EXPERIMENTS ON ANIMALS 
63 
are indebted to Pasteur. He found that when a minute particle 
of blood from chickens dying from cholera was placed in steril¬ 
ized chicken broth, and kept at the temperature of about 30 deg. 
C., immense numbers of micrococci developed for twoor three days, 
and then settled as an opaque deposit to the bottom of the cul¬ 
ture fluid, and there remained without change, provided access 
from all external impurity was prevented. Thus treated, a drop 
of the culture fluid, after shaking, is as virulent as the original 
blood, and injected into chickens or rabbits, rapidly produces fatal 
cholera. 
But if a series of cultures are made, allowing intervals of sev¬ 
eral weeks or even months to elapse between the periods when 
active development takjes place and the commencement of the 
next cultivation, there is a gradually decreasing energy in the 
virulence of the fluids, so that a point is finally reached when the 
injection of a drop of this attenuated virus into the muscles of a 
chicken produces either a transitory local inflammation, which 
never proves fatal and soon passes off, or else no visible effect; 
while complete immunity to cholera, lasting for a year or more, 
is acquired. 
It was further proved that this reduction in the energy of the 
choleraic virus was due to the action of the oxygen of the atmos¬ 
phere ; for when the cultures are carried on in hermetically sealed 
tubes, no decrease in virulence takes place, no matter what the 
interval between the different series of cultivations may be. 
By this means chickens may be vaccinated against cholera 
with even more positive results than is the case in human small¬ 
pox. 
Contagious pleuro-pneumonia in cattle is a disease with which 
we in America are unfortunately but too familiar; and yet in 
1.850 the question as to its contagiousness was still undecided. 
At the suggestion of Dumas, a commission was then appointed 
to investigate the subject, and the result of these experiments 
was to prove that the disease was communicable, not only by co¬ 
habitation in 50 per cent, of the exposed cases, and of these 70 
per cent, proved fatal, but that it was also transmissable without 
contact, through the atmosphere. The experiments of this com¬ 
mission further showed that when once affected with this disease, 
