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to. L. PASTEUR. 
nothing unnatural, when one considers that every culture gener¬ 
ally modifies the medium in which it takes place; for example, 
the modification of the earth if for ordinary plants ; of the plants 
and animals, if for their parasites; and of our liquids of culture, 
if for mucedinae, vibrios or foments. 
The modifications are manifested and characterized by the 
circumstances that new cultures of the same species in those 
media become rapidly difficult or impossible. Let us introduce 
into a bouillon of chicken, the microbe of cholera, and after 
three or four days, filtrate from the liquid all signs of the microbe ; 
and again into this filtrated liquid introduce parasites: this last 
will prove wholly incapable of sustaining the slightest growth. 
Perfectly limpid after filtration, the liquid will so continue. 
Can wo then doubt that, by the culture in the chicken of the 
attenuated virus, the body of the bird is placed in a condition ana¬ 
logous to that of the filtrated liquid, which has been rendered in¬ 
capable of cultivating the microbe ? The comparison can extend 
still farther, for if the bouillon of culture is filtrated, not the 
fourth day of the culture, but the second, the filtrated liquid will 
yet remain good for the cultivation of the microbe, but with less 
force. Thus one will comprehend that after a culture of the 
weaker microbe in the body of the hen, the food of the microbe 
may not have been removed from all the different parts of the 
body. What remains will allow, then, a new culture, but equally 
in a limited degree. This is the effect of a first vaccine. Subse¬ 
quent inoculations will by degrees remove all the materials of 
culture of the parasite. Consequently, by the action of the cir¬ 
culatory system, a moment will come when all new culture will 
become sterile. It is only then that the recidivity becomes im¬ 
possible, and that the animal is entirely and effectually vaccinated. 
One might be surprised to see the first culture of the weak 
virus cease before the nutritive elements of the microbe are con¬ 
sumed. But it must be remembered, that the microbe, aerobic 
being, is not at all in the body of the animal, in the same condi¬ 
tion as in an artificial medium of culture. Here there is no ob¬ 
stacle to its multiplication; in the body, on the contrary, it is con¬ 
tinually in conflict with the cells of organs, while they also, as 
