sizes of their particles give equally steep curves, regardless of the absolute sizes of the particles, thus greatly facilitating a comparison of different materials. This scale also shows adequately every grade of material from 0.01 to above 10 millimeters in a small space, and without unduly extending any portion of the scale. It is assumed

for the purpose of plotting that the particles of organic matter (determined by the loss due to heating that portion of the material finer than the 140 mesh sieve to a dull red heat) are less than 0.01 millimeters in diameter.

These materials may be said to include the whole range of sands available for sewage purification. Anything as fine as No. 5* is too

For convenience the different materials are numbered to correspond with the filter tanks in which they were first used.

fine for advantageous use, while, at the other end, it would hardly be safe to depend upon a gravel coarser than No. 16 with a filtering stratum not over five or six feet in thickness.

With the mixed materials, Nos. 5 and 6, the smaller particles fill the spaces between the larger, and these finer portions determine. the capillary attraction of the filter, its resistance to the passage of sewage, and, in fact, its action in every way. The appearance of No. 6 is coarser than No. 1, and the average size of its particles is greater, but its finest portion determines its character as a filter, so that it is practically finer than No. 1. It has been found as the result of a careful study that the points where the curves in the diagram cut the 10 per cent. line give the best idea of the total effect of the various materials. By measurements of the diagram we find that with the various materials 10 per cent. by weight of the particles are smaller than the sizes given in the following table. This gives as good an idea of the relative effective sizes of the materials as can be condensed into a single figure for each.

To obtain a definite basis of comparison of the uniformities of the sizes of the grains of different materials, the ratios between the diameters of the particles at the 10 per cent. line, as given above, and the diameters at the 60 per cent. line are given in the table under the heading "Uniformity Coefficient." If all the grains of a sand were absolutely of the same size, the coefficient would be 1; with a majority of our comparatively even-grained sands the coefficient ranges from 2 to 3; with No. 6 and No. 5, the figures are 8 and 9 respectively, and some extremely uneven sands have coefficients as high as 20 or 30, but our data in regard to the action of such materials is as yet very limited.

Air and Water Capacities of Materials when Drained. The average per cent. of sewage by volume which these materials hold, as they are in daily use for sewage filtration, at the time just before sewage is applied, that is, when they are as completely drained as is possible during regular use, is shown by the following diagram. The curves show the per cent. by volume of water retained at various depths in a bed five feet deep; the dotted lines indicate the total open space, and the distances between the curves and the corresponding dotted lines represent the amount of air.

These curves showing the open space and the volume of water held when drained must be taken as general averages, under as nearly normal conditions as we have been able to obtain. The materials can be so packed as to contain either more or less open space than the figures given. The amount of water held depends not only upon the closeness with which the material is packed, but also upon its uniformity, any stratification tending to increase in places the water capacity, and also upon the amount of organic matter stored from the sewage. The curves give a correct idea of the general distribution of water and air in the various materials, but the daily variations, in addition to the above-mentioned differences, are so great that it is impossible to draw curves showing accurately the amounts under various conditions.

The amount of open space depends upon the shape and uniformity in size of the particles of sand, and is independent of their absolute size. The materials which have the sharpest rise on the diagram of mechanical composition (indicating greatest uniformity in size) have the greatest open space, while the sands having a more gradual rise pack more closely; the finer particles occupy the spaces between the larger stones, greatly reducing the open space.

While the total open space depends almost entirely upon the uniformity, and is independent of the absolute size, the amount of water held when drained depends very largely upon the size of the particles, the finer sands holding much water, especially at the bottom. With the coarser sands the percentage held is practically constant from top to bottom, with the exception of a few inches at each limit. The increase at the bottom is due to capillary attraction, and with the finer sands this acts for longer and longer distances, approxi

*The term "water capacity" is used in this report to designate the amount of water retained in the interstices of the filtering material after it has been thoroughly drained.

mately in inverse proportion to the square of the size of the sand particles, holding the material saturated, or nearly so, for 1 inch with No. 9; 10 inches with No. 2; 24 inches with No. 4, and 60 inches with No. 5. The increase at the top is due to the capillarity at that point being increased by the storage of organic matter from the sewage, and this portion of the curve depends entirely upon the amount of such material stored.

If the material was more than five feet deep above its drainage level, the water curves for the coarse materials would be lengthened by the insertion of a straight line above the curve due to capillarity as long as the increase in depth above five feet. The curve due to capillarity is constant from the bottom, and is independent of the depth of material. Thus, it has been found by experiment that if No. 2 sand was only a foot deep, that foot would hold the water shown by the lower foot on the diagram. In the same way the increase of water held by the organic matter at the top is independent of the depth of material.

The action of No. 4, and especially of No. 5, is influenced by the organic matters originally in these materials. This organic matter is best shown by the albuminoid ammonia given in the table (page 431). Its effect is to render the particles sticky, greatly increasing the capillarity, and consequently the water capacities, while the air space is correspondingly reduced. We have not as yet sufficient data to accurately formulate a general correction for organic matter. Organic matters from different sources will doubtless have different. properties, and in different materials the same matter will probably have somewhat different effects. When coarse sands accumulate organic matters from sewage in their upper layers, the increase in water capacity is usually about one per cent. of water by volume for every three parts per 100,000 by weight of albuminoid ammonia. This ratio varies considerably in different cases, but any material containing organic matter will act as if it contained much more fine material than is indicated by its mechanical analysis.

Limitations of the Size of Single Doses of Sewage.

Each of the materials shown in the above diagrams has been made the subject of special study to determine the most advantageous method of applying sewage. With the coarser materials the amount of sewage which can be applied at one time is limited by the slight