This original water capacity is, however, not maintained in regular work, as has been pointed out already. Basing the observations on regular work only, the original capacity decreases at first, after a new bed has been started or after an old reconstructed bed has been taken in hand, rapidly for some time and afterwards more slowly. Graphically expressed, this decrease is not represented by a straight line but approaches more nearly a parabolic curve. This initial rapid decrease is chiefly due to the consolidation of the bed. [Sidenote: Disintegration of filling material.] In connection with the movements in the bed tending towards its consolidation, it is also clear that the continual filling and emptying operations cause the smaller particles to be washed out of their original position and to be placed in the larger passages between the filling material, and if this process is assisted by the gradual disintegration of the particles composing the filling material, it is clear that the pores must become smaller and smaller in time, i.e. choked. From these observations it follows that the filling material should be a hard substance, which will only to a limited extent be subject to this crumbling away process. But besides these there are, as has already been pointed out, other silting up agencies at work. [Sidenote: Water-retentive power of filling material.] Of the total quantity of sewage which has entered the bed a small portion will always remain in it owing to the water retaining power of the material. This power has sometimes been called “minimum water capacity,” but as this name is liable to be misunderstood, it is better to adopt here the term “water-retentive power” of material. The quantity of the sewage retained by the bed varies with the material and pore-volume, and is due to adhesion and capillary attraction. The greater the pore-volume, and the greater the percentage of fine pores, the greater is the quantity thus retained. Clean gravel retains about 12 per cent. and fine sand about 84 per cent. of its water capacity—i.e. expressed per cubic yard of filling material, one cubic yard of clean gravel will retain about 10 gallons and one cubic yard of fine sand about 70 gallons of water. Through draining a bed for several hours through evaporation and other atmospheric influence, a portion of the sewage retained is lost, but the quantity so lost will vary continually with the circumstances under which the bed is worked. The water-retentive power of the filling material does not decrease with the working of the bed, but increases, which in a large measure is probably due to the slimy coat which forms round the surface of the component particles, and to which reference is made in the following paragraph. [Sidenote: Slimy surface coating of component particles.]