Unoxidized liquid sewage being in a condition to flow readily thru the earth, is more dangerous than fresh sewage because it is more likely to seep into wells. [Illustration: FIG. 81-_a_. Septic tank.] =153. Construction of the Septic Tank.= The septic tank is composed of two chambers--one the liquefying chamber and the other the oxidizing chamber. Both are water-tight (Fig. 82). The fresh sewage comes into the liquefying chamber thru a pipe placed near the top of the tank. Here it stands and liquefies, which is a process of decomposition. The solids fall to the bottom as they come into this chamber, and the liquid formed rises to the top and flows into the oxidizing chamber (_B_, Fig. 82), when it reaches a point a little below the height of the inlet pipe. It either does this by flowing over a partition or thru a pipe leading from one compartment to the other. The second compartment is usually slightly smaller than the first. Here the sewage is held until the process of oxidation takes place, which renders it less dangerous. When the sewage in the second chamber reaches a certain height, it siphons out into a tile which distributes it over a plot of ground (Fig. 81). Various kinds of siphons are used, the important feature of them being that they are constructed so that they drain the tank often enough to remove the oxidized sewage and not so often as to remove it before it has become oxidized. =154. The Size of Tank.= Because the liquid must be drained from the tank at certain intervals, it is important that the size of the tank be adapted to the amount of waste it will receive. [Illustration: FIG. 82. Details of septic tank.] Septic tanks are kept warm by the heat generated in the oxidizing process, which is simply slow burning of the waste, so that they rarely freeze in winter. Run waste water from the kitchen sink and laundry tubs into a catch basin to collect the grease from the water, as grease or oil on the surface of the sewage of a tank will stop the action of the microbes in the tank by smothering them. When too much grease does get into it, the tank must be thoroly cleaned. Do not use lye, chloride of lime, carbolic acid and other chemicals in drains and septic tanks. Disinfectants of this type put into pipes leading to a septic tank will kill the useful bacteria which decompose the sewage. Use clear boiling water to clean the pipes. This will be cooled by the time it reaches the tank so that it will not kill the useful bacteria. Insoluble mineral matter gradually accumulates in septic tanks, so that they must be cleaned once every few years. Care will postpone the times for cleaning. Do not wash vegetables with much earth adhering to them in sinks leading to cesspools or septic tanks. Shake or rinse off the dirt before washing them.