THE ATTENUATED VIEUS, OR VACCINATION. 225 



among their poultry. Some trials were made, and 

 all succeeded beyond expectation. To preserve this 

 vaccine it must be secured from contact with the air, 

 the cultures being enclosed in tubes, the extremities 

 of which are sealed by the flame of a blowpipe. 



What takes place during that interval of time in- 

 tentionally placed between two successive cultivations 

 of the cholera microbe that interval which is em- 

 ployed in effecting the attenuation and producing the 

 vaccine ? What is the secret of this result ? The 

 agent which intervenes is no other than the oxygen 

 of the air. Here is the proof. If the cultivation of 

 this microbe is carried on in a tube containing very 

 little air, and if the tube is then closed by the flame 

 of a lamp, the microbe, by its development and life, 

 quickly appropriates all the free oxygen contained in 

 the tube, as well as the oxygen dissolved in the liquid. 

 Thus, completely protected from contact with oxygen, 

 the microbe does not become sensibly weakened for 

 months, sometimes even for years. 



The oxygen of the air, then, appears to be the 

 cause of modification in the virulence of the microbe. 



But how, then, is the absence of influence on the 

 part of the atmospheric oxygen, in the successive cul- 

 tures which are practised every twenty- four hours, to 

 be explained ? There is, in Pasteur's opinion, but one 

 possible explanation ; it is that the oxygen of the air 

 in this latter case is solely employed in the life of the 



