tituting the seal in the trap of the fixture will
evaporate; the seal will thus be lost, and ingress of sewer air will
result. To guard against evaporation, fixtures must be frequently
flushed; and during summer, or at such times as the house is
unoccupied and the fixtures not used, the traps are to be filled with
oil or glycerin, either of which will serve as an efficient seal.

(2) _Momentum._--A sudden flow of water from the fixture may, by the
force of its momentum, empty all water in the trap and thus leave it
unsealed. To prevent the unsealing of traps by momentum, they must be
of a proper size, not less than the waste pipe of the fixture, the
seal must be deep, and the trap in a perfectly straight position, as a
slight inclination will favor its emptying. Care should also be taken
while emptying the fixture to do it slowly so as to preserve the seal.

(3) _Capillary Attraction._--If a piece of paper, cotton, thread,
hair, etc., remain in the trap, and a part of the paper, etc.,
projects into the lumen of the pipe, a part of the water will be
withdrawn by capillary attraction from the trap and may unseal it. To
guard against unsealing of traps by capillary attraction, traps should
be of a uniform diameter, without nooks and corners, and of not too
large a size, and should also be well flushed, so that nothing but
water remains in the trap.

_Siphonage and Back Pressure._--The water in the trap, or the "seal,"
is suspended between two columns of air, that from the fixture to the
seal, and from the seal of the trap to the seal of the main trap on
house drain. The seal in the trap is therefore not very secure, as it
is influenced by any and all currents and agitations of air from both
sides, and especially from its distal side. Any heating of the air in
the pipes with which the trap is connected, any increase of
temperature in the air contents of the vertical pipes with which the
trap is connected, and any evolution of gases within those pipes will
naturally increase the weight and pressure of the air within them,
with the result that the increased pressure will influence the
contents of the trap, or the "seal," and may dislodge the seal
backward, if the pressure is very great, or, at any rate, may force
the foul air from the pipes through the seal of the traps and foul the
water therein, thus allowing foul odors to enter the rooms from the
traps of the fixtures. This condition, which in practice exists
oftener th