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41
filtration of organic solutions through a porous soil.*
Let that soil have frequent fissures in its substance;
or let its thickness be scanty in proportion to the
organic matters to be acted on; and the water,
imperfectly filtered, would run off foul and putrescent.
Now this risk, more or less, belongs to all
pumps within the City of London. They draw
from a ground excavated in all directions by
sewers, drains, cesspools, gas-pipes, burial-pits.
The immense amount of organic matter which infiltrates
the soil does, undoubtedly, for the greater
part suffer oxidation, and pass into chemical repose:
* This very important influence, exerted by the earth on
various organic infiltrations, is referred to in the text only
under one point of view ; only as it occasions the deterioration
of land-springs in urban districts, and renders their water unfit
for consumption. But the subject has another equally important
side. Such springs, having their waters laden with
nitrates, represent the continuous removal of organic impurities
which otherwise would contaminate the air. The evil of
spoiled springs, therefore—while it necessitates for every urban
population that their water-supply shall be artificially furnished
from a distance, has great countervailing advantages. A given
organic soakage will cease to vitiate the atmosphere by evaporation,
in proportion as it gravitates to lower levels, and undergoes
those chemical changes which accompany filtration through the
soil* Hence it is evident that, for the healthiness of inhabited
districts (where extensive soakage of organic matters is almost
invariable) it becomes most important to maintain, or by
artificial measures to accelerate, this down-draught through the
soil; and the reader will scarcely need to be reminded, that, in
those improvements of metropolitan sewerage, which it is a
chief object of this Report to advocate, complete provision for
the continuous drainage of soil is implied as an essential part.