l6 BACTERIA, YEASTS, AND MOLDS 



ing but a guess; but here in the case of chicken cholera 

 Pasteur had shown definitely that a weakened culture could 

 give immunity without causing the disease. It was a mo- 

 mentous discovery, not because chicken cholera is a disease 

 of much importance, but because the same principle could 

 be applied to other diseases. 



He first applied this same principle to anthrax. In this 

 case he found that old cultures were just as virulent as 

 fresh ones ; but he soon discovered that he could weaken 

 the anthrax germ by growing it at a high temperature. A 

 culture weakened in that way could be used to vaccinate 

 cattle against the disease. He proved this to his own satis- 

 faction ; but he had found other scientists so skeptical 

 about his earlier discoveries that he decided to arrange a 

 public experiment to show that he had conquered anthrax. 

 For this purpose he obtained fifty sheep, and in the pres- 

 ence of an assembly that had come to watch the experi- 

 ment he inoculated them all with virulent anthrax bacteria. 

 Now, anthrax is a severe disease, and any sheep thus in- 

 oculated could hardly be expected to escape ; but, as 

 Pasteur informed the crowd, twenty-five of the sheep had 

 been vaccinated against anthrax and were immune. He 

 urged the assemblage to return in two days to see the 

 result. On the third day, therefore, a large crowd, com- 

 posed of farmers, physicians, veterinarians, scientists, and 

 journalists, was present, and their impressions of the 

 sight that greeted them were so strong that the world is 

 never likely to hear the last of Pasteur's experiment. 

 Twenty-five sheep were dead or dying ; the other half of 

 the fifty were in perfect health. The success of the vac- 

 cination was perfectly demonstrated. 



