We must not think, however, that all earth containing this ferment is
capable of poisoning the superjacent atmosphere. Popular experience,
certain modern scientific investigation, and the facts which one can often
verify when the soil, which was malarious in ancient times and which has
since ceased to be so, is turned up to a great depth, all agree in proving
that the ground remains inoffensive as long as it is not placed in certain
conditions indispensable for the multiplication of this specific ferment.
Up to this point the organism lives, so to speak, in an inert state, and
may remain so during centuries without losing any of its deleterious
power. There is nothing in this fact that ought to surprise us, since we
know that the life and the power of evolution belonging to the seeds of
plants of a much higher order than these vegetable organisms constituting
ferments, may remain latent for centuries, and may then revive at once
when these grains are placed in the conditions suitable for their
germination.
Among the conditions favorable to the multiplication of the malarial
ferment contained in the soil, and to its dispersion through the
superjacent atmosphere, there are three which are absolutely essential,
and the concurrence of which is indispensable for the production of bad
air (malaria).
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