bustion of the tissues in the absence of food, and conse-
quently as the agent of enervation, is, at the same time,
an agent of attenuation. Attenuation and weakening
are synonymous, and we have here a conception which
harmonizes well with our idea of strife in the microbial
diseases. That which is harmful to the microbe is of
value to its host.

We are then justified in asking ourselves if all these
causes of weakening on the part of the microbe, all these
factors which contribute more or less quickly to its
death, do not first cause it to pass through a series of suc-
cessive attenuations, that is, transform it into vaccines.
To this new question, experiment replies without hesi-
tation, "Yes." In a general way, attenuation is one of
the forms of the gradual weakening of a microbial cell
which is on its way to death, and every action harmful
to the microbe begins by diminishing its virulence.
Such, for example, is heat, too high a degree of which
kills the microbe, as we know. Between the optimum
temperature for culture and the death point exists a
zone of attenuation, observed by M. Toussaint and
carefully studied by M. Chauveau, for the anthrax
bacteridium. The duration of the heating should be in
inverse ratio to the elevation of the temperature and, for
a given temperature, directly proportional to the degree
of attenuation to be obtained.

Next to the action of heat naturally comes that of the
light of the sun. It kills the microbe after a certain
length of exposure to it, but before killing, it causes
attenuation. This is the conclusion from my experi-
ments, followed by those of M. Arloing.

So much for the physical agents. Now for the
chemical ones. Oxygen is a physiological factor of
the greatest importance, and we have already examined
its rôle in this relation. But it plays also a rôle more

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