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13
to discover that if putrid air were slowly passed through
charcoal, the poisonous and offensive gases would be
absorbed and completely decomposed by the oxygen contained
in the pores of the charcoal. In this way the gases
are entirely destroyed, as if they passed through a fire.
This discovery led to the construction of the now wellknown
charcoal air-filter, of which I have already given a
detailed description in your journal in 1854 as well as in
various other publications. It consists essentially of a
layer of coarsely-powdered charcoal interposed between
two sheets of wire gauze, and supported on a suitable
frame of metal or wood. When an air-filter similar to
that above described is closely fitted into the ventilating
opening of any drain, all pressure will be taken off the
water-traps, the deleterious gas will be removed, and only
pure air allowed to pass out through the filter, which will
remain in an effective state for several years, provided only
that the charcoal be kept dry. These air filters have been
very extensively employed during the last fourteen years
in London, Glasgow, Nottingham, and numerous other
large towns."
The sine qud non of keeping the charcoal dry appears
to have operated to throw this method into disuse, but it
is yet possible it may be revived by the discovery of some
means of overcoming the present objection to its employment.
The utilisation of the lamp-posts in streets and exposed
places, for carrying up, at certain distances, ventilating
pipes or small shafts, so that the sewer-air escaping into a
moving atmosphere, high above the heads of the passersby,
might become practically harmless, is another suggestion
that has found favour with some. Whether this plan
has ever been tried on a sufficiently large scale to test its
practicability and usefulness does not appear, but be this as
it may, it is believed by the Medical Officers of Health
that nothing is likely to prove more satisfactory and effectual
than the adoption of a constant water-flow through