t this fanatical prejudice in favor of the
eucalyptus tree. These plants are, in fact, very capricious in their
growth. In full vegetation during the winter in our climate, they are
often killed instantly by a sharp winter frost, by damp cold, by the
frosts of spring, or by other causes which the botanists have not yet been
able to determine. At other times, if the winters are very mild, these
plants grow too rapidly in height, and then are broken short off by
moderately strong winds. It should further be mentioned that these
plantations are sometimes very expensive. In fact, if the earth contains
too much water, it must be drained under penalty of seeing the roots of
the eucalyptus rot. Then again, if the subsoil is compact, it is necessary
to dig deep trenches in order to give room to the long roots of these
trees, and often indeed these trenches must also be drained, as is done
for olive trees. The conclusion evidently is that it is better to confine
ourselves to hydraulic methods of promoting the health fulness of a
locality, the immediate effects of which are less uncertain. And then,
when the local conditions are such as to make it desirable to try the
effects of plants possessed of strongly absorbing powers, it is better to
choose them from among the flora of our own hemisphere. This is more sure,
and will cost less.

Simple hydraulic methods of purification, even the most perfect, do not,
however, produce permanent hygienic effects, since the moisture necessary
for the multiplication of the malaria in the soil is so slight that these
effects may be compromised by anything whatever that is capable of
restoring a moderate degree of humidity to the ground during the hot
season. It has often been thought that a suspension of malarial production
would be better assured by suppressing at the same time the humidity of
the soil and the direct action of the oxygen of the air upon the
superficial strata of earth which contain the ferment. This has been
successfully accomplished by the system of overlaying (_comblees_). This
consists in covering the infected soil by thick layers of uninfected
earth, carried there either by the muddy waters of rivers or by the hand
of man. At the same time the steady drainage of the surface and
underground water is provided for. Last year I advised our Minister of War
to undertake in another form a hydraulico-atmospheric purification of the
district of the Janiculum surrounding the Salviati