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about in his round-bellied bowl. The frogs croaked unconcernedly and the eels kept all of their slippery liveliness; the tortoise now and then stuck his head out of his shell and seemed to wink an eye at Koch as if to say: "Your tubercle bugs are food for me—give me some more."

But while his injections worked no harm to these creatures, that do not in the course of nature get consumption anyway—at the same time the guinea-pigs began to droop, to lie pitifully on their sides, gasping. One by one they died, their bodies wasting terribly into tubercles. . . .

Now Koch had forged the last link of the chain of his experiments and was ready to give his news to the world: The bacillus, the true cause of tuberculosis, has been trapped, discovered! When suddenly he decided there was one more thing to do.

"Human beings surely must catch these bacilli by inhaling them, in dust, or from the coughing of people sick with consumption. I wonder, will healthy animals be infected that way too?" At once Koch began to devise ways of doing this experiment—it was a nasty job. "I'll have to spray the bacilli from my cultivations at the animals," he pondered. But this was a more serious business than turning ten thousand murderers out of jail. . . .

Like the good hunter that he was, he took a chance with the dangers that he couldn't avoid. He built a big box and put guinea-pigs and mice and rabbits inside it and set this box in the garden. Then through the window he ran a lead pipe that opened in a spray nozzle inside the box, and for three days, for half an hour each day, he sat in his laboratory, pumping at a pair of bellows that shot a poisonous mist of bacilli into the box—to be breathed by the cavorting beasts inside it.

In ten days three of the rabbits were gasping, fighting for that precious air that their sick lungs could no longer give them. In twenty-five days the guinea-pigs had done their humble work—one and all they were dead, of tuberculosis.

Koch told nothing of the ticklish job it was to take these