WATER SUPPLY 



285 



but one cannot be absolutely sure that the water is free from con- 

 tamination. 



Sewer Gas. There is not as much danger in sewer gas as is 

 sometimes supposed, since analysis of the gas shows that it is often 

 freer from germs than is street air. Sewer gas, however, is very 

 unpleasant to have in the house. Therefore vents must be so con- 

 structed as to allow all gases to pass through the house without 

 entering any of the rooms. Fig. 187 shows the arrangement 

 for allowing the sewer gas to escape by means of a soil stack 



FIG. 182. A 

 hand basin 

 and water 

 trap. Why 

 is material 

 collected at 

 d? Why is 

 there a nut 

 at e? 



FIG. 183. A tank for flushing a 

 closet. By pulling the string the 

 trap is opened, causing water to 

 run down the pipe/. This causes 

 a partial vacuum at 6, since some 

 of the air will go out with the 

 water. Water rises in a, flowing 

 into 6 it produces a siphon as soon 

 as the trap is closed. 



FIG. 184. An- 

 other type of 

 tank for flush- 

 ing a closet. 

 The bulb at ss 

 is pulled up, 

 allowing water 

 to run down 

 the pipe /. 



or vent stack through the roof. The soil pipe passes from the 

 sloping drain pipe up through the house and extends above the 

 roof. 



Traps. Traps must be provided for all fixtures which are 

 attached to drain pipes. The bent part of the pipe is called a trap. 

 A small amount of water must remain in this in order to prevent 

 gases from passing into the house. 



Peppermint Test. Sometimes sewer gas without odor enters the 

 house because the soil pipes are not perfectly sound. If there is 

 any suspicion of this, a very simple test may be made by pouring 

 two ounces of peppermint, followed by a pail of hot water, down the 



