"8th. It may be developed, in previously healthy places, by turning up the
soil; as in making excavations for foundations of houses, tracks for
railroads, and beds for canals.
"9th. In certain cases it seems to be attracted and absorbed by bodies of
water lying in the course of such winds as waft it from the miasmatic
source.
"10th. Experience alone can enable us to decide as to the presence or
absence of malaria, in any given locality.
"11th. In proportion as countries, previously malarious, are cleared up
and thickly settled, periodical fevers disappear--in many instances to be
replaced by the typhoid or typhus."
La Roche, in a carefully prepared treatise on "Pneumonia; its Supposed
Connection with Autumnal Fevers," recites various theories concerning the
mode of action of marsh miasm, and finds them insufficient to account for
the phenomena which they produce. He continues as follows:--
"All the above hypotheses failing to account for the effects in question,
we are naturally led to the admission that they are produced by the
morbific influence of some special agent; and when we take into
consideration all the circumstances attending the appearance of febrile
diseases, the circumscribed sphere of their prevalence, the suddenness of
their attack, the character of their phenomena, etc., we may safely say
that there is nothing left but to attribute them to the action of some
poison dissolved or suspended in the air of the infected locality; which
poison, while doubtless requiring for its development and dissemination a
certain degree of heat, and terrestrial and atmospheric moisture, a
certain amount of nightly condensation after evaporation, and the presence
of fermenting or decomposing materials, cannot be produced by either of
these agencies alone, and though indicated by the chemist, betrays its
presence by producing on those exposed to its influence the peculiar
morbid changes characterizing fever.
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