hing out the
soil-pipe, its sides will always be more or less coated with organic
filth; and, however slight this coating, there will be a certain amount
of decomposition. The decomposition of all such matters must be rapid
and complete, not slow and partial. A necessary condition of complete
destructive decomposition is an abundance of atmospheric air to supply
the oxygen which complete decomposition demands. If the soil-pipe is
closed at its top, or if it is obstructed by a trap in the lower part of
its course, there can be no such circulation of air as safety requires.

If there is an opportunity for the free admission of air from the
well-ventilated sewer to feed the upward current almost constantly
prevailing in a soil-pipe open at both ends, the gases resulting from
the decomposition will be of a different and less injurious character
than where the air is confined,--and by the mere volume of air passing
through the pipe they will be so diluted that even were they originally
poisonous their power for harm will be lessened.

The gases formed by the decomposition of organic matter in the sewer
itself, or in the soil-pipe, have a certain expansive force which is
greatly increased by the elevation of temperature, caused, for example,
by the discharge of hot water into the pipe or sewer. If the soil-pipe
is open at its upper end this expansion will be at once relieved; but if
the top of the pipe be closed there will always be danger of the forcing
of the feeble barrier offered by the ordinary water-seal trap of a
branch pipe leading from a wash-basin or sink. Then, too, the
sealing-water of the trap readily absorbs any foul gases presented at
its outer end, toward the soil-pipe, and gives it off in an unchanged
condition at the inner or house end. Such traps retard, but do not
prevent, the entrance of sewer gases into the house. Water-seal traps
which are unused for any considerable time are emptied by evaporation,
and thus open a channel through which the air of the soil-pipe may find
its way into the house.

It is usual in modern plumbing to relieve the pressure of gas in the
soil-pipe by what is called a "stench-pipe." This is a pipe from one to
two inches in diameter, leading from the highest point of the soil-pipe
to the outside of the roof, where it is bent over to prevent the
entrance of foreign matter, or is closed at the top and perforated with
holes to allow the gas to escape. This small stench-pipe is inad