We have thus far considered only the material, size, general arrangement and manner of jointing the drain, soil and waste pipes in a house. We must now consider what the essentials of the system are, in order to secure to the house perfect immunity from sewer gas. Briefly stated, these essentials are as follows:

1. Extension of all soil and waste pes through and above the roof.

2. Providing a fresh air inlet in the drain at the foot of the soil and waste pipe system.

3. Trapping the main drain outside of the fresh air inlet, in order entirely to exclude the sewer air from the house.

4. Providing each fixture, as near as possible to it, with a suitable trap.

5. Providing vent pipes to such traps under fixtures as are liable to be emptied by siphonage.