364 BIOLOGY: GENERAL AND MEDICAL 



From chemistry Pasteur was led into a study of fer- 

 mentation and putrefaction which he discovered to be 

 due to microorganismal life, the source of which he 

 quickly traced to the spores or seeds of minute plants 

 abounding in the atmosphere. The source of fermenta- 

 tion being thus traceable to living entities in the air, 

 he conjectured that the source of fermentation in wounds 

 might be the same, and an investigation of the discharges 

 from fetid wounds showed them to be teeming with 

 microorganismal life capable of infecting the small 

 animals used for inoculation experiments. Convinced 

 that these microbes were the cause of the disturbances, 

 the investigation was pursued, and for various maladies 

 different microbes were found. The first investigations 

 bearing directly upon the subject of immunity were 

 made with the bacillus of chicken cholera and came 

 about in a peculiar manner. " A chance such as happens 

 to those who have the genius of observation was now 

 about to mark an immense step in advance and prepare 

 the way for a great discovery. As long as the culture 

 flasks of the chicken-cholera microbes had been sown 

 without interruption, at twenty-four hours' interval, 

 the virulence had remained the same; but when some 

 hens were inoculated with an old culture, put away 

 and forgotten a few weeks before, they were seen, with 

 surprise, to become ill and then to recover. These 

 unexpectedly refractory hens were then inoculated with 

 some new culture, but the phenomenon of resistance 

 had occurred. What had happened? What could 

 have attenuated the activity of the microbe? Re- 

 searches proved that oxygen was the cause, and, by 

 putting between the cultures variable intervals of days, 

 of one, two, or three months, variations of mortality 

 were obtained, eight hens dying out of ten, then five, 

 then only one out of ten, and at last, when, as in the first 

 case, the culture had had time to get stale, no hens 

 died at all, though the microbe could still be cultivated." 



" Finally," said Pasteur, eagerly explaining this phe- 





