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37
It will be noted that only 12.1 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 43.3 per cent.
shewed 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), it will be noticed that the percentage
of houses showing defective drains has decreased from
22.3 in 1891 to 12.1 in 1904. Table L gives full particulars,
with the averages for the 10 years worked out, and on comparing
these (which relate to the old Parish of Lambeth) with those
for 1904 (relating the new Borough of Lambeth), it will be
seen that the latter show a decrease of 37.9 per cent. below the
average in respect of defective drains.