Another type of sanitary privy, known as a liquefying closet, makes use of bacterial action as an aid to disposal. The excretions are deposited in a tight receptacle containing water, where fermentation and decomposition reduce a large part of the organic solids to liquid and gaseous forms. Much of the liquid evaporates and the gases diffuse, so that the volume of sewage is reduced materially. More or less insoluble and undigested residue, known as sludge, gradually accumulates at the bottom of the receptacle, which from time to time must be cleaned out. Disposal of the partially clarified liquid and the sludge, however, involves much less labor than would be needed to handle the untreated excrements. Liquefying closets have been used many years with fair satisfaction. The receptacle sometimes is a tight brick vault, but more frequently a barrel or hogshead with one end nearly flush with the ground. Over this is mounted the seat, sometimes with iron bars beneath to prevent accident to small children, and the whole is inclosed in a small frame house. The vault usually is bailed or pumped out two or three times a year. Upon farms where slope, soil, and drainage conditions are favorable the effluent from liquefying closets may be distributed and aerated by means of drain tile laid in the top soil or in shallow beds filled with cinders, coke, gravel, or stone. Figure 17 shows a simple one-chamber liquefying closet with shallow distribution of the effluent in a stone-filled trench. The vault or tank consists of vitrified sewer pipe, a simple and cheap construction. Where a larger vault is required concrete or brick may be used, the usual capacity being 12 or 13 gallons to a person. [Illustration: Fig. 17.--Liquefying closet. _A_, Excavation about 3 feet 3 inches in diameter; _B_, 3-foot length vitrified Y branch, 24 by 4 inches; _C_, 2-foot length of 24-inch hard burned drain tile or vitrified sewer pipe; _D_, 4 by 4 inch Y branch; _E_, 1-foot length of 4-inch cast-iron soil pipe; _F_, concrete bottom making water-tight seal; _G_, joints made water tight by use of a strand of jute or oakum and rich Portland cement mortar or hot bituminous jointing compound; H; submerged outlet; _I_, removable strainer with openings 1/4 inch or larger; _J_, 4-inch removable plug; _K_, 4-inch drain tile laid on good slope in trench about 15 inches deep, ends of tile butting, joints covered with strips of tarred paper extending three-fourths of the way around the tile; _L_, removable seat supported by end cleats; _M_, 4 by 4 inch ventilating flue, bottom portion removable; _N_, hinged door to facilitate bailing out sludge.]