r. Metcalfe states,
that all hypotheses, even the most plausible, are entirely unsupported by
positive knowledge, and he says:--

"This confession of ignorance still leaves us in possession of certain
knowledge concerning malaria, from which much practical good may be
derived.

"1st. It affects, by preference, low and moist localities.

"2d. It is almost never developed at a lower temperature than 60 deg.
Fahrenheit.

"3d. Its evolution or active agency is checked by a temperature of 32 deg..

"4th. It is most abundant and most virulent as we approach the equator and
the sea-coast.

"5th. It has an affinity for dense foliage, which has the power of
accumulating it, when lying in the course of winds blowing from malarious
localities.

"6th. Forests, or even woods, have the power of obstructing and preventing
its transmission, under these circumstances.

"7th. By atmospheric currents it is capable of being transported to
considerable distances--probably as far as five miles.

"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 infec