rom a sickness so produced, with the bacilli in their
strongest and most virulent form, and the hen showed no effect
whatever. Then he took two hens, one fresh from the coop and the
other well again after the sickness produced by the inoculation with
the exposed bacilli, and inoculated both with the blood of a hen
that was dying of chicken cholera. The first died, the second was
affected. In other words, Pasteur had made the greatest discovery in
physiology of this century. He had found it is possible to attenuate
the virus of a virulent disease, and to use that virus so attenuated
as a vaccine matter which will guard the animal vaccinated against
the disease. He had taken Jenner's discovery, and proved it applied
to other diseases besides small-pox.

Pasteur's theory of the reason why any vaccine matter will have its
prophylactic effect, is this: He believes there is in the blood of
any animal subject to a disease caused by bacilli some substance
which is necessary to the sustenance of those bacilli; and when the
bacilli, having an attenuated virus, are introduced, they slowly
consume all of this substance.

The substance being one which nature creates very slowly, no
subsequent introduction of the bacilli, however virulent, can
produce the disease until such time shall have elapsed that a new
supply of the substance shall have been secreted. In this way he
accounts for the fact that vaccination will protect from small-pox
for a more or less defined period of time.

Pasteur hastened to apply his discovery of the attenuation of the
virus of chicken cholera to the virus of splenic fever. Here,
however, he was met with a serious difficulty. The microbes of
splenic fever, if left in the flask for forty-eight hours, developed
bright spots, and gradually into these spots the bacilli themselves
seemed to be absorbed. Pasteur found these spots were the spores or
seeds of the microbes, and he also found that, while the bacilli
could be killed easily in various ways, the spores possessed a much
greater resistance. They could be dried, for example, and preserved
in that state indefinitely. It was apparent that the oxygenation
which attenuated the venom of the bacilli of chicken cholera was
impossible with those of splenic fever if the bacilli of the latter
disappeared within a week, leaving the spores behind. But Pasteur
had discovered before this that, unless the temperature of a fowl
were lowered artificially, inoculation w