should be gas-tight.

=Sewer Air and Gas.=--Sewer gas is not a gas at all. What is commonly
understood by the term is the air of sewers, the ordinary atmospheric
air, but charged and contaminated with the various products of organic
decomposition taking place in sewers. Sewer air is a mixture of gases,
the principal gases being carbonic acid; marsh gas; compounds of
hydrogen and carbon; carbonate and sulphides of ammonium; ammonia;
sulphureted hydrogen; carbonic oxide, volatile fetid matter; organic
putrefactive matter, and may also contain some bacteria, saprophytic
or pathogenic.

Any and all the above constituents may be contained in sewer air in
larger or smaller doses, in minute or toxic doses.

It is evident that an habitual breathing of air in which even minute
doses of toxic substances and gases are floating will in time impair
the health of human beings, and that large doses of those substances
may be directly toxic and dangerous to health. It is certainly an
error to ascribe to sewer air death-dealing properties, but it would
be a more serious mistake to undervalue the evil influence of bad
sewer air upon health.

=Ventilation.=--To guard against the bad effects of sewer air, it is
necessary to dilute, change, and ventilate the air in sewers. This is
accomplished by the various openings left in the sewers, the so-called
lamp and manholes which ventilate by diluting the sewer air with the
street air. In some places, chemical methods of disinfecting the
contents of sewers have been undertaken with a view to killing the
disease germs and deodorizing the sewage. In the separate system of
sewage disposal, where sewer pipes are small and usually
self-cleansing, the late Colonel Waring proposed to ventilate the
sewers through the house pipes, omitting the usual disconnection of
the house sewer from the house pipes. But in the combined system such
a procedure would be dangerous, as the sewer air would be apt to enter
the house.

Rain storms are the usual means by which a thorough flushing of the
street sewers is effected. There are, however, many devices proposed
for flushing sewers; e. g., by special flushing tanks, which either
automatically or otherwise discharge a large volume of water, thereby
flushing the contents of the street sewers.

CHAPTER VI

=Plumbing=

=Purpose and Requisites for House Plumbing.=--A system of house
plumbing presupposes the existence of a street sewer, and a
water-su