The injuriousness of fogs and low temperatures will
be presently dwelt upon at greater length.
Variations in the pressure and temperature of the atmosphere exert a
considerable influence on the circulation of air contained in the soil,
which is called ground air. As all the interstices of the ground are
filled with air or water, the more porous the soil, the greater is the
bulk of air. The quantity of air contained in soil varies very much
according to the material of which the soil is composed, as it is evident
that in a gravelly or sandy soil it must be greater than when the ground
consists of loam or clay. The estimates vary from 3 to 30 per cent., but
the latter is probably too high. If, therefore, a cesspool leak into the
ground, the offensive effluvia, if in large quantities, will escape into
the soil, and are given off at the surface of the ground, or are drawn
into a house by the fire; but, if small, they are rendered innocuous by
oxidation. The distance to which injurious gases and suspended or
dissolved organic matters may travel through a porous soil is sometimes
considerable, as I have known it pass for 130 feet along a disused drain,
and above 30 feet through loose soil.
Winds exercise a great effect on health both directly and indirectly.
Pages:
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159