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muttered (thinking, perhaps of the lightning move of the paw of one of his cats jabbing the germ-filled syringe needle into his own hand). For Koch, hunting his invisible foes alone, there were so many disagreeable and always imminent possibilities of excitement—of something tragically worse than mere excitement. . . .

But the hand of this completely unheroic looking little microbe hunter never slipped, it just grew drier and more wrinkled and blacker from its incessant baths in the bichloride of mercury—that good bichloride, with which in those old days the groping microbe hunters used to swab down everything, including their own persons. Then, week by week, in all of Koch's meaouwing, crowing, barking, clucking menagerie of beasts those small curved bacilli grew into their relentless millions—and one by one the animals died, and gave eighteen-hour-days of work to Robert Koch in post-mortems and blear-eyed peerings through the microscope.

"It is only when a man or beast has tuberculosis that I can find these blue-stained rods, these bacilli," Koch told Loeffler and Gaffky. "In healthy animals—I have looked, you know, at hundreds of them—I never find them."

"That means, without doubt, that you have discovered the bacillus that is the cause, Herr Doktor——"

"No—not yet—what I have done might make Pasteur sure, but I am not at all convinced yet. . . . I have to get these bacilli out of the bodies of my dying animals now . . . grow them on our beef broth jelly, pure colonies of these microbes I must get, and cultivate them for months, away from any living creature . . . and then, if I inoculate these cultivations into good healthy animals, and they get tuberculosis . . ." and Koch's sober wrinkled face smiled for a moment. Loeffler and Gaffky, ashamed of their jumping at conclusions, went back awed to their own searchings.

Testing every possible combination that his head could invent, Koch set out to try to grow his bacilli pure on beef-broth jelly. He made a dozen different kinds of good soup for them,