INOCULATION WITH DILUTED VIRUS. 
65 
When, in the summer of 1880, I commenced my investiga¬ 
tions of the effect of inoculations with diluted virus, I soon be¬ 
came convinced that, to obtain definite and comparable results, it 
was necessary to obtain a virus of a standard strength, and one 
free from coagula and other foreign particles which prevent the 
even diffusion of the virulent granules. In these experiments, 
fowl-cholera was the disease selected, not only because it is an 
excellent type of the non-recurrent. contagious fevers; but also 
because the virus is easily cultivated outside of the body, the sub¬ 
jects are cheap and easily obtained, and the Department of Agri¬ 
culture, with which I am connected, was anxious for an investiga¬ 
tion in the interests of our agricultural population. 
The production of a virus which should contain a practically 
constant number of disease-germs in every drop, which could be 
obtained at will and in any desired quantity, which should be free 
from foreign particles, certainly seemed, at the time I am refer¬ 
ring to, a most difficult question to resolve. But, fortunately, the 
obstacles to our success, as so frequently happens, did not prove 
so insurmountable when we came to grapple with them as they 
had appeared when contemplated from a greater distance. The 
germs of this disease were easily cultivated in a broth made from 
the flesh of fowls, which was carefully filtered until perfectly 
limpid and sterilized by heat. A few germs placed in a flask of 
this liquid, multiplied for a certain time and then became in¬ 
active. Pasteur demonstrated that this cessation of activity was 
due to the exhaustion of the available nutriment. What, then, 
would be easier than to make a broth of a definite strength by 
extracting the soluble parts of a given weight of flesh with a 
given quantity of distilled water ? If we cultivate our germs in 
such a broth, at a favorable temperature, we should always ob¬ 
tain, at the moment when the nutriment is exhausted, a virus of 
practically identical strength. This reasoning, I may add, has 
been fully justified by the many experiments which I have made 
with a standard virus prepared in this way. 
The number of germs in a drop of such standard virus, 1 have 
never accurately determined, but I assured myself at an early 
stage of these researches that there were over one million six hun¬ 
dred thousand. 
