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 been cured from it either, and this serum did not hinder one bit the murderous action of the poison. He shot this mixture into new guinea-pigs; in three days they grew cold; when he laid them on their backs and poked them with his finger they did not budge. In a few hours they had coughed their last sad hiccup and passed beyond.... “It is only the serum of immune animals--of beasts who have had diphtheria and have been cured of it--it is only such serum kills the diphtheria poison!” cried Behring. Healer that he was, you can hear him muttering: “Now, maybe, I can make larger animals immune too, and get big batches of their poison-killing serum, then I’ll try that on children with diphtheria ... what saves guinea-pigs should cure babies!” By this time nothing could discourage Behring. Like some victorious general swept on by the momentum of his first bloody success, he began shooting diphtheria microbes, and iodine tri-chloride, and the poison of diphtheria microbes, into rabbits, into sheep, into dogs. He tried to turn their living bodies into factories for making the healing serum, the toxin-killing serum. “Antitoxin” he called such serum. And he succeeded, after those maimings and holocausts and mistakes, always the necessary preludes to his triumphs. In a little while he had sheep powerfully immune, and from them he got plenty of blood. “Surely the antitoxin [he hadn’t the faintest notion what the chemistry of this mysterious stuff was] certainly it will prevent diphtheria,” said Behring.