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Then in a flash Koch saw the beautiful experiment nature had done for him. "Every one of these droplets is a pure culture of one definite kind of microbe—a pure colony of one species of germs. . . . How simple! When germs fall from the air into the liquid soups we have been using—the different kinds of them get all mixed up and swim among each other. . . . But when different bugs fall from the air on the solid surface of this potato—each one has to stay where it falls . . . it sticks there . . . then it grows there, multiplies into millions of its own kind . . . absolutely pure!"

Koch called Loeffler and Gaffky, his two military doctor assistants, and soberly he showed them the change in the whole mixed-up business of microbe hunting that his chance glance at an abandoned potato had brought. It was revolutionary! The three of them set to work with an amazing—loyal Frenchmen might call it stupid—German thoroughness to see if Koch was right. There they sat before the three windows of their room, Koch before his microscope on a high stool in the middle, Loeffler and Gaffky on stools on his left hand and his right—a kind of grimly toiling trinity. They tried to defeat their hopes, but quickly they discovered that Koch's prophecy was an even more true one than he had dreamed. They made mixtures of two or three kinds of germs, mixtures that could never have been untangled by growing in flasks of soup; they streaked these confused species of microbes on the cut flat surfaces of boiled potatoes. And where each separate tiny microbe landed, there it stuck, and grew into a colony of millions of its own kind—and nothing but its own kind.

Now Koch, who, by this simple experience of the old potato, had changed microbe hunting from a guessing game into something that came near the sureness of a science—Koch, I say, got ready to track down the tiny messengers that bring a dozen murderous diseases to mankind. Up till this time Koch had bad very little criticism or opposition from other men of science, mainly because he almost never opened his mouth until he was sure of his results. He told of his discoveries with