This may continue
for a long time (as happened in the Roman Campagna during the years 1881
and 1882), but may also be completely destroyed by a single shower. Nature
also sometimes renders a district healthy in a manner _purely
atmospheric_, by covering a malarious soil with earth which does not
contain the malarial ferment, or with a matting formed of earth and the
roots of grasses growing closely together in a natural meadow.
In the attempts of purification by suspending the malarial action, which
have been devised by man, the same thing has been done; that is to say, it
has been sought, to eliminate at least one of the three conditions
essential to the development of the specific ferment contained in the
infected soil. Naturally, they have not thought of bringing about a
thermic purification, such as nature produces in winter, because of the
impossibility of moderating the action of the sun; but they have tried
from all time to procure hydraulic or atmospheric purifications, and
sometimes to combine these together in a very happy way.
The hydraulic systems are very numerous, for the problem which is
presented, namely, that of depriving the ground of its humidity during the
hot season, necessitates different solutions according to the nature and
the bearing of the soil.
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