to do in vain--in forcing the rods of the anthrax to
produce spores. Furthermore he gave to this spore an
important place, which it has not since lost, in the eti-
ology of the disease. He did this by showing that it
always forms in the blood and tissues of an animal dead
of anthrax, if the temperature is suitable and there is
sufficient oxygen.

These two conditions are necessary. Below 18°C.
spores are not formed; at 30°C. they occur at the end of
30 hours; at 35°C., in 20 hours. The rapidity with which
spores are formed is, therefore, directly proportional to
the amount of heat. Oxygen is also indispensable.
Anthrax blood, if deprived of this gas, ceased to be vir-
ulent in 24 hours without putrefaction. When the
blood is allowed to putrefy, the virulence also disap-
pears if putrefaction exhausts the oxygen quickly
enough so that the spores have not time to form at the
temperature to which they are exposed. If the spores
have already formed, putrefaction does not kill them
or prevent them from developing ultimately on the same
field or in the same region if circumstances are favor-
able. All the contradictory results of previous investi-
gators on the duration of the virulence of the blood
or of diseased organs, some saying that it could persist
others that it was lost immediately, were at once
explained. The persistence of the disease and its
return in an infected country was also explained,
and in an entirely natural way. It was the spore which
was the agent of preservation, which persisted where
the conditions of temperature and of aëration had per-
mitted it to form, and where it always held itself in
readiness to make new victims.

Koch was not satisfied in thus broadly explaining
the etiology of the disease. He studied the mode of
transmission, proved that the symptoms of natural

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