ied composition, and without the presence of which neither
marshes nor stagnant pools of water are capable of producing malaria.

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). First, a temperature which does not fall below 20 deg.C.
(67.5 deg.F.); next, a very moderate degree of permanent humidity of the soil;
and finally, the direct action of the oxygen of the air upon the strata of
earth which contain the ferment. If a single one of these three conditions
be wanting, the development of malaria becomes impossible. This is a point
of prime importance in the natural history of malaria, and it gives us the
key to most of the methods of sanitary improvement attempted by man.

Let us see first what can be done in this direction without the labor of
man. For nature herself makes localities salubrious by _suspending_ for a
greater or less time the production of malaria. It is thus that winter
brings about in every country a freedom from malaria which is _purely
thermic_, for it is due simply and entirely to a sinking of the
temperature below the required minimum. Indeed, if the temperature in
winter rises above this minimum, there are often sudden outbreaks of
malaria. Sometimes,