Ivi 



busy period which preceded the discovery of the attenuation of 

 viruses, numbers of impossible experiments were gravely discussed 

 amongst them, to be only laughed over the following day. 



Yet another living virus must, however, be mentioned, which 

 Pasteur successfully identified in these earliest days of his patho- 

 logical researches, this was the specific agent of fowl cholera, the 

 bacillus Gholerce gallinarum. 



True it is that Perducito, in 1878, and Toussaint in the following 

 year, had discovered and described the presence of this particular 

 micro-organism in the blood of birds afflicted with this malady, but 

 Pasteur was able to carry these observations a great and important 

 step farther, for he succeeded in separating out this bacillus, and in 

 cultivating it outside the animal's body, and in artificially inducing 

 the disease in other fowls by the inoculation of such cultures. 

 Interesting and important as was this achievement, it was destined 

 to become of yet greater significance, for it was in the study of these 

 fowl cholera organisms that Pasteur made that epoch-making dis- 

 covery of the attenuation of virus and the artificial production of 

 immunity. 



On returning to his laboratory from a holiday which had inter- 

 rupted his researches on fowl cholera, Pasteur, to his dismay, found 

 all his cultivations of the microbes of this disease either dead or 

 nearly so, and that many of the animals which he inoculated with 

 these exhausted cultures appeared to suffer no ill effects whatever, a 

 condition which contrasted painfully , from the experimenter's point of 

 view, with the unerring fatal termination which had accompanied 

 such inoculations before the vacation. Having at length succeeded 

 in obtaining a virulent culture, the idea occurred to Pasteur of re- 

 inoculating the birds which had survived the previous treatment 

 with the exhausted cultures. "What was his astonishment on seeing 

 that these birds resisted the attack of the virulent organisms which 

 proved rapidly fatal to those which had undergone no previous in- 

 oculation with the exhausted cultures. Convinced that this was no 

 chance circumstance, but that he was here face to face with an 

 entirely new phenomenon, Pasteur repeated the experiment in 

 various ways, and found that he had indeed realised his great 

 ambition of "immunising against an infectious disease, of which 

 they cultivated the virus," and the microbe, which had hitherto 

 only proved a malignant foe, was constrained to become the bene- 

 ficent protector of its prey. 



Numerous investigations were now made to determine upon what 

 factors this conversion of the virus into a vaccine depended, and 

 before long Pasteur was able to announce that the secret lay in 

 the prolonged action of the air upon the culture at a suitable 

 temperature. It was in. the following year, 1881, on the occasion 



