286 INTRODUCTION TO GENERAL SCIENCE 



that is, pipes without certain bends, or cavities, which are 

 called traps. Under those conditions the gases from the 

 decomposing material rose in the pipes and entered the build- 

 ings, carrying bacteria. The common name for this gas 

 is sewer gas, and it should not be allowed to enter a building 

 under any circumstances. 



The first improvement was to bend the pipe into an S-shape, 

 which, turned on its side, produced a U in the pipe. In this 

 bend water was expected to remain, and thus prevent the 

 entrance of the sewer gas. Under favorable conditions, this 

 form of trap operated successfully, but often the sewer gas 

 came up the drainpipe, notwithstanding its shape. It was 

 then discovered that the traps acted as siphons and were 

 entirely emptied by the outgoing stream. To prevent the 

 siphon effect, a vent pipe was connected to the top of the bend, 

 and this pipe was run up through the roof of the building. 

 Not only is the trap thus prevented from emptying, but any 

 accumulation of sewer gas is led off from the pipes without 

 harm to the dwellers in the building. 



Every drain should have a trap, and every trap should have 

 a vent pipe, running up through the roof. The^re is a dan- 

 gerous habit of using one trap for two or more near-by drains, 

 and it should be discouraged. On the other hand, one vent 

 pipe can be safely used for many traps. 



There should be a large trap in the main drain where it 

 leaves the building. This trap may be what is called drum 

 trap, and need not be ventilated. All traps should have a 

 tightly fitting cover, which can be removed for purposes of 

 cleaning out any solid material which has accumulated. 



Cesspools should be vented, and the vent pipe should run 

 up high enough to prevent any annoyance from the gases. 



It must be remembered, however, that even sanitary plumb- 



