-porous sub-drains are not often necessary, but ditches are carried through the farm at intervals to receive the drainage. If the sewage is not clarified before being applied to the soil, an impervious skin shortly forms, composed of filaments of paper, rags, and similar matters, together with grease and the more stable organic matter; and this must be removed frequently if the ground is to be reaërated and kept absorptive. This matter, which has little odor, can be piled in a dry spot and burned occasionally.

There are few, if any, places in this country where irrigation is used for its fertilizing value only. Where used, it is because water of any kind has a value for irrigation. In general it is applied like any irrigating water, subject to the qualifications just stated. The sewage of from 50 to 200 persons can be used for irrigating one acre, depending upon the quality of the soil.

Crops of all kinds are grown on sewage farms, but corn, English walnuts, alfalfa, Italian rye-grass, and timothy and other grasses are most common.

Filtration may be effected through natural soil if porous, or through specially prepared beds of sand, gravel, or other substances.

If natural soil is used, care must be taken to keep it as free and open on top as possible. The beds are made level, and generally surrounded with high banks and flooded, but are sometimes plowed into ridges and furrows. If the soil is very porous, there is a tendency for all the sewage to enter it near the carrier outlets. Under such conditions numerous secondary carriers may be used, composed of boards formed into shallow troughs. Uni

form distribution may also be assisted by giving considerable slope to the surface of the beds. Great care must be used to

prevent the formation of puddles in which the sewage will stand and putrefy. The surface of the ground in furrows will shortly become clogged with organic matter which resists immediate decomposition, but would be

broken down and oxidized if given time. Furrows should

then be opened in the ridges where the soil is probably unclogged, the earth being thrown into the old furrows. In time a considerable amount of undecomposed organic matter will collect throughout the interstices of the filter, and this should then be given

a rest for several days or weeks, for which purpose the filtration area should be divided into three or more beds, one of which is always resting. Those in use should be allowed to drain out after each dose, that they may be reaërated; the

sewage generally flowing onto drained beds while the ones previously used are draining. In some small plants, however, the sewage is received in settling tanks and the effluent discharged upon all the beds at intervals of several hours, or even only once a day.

On such natural filters the sewage of from 800 to 2000 people may be treated and 85 to 99 per cent of the albuminoid ammonia removed. If it is desired to further economize space, artificial filters may be constructed. These are generally of sand, of an "effective size of about .or inch, over coarse sand or fine gravel, which in turn rests upon a layer of medium-sized gravel, at the bottom of which the drains are placed. The greater part of the purification appears to be done in the upper layer, since 1,118,000 bacteria have been found per gram of sand in the upper inch, while at 4 inches depth but 125,cco were found.† The purpose of the finer top layer is to regulate the velocity of flow, to insure a more minute subdivision of the water and thorough oxidation, and to support the gelatinous top coating which materially assists in the straining and probably in the removal of bacteria. Care must be used to insure that in no place does the sewage pass from a coarse to a fine sand, since organic matter would be deposited here and clog the filter. By having the finest sand on top all clogging is at the surface, where it can le reached.

By intermittent filtration through clean, coarse sand 50,cco to 75,000 gallons per day of American sewage can be treated on one acre, and 97 per cent to 99 per cent of the organic matter therein removed. With fine sand or sedimentary deposit the same result can be obtained with 30,000 gallons or less per day if care is taken to allow thorough drainage between doses.

Low rates are obtained with very fine soil because capillary attraction not only prevents the actual passing of liquid at a high

*The effective size of a material "is such that 10 per cent of the material is of smaller grains and 90 per cent is of larger grains than the size given. The results obtained at Lawrence indicate that the finer 10 per cent have as much influence upon the action of a material in filtration as the coarser 90 per cent." (24th Annual Report State Board of Health of Mass.).

† It is probable that a large percentage of the great number of bacteria found in the upper inch are those strained out of the sewage, only a few of which are nitrifying.

rate, but it retains a part in the lower portion and prevents complete reaëration and hence full oxidation.

It is probable that if sewage is applied without preliminary clarification, there will be strained out on the surface as much suspended matter as though it were collected in a settling tank; in other words, the same amount of solids remains to be disposed of. And collection on the surface interferes with aëration of the filter and even lessens the amount of sewage which can be passed through it. If too much accumulates it will even waterproof the surface in places and cause pools of sewage to collect and putrefy. It is therefore generally advisable to remove as much suspended matter as possible before filtration; and the greater the amount removed the higher the rate of filtration possible. Perhaps double the rate can be maintained with septic sewage as with crude; but the clogging of the body of the filter will be more rapid, requiring frequent renewal of the sand, because of the fine division of the matter.

The amount of oxygen introduced by each aëration of the bed can nitrify only a given amount of sewage, and if more be applied before reaëration an unsatisfactory effluent must result. For example, to oxidize five parts of nitrogen per 100,000 requires a volume of air one-half as great as that of the sewage treated.

Nitrification is favored by certain constituents of soil, such as carbonate of lime, and impeded by others.

Polarite (magnetic oxide of iron 54 per cent, silica 25 per cent, lime, alum, magnesia, carbonaceous matter and moisture 21 per cent) is a (patented) granular substance used for filtration, but there seems to be little evidence that it is more efficient than sand of a similar size of grain, or finely broken coke breeze. Polarite is generally placed in a thin layer between an upper and a lower bed of sand.

On the care of filtration areas or beds Mr. Geo. W. Fuller has given the following suggestions:

"(1) Systematic raking, with occasional harrowing or plow ing, is very satisfactory, particularly for coarse materials.

"(2) Systematic scraping (removal of clogged material) at

regular intervals (followed by raking to loosen the material) gives very good results, especially for fine materials.

"(3) Systematic scraping when necessary, without raking or harrowing, is not advisable.

(4) The efficiency of very fine material (clogged or not clogged) is much increased by trenching with coarse material. (Digging trenches through the bed and filling them with other material, generally coarse sand.)

(5) Such trenches should contain carefully graded materials at the bottom to prevent clogging at the junction of the coarse and fine sand.

"(6) When new material is put onto old to replace clogged material removed by scraping, it is always advisable to mix the old and the new together in order to prevent clogging at the junction of layers of unlike capillary attraction.

"(7) The removal of stored organic matter by resting for a limited period is sufficiently great to render this simple and inexpensive method worthy of careful consideration in cases of clogging where the available area is not too limited.

"(8) It is important that the treatment of filters be such that the condition of operation be as favorable as possible during the cold winter weather.

'(9) Great care should be taken, especially in the case of filters of fine material, that the capacity of the filter be not taxed during the winter months to such an extent that more organic matter is stored throughout the sand that can be removed during the spring and early summer, which is the period of highest nitrification."

Qualitative deterioration is a serious matter in winter, because when a period of biological reconstruction is necessary, nitrification cannot be promptly re-established, as is the case in summer, but requires a period of several weeks and possibly months." (Report Massachusetts State Board of Health, 1894.)

With reference to the effect of cold and snow upon irrigation or filtration beds, it is found that if snow falls before the ground is frozen, there is generally little trouble; but if the