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bothered them no more than had the bacilli which made it. Here was something entirely new in microbe hunting, something Roux maybe dreamed of but couldn't make come true. Pasteur had guarded sheep against anthrax, and children from the bites of mad dogs, but here was something incredible—Behring, giving guinea-pigs diphtheria and then nearly killing them with his frightful cure, had made them proof against the microbe's murderous toxin. He had made them immune to the stuff of which one ounce was enough to kill seventy-five thousand big dogs. . . .

"Surely it is in the blood I will find this antidote which protects the creatures!" cried Behring.

He must get some of their blood. There were hardly any of the battered but diphtheria-proof guinea-pigs left now, but he must have blood! He took one of the veterans, and cut into its neck to find the artery; there was no artery left—his numerous blood lettings had obliterated it. He poked about (let us honor this animal!) and finally got a driblet of blood out of a vessel in its leg. What a nervous time it was for Behring, and I do not know whether it is Behring or his beasts who is most to be pitied, for every morning he came down to the laboratory wondering whether any of his priceless animals were left alive. . . . But he had a few drops of serum now, from a cured guinea-pig. He mixed this, in a glass tube, with a large amount of the poisonous soup in which the diphtheria microbes had grown.

Into new, non-immune guinea-pigs went this mixture—and they did not die!

"How true are the words of Goethe!" cried Behring. "Blood is an entirely wonderful sap!"

Then, with Koch the master blinking at him, and with the entire small band of maniacs in the laboratory breathless for the result, Behring made his famous critical experiment. He mixed diphtheria poison with the serum of a healthy guinea-pig who was not immune, who had never had diphtheria or