As I have told you before, Pasteur had a great deal of the mystic in him. Often he bowed himself down before that mysterious Infinite--he worshiped the Infinite when he was not clutching at it like a baby reaching for the moon; but frequently, the moment one of his beautiful experiments had knocked another little chunk off that surrounding Unknown, he made the mistake of believing that all mysteries had dissolved away. It was so now--when he saw that he could really protect chickens perfectly against a fatal illness by his amazing trick of sticking a few of their own tamed assassins into them. At once Pasteur guessed: “Maybe these fowl-cholera microbes will guard chickens against other virulent diseases!” and promptly he inoculated some hens with his new vaccine of weakened fowl-cholera germs and then injected them with some certainly murderous _anthrax_ bacilli--and the chickens did not die! Wildly excited he wrote to Dumas, his old professor, and hinted that the new fowl-cholera vaccine might be a wonderful Pan-Protector against all kinds of virulent maladies. “If this is confirmed,” he wrote, “we can hope for the most important consequences, even in human maladies.” Old Dumas, greatly thrilled, had this letter published in the Reports of the Academy of Sciences, and there it stands, a sad monument to Pasteur’s impetuousness, a blot on his record of reporting nothing but _facts_. So far as I can find, Pasteur never retracted this error, although he soon found that a vaccine made from one kind of bacillus does not protect an animal against all diseases, but only--and then not absolutely surely--against the one disease of which the microbe in the vaccine is the cause. But one of Pasteur’s most charming traits was his characteristic of a scientific Phœnix, who rose triumphantly from the ashes of his own mistakes. When his imagination carried him into the clouds you find him presently landing on the ground with a bump--making clever experiments again, digging for good true hard facts. So it is not surprising to find him, with Roux and Chamberland, in 1881, discovering a very pretty way of taming vicious anthrax microbes and turning them into a vaccine. By this time the quest after vaccines had become so violent that Roux and Chamberland hardly had their Sundays off, and never went on vacations; they slept at the laboratory to be near their tubes and microscopes and microbes. And here, Pasteur directing them, they delicately weakened anthrax bacilli so that some killed guinea-pigs, but not rabbits, and others did mice to death, but were too weak to harm guinea-pigs. They shot the weaker and then the stronger microbes into sheep, who got a little sick but then recovered, and after that these sheep could stand, apparently, the assaults of vicious anthrax germs that were able to kill even a cow.