not, knock on the trap with a piece of metal; if the trap is empty,
a hollow sound will be given out; if full, the sound will be dull.
This is not reliable in case the trap is full or half-full with slime,
etc. Another test for the same purpose is as follows: hold a light
near the outlet of the fixture; if the light is drawn in, it is a sign
that the trap is empty.

(2) Defects in leaded joints can be detected if white lead has been
used, as it will be discolored in case sewer gas escape from the
joints.

(3) The connection of a waste pipe of a bath tub with the trap of the
water-closet can sometimes be discovered by suddenly emptying the bath
tub and watching the contents of the water-closet trap; the latter
will be agitated if the waste pipe is discharged into the trap or on
the inlet side of trap of the water-closet.

(4) The presence of sewer gas in a room can be detected by the
following chemical method: saturate a piece of unglazed paper with a
solution of acetate of lead in rain or boiled water, in the proportion
of one to eight; allow the paper to dry, and hang up in the room where
the escape of sewer gas is suspected; if sewer gas is present, the
paper will be completely blackened.

The main tests for plumbing are: (1) the _Hydraulic_ or water-pressure
test; (2) the _Smoke_, or sight test, and (3) the _Scent_, or
peppermint, etc., test.

The _Water-pressure Test_ is used to test the vertical and horizontal
pipes in new plumbing before the fixtures have been connected. It is
applied as follows: the end of the house drain is plugged up with a
proper air-tight plug, of which there are a number on the market. The
pipes are then filled with water to a certain level, which is
carefully noted. The water is allowed to stand in the pipes for half
an hour, at the expiration of which time, if the joints show no sign
of leakage, and are not sweating, and if the level of the water in the
pipes has not fallen, the pipes are water-tight. This is a very
reliable test, and is made obligatory for testing all new plumbing
work.

_The Smoke Test_ is also a very good test. It is applied as follows:
by means of bellows, or some exploding, smoke-producing rocket, smoke
is forced into the system of pipes, the ends plugged up, and the
escape of the smoke watched for, as wherever there are defects in the
pipes the smoke will appear. A number of special appliances for this
test are manufactured, all of them more or less ingeniou