“I will stain live animals!” he cried. “The chemistry of animals is like the chemistry of my dyes--staining them while they are still alive--that will tell me all about them!” So he took his favorite dye, which was methylene blue, and shot a little of it into the ear-vein of a rabbit. He watched the color flow through the blood and body of the beast and mysteriously pick out and paint the living endings of its nerves blue--but no other part of it! How strange! He forgot all about his fundamental science for a moment. “Maybe methylene blue will kill pain then,” he muttered, and he straightway injected this blue stuff into groaning patients, and maybe they were eased a little, but there were difficulties, of a more or less entertaining nature, which maybe frightened the patients--who can blame them? He failed to invent a good pain-killer, but from this strange business of methylene blue pouncing on just one tissue out of all the hundred different kinds of stuff that living things are made of, Paul Ehrlich invented a fantastic idea which led him at last to his magic bullet. “Here is a dye,” he dreamed, “to stain only one tissue out of all the tissues of an animal’s body--there must be one to hit _no_ tissue of men, but to stain and kill the microbes that attack men.” For fifteen years and more he dreamed that, before ever he had a chance to try it.... In 1890 Ehrlich came back from Egypt; he had not died from tuberculosis; Robert Koch shot his terrible cure for consumption into him, still he did not die from tuberculosis--and presently he went to work in the Institute of Robert Koch in Berlin, in those momentous days when Behring was massacring guinea-pigs to save babies from diphtheria and the Japanese Kitasato was doing miraculous things to mice with lockjaw. Ehrlich was the life of that grave place! Koch would come into his pupil’s crammed and topsy-turvy laboratory, that gleamed and shimmered with rows of bottles of dyes Ehrlich had no time to use--for you may be sure Koch was Tsar in that house and thought Ehrlich’s dreams of magic bullets were nonsense. Robert Koch would come in and say: “Ja, my dear Ehrlich, what do your experiments tell us to-day?” Then would come a geyser of excited explanations from Paul Ehrlich, who was prying then into the way mice may become immune to those poisons of the beans called the castor and the jequirity: