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26
wards dispersed by the wind, and probably rendered
innocuous. Let us hope that any seeds of disease
carried into the atmosphere by these gases are, like
other organic particles, dried, disintegrated, oxidised,
or, if brought down again to the earth by the rain,
are, in the ceaseless chemistry of nature, decomposed
and appropriated by the vegetable world.* The external
air will, moreover, pass into the sewers. Temperature
largely determines these movements of the
air, both within and without the sewers. The higher
the temperature in the sewers, the greater is the expansive
force of the gases therein, and the more readily,
therefore, do they pass into the atmosphere. When
the temperature of the sewer is low, a contraction of
the gases takes place, and favours the entrance of
atmospheric air. But while the sewer air is confined
in the sewer, it may be subjected to an increase of
tension. This may be caused by heavier bodies, such
as soil or water, passing into the sewer, or by gases generated
in the sewer wherever there is any deposit for
any length of time. Such deposit may be due to a
continuance of dry weather, or to house drains being
inadequately flushed, or, lastly, to an insufficient fall
in the sewers themselves. I hope to be able, by
*So far as I am aware, we have no positive evidence of small-pox,
scarlet fever, or measles, having been propagated by sewer air. This
remark does not apply to diarrhoea, cholera, or typhoid fever. I have
known children to suffer from choleraic diarrhoea, induced by playing
over and around the gullies in the streets. In this country the occurrence
of typhoid fever has been traced, whenever the cause has been
discovered, to drinking water or milk poisoned with typlioidal excretions'
or to the emanations from sewers, or possibly to personal contagion.