152 HOW WASTES ARE REMOVED FROM THE HOME 



caying substances or sewer gas from getting into the 

 house, fixtures known as traps are necessary. A study 

 of the figure on page 151 shows that the trap is an inverted 

 siphon, and forms a water seal which prevents any gas 

 from going by the water. A good example of such a water 

 seal is seen in the refrigerator trap, where the water seal 

 is used to prevent warm air from getting in to melt the ice. 



Why well water is sometimes unsafe. 



In the water seal or traps in our houses, the vent (see fig- 

 ure) is usually necessary to keep the air pressure equal 

 on each side of the trap, so that the water will not be all 

 sucked out, thus leaving no protection against odors. 



Under the kitchen sink we often find a grease trap. This 

 is fitted, as are all other traps, with a cap which is easily 

 accessible, and which can be unscrewed to remove the 

 grease which otherwise might clog the drain pipes. 



Disposal of sewage. Where there is no city sewage 

 system the disposal of sewage is a most important ques- 



