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(c) 725 (i.e., 38.2 per cent.) gave no results with the
tests.
It will be noted that only 12.6 per cent. of the total infected
houses showed, with the test, defective drains, no results being
obtained from the tests in the other cases ; and 49.2 per cent.
showed defective traps, fittings and appliances.
Drains and sanitary fittings are tested, as a routine, in all
infected houses, though it does not follow that, when defects are
found, such defects are the causes of the diseases under investigation.
They may indirectly be so.
The test used is the chemical test (Kingzett.s), and, when a
result is obtained, the drain (tested) is defective, but, when no
result is obtained, it would be unsafe to state that, therefore, the
drain (tested) was sound. A negative result proves nothing
with the chemical (or smoke) test. The hydraulic (water) and
pneumatic (air) tests are the only reliable tests, but too severe for
the routine testing of drains of old buildings. The pneumatic
(air) test is comparatively new, and most delicate, but is not
much used in London at the present day.
Bearing this explanation in mind, the statistics are still useful
as showing that, year by year, the number of defects found
decreases. Taking the statistics for the Parish of Lambeth
during 10 years (1891.1900), and those for the Borough of
Lambeth during 5 years (1901.6), it will be noticed that the percentage
of houses showing defective drains has decreased from
22.3 in 1891 to 12 6 in 1906. Table L gives full particulars,
with the averages for the 10 years worked out, for the old Parish
of Lambeth, and comparing these with those for 1906 for the
Borough of Lambeth, it will be seen that the latter show a
decrease of 35.4 per cent. below the average for the old Parish
(1891.1900) in respect of defective drains.