iftieth successive inoculation. "Rabbits
inoculated with a brain suspension of rabbit number fifty all died in
seven days." This caused Pasteur to name the virus of number fifty "virus
fixe," a virus of definite length. He now had obtained a virus of definite
strength and the next question was, how could the virulence be gradually
and definitely reduced.

[246 MOTHERS' REMEDIES]

This he accomplished after many experiments. He proved that pieces of the
"medulla oblongata" suspended in sterile tubes which contained fragments
of caustic potash, steadily and gradually reduced their virulence as they
dried, till the fourteenth day, when they were practically inert. New
specimens were prepared each day and cords which had dried in one day
Pasteur called "one-day virus;" cords which had dried in two days, "two
day's virus," and so on up to the fourteenth day. With this graduated
virus he now experimented on dogs, and the injection he used on the first
day consisted of an emulsion of fourteen-day virus; for the second day,
the thirteen-day virus, thus using a stronger virus each day, until on the
fourteenth day he used the full strength virus. This treatment produced
what is called immunity in the dog, and even the direct inoculation into
the brain of the strong virus would not produce death.

After Pasteur had thoroughly satisfied himself by repeated trials, he
announced his wonderful discovery, and it was in 1886 that Pasteur
considered the preventive inoculation in human beings as resting upon a
satisfactory experimental basis. During these five years this eminent man
proved that it was possible to protect or immunize the lower animals,
rabbits and dogs, against inoculation with the virulent virus.

The efficiency of this immunity was given trials by different methods of
inoculation. It was found that sixty per cent of dogs inoculated under the
"dura" (a membrane of the brain) were saved if treatment was given the
second day. This test is more severe than is required to meet the ordinary
infection of rabies. Pasteur, after a series of these final tests were so
convincing, prescribed the preventive inoculations in human beings and on
July 6th, 1886, the first human patient received the first treatment of
his series of inoculations.

The method of obtaining the attenuated virus used in the treatment is as
follows: A rabbit is inoculated by the brain method before described, each
day, with suspension of the fresh, fixed vi