“No--not yet--what I have done might make Pasteur sure, but I am not at all convinced yet.... I have to get these bacilli out of the bodies of my dying animals now ... grow them on our beef-broth jelly, pure colonies of these microbes I must get, and cultivate them for months, away from any living creature ... and _then_, if I inoculate these cultivations into good healthy animals, and they get tuberculosis ...” and Koch’s sober wrinkled face smiled for a moment. Loeffler and Gaffky, ashamed of their jumping at conclusions, went back awed to their own searchings. Testing every possible combination that his head could invent, Koch set out to try to grow his bacilli pure on beef-broth jelly. He made a dozen different kinds of good soup for them, he kept his tubes and bottles at the temperature of the room and the temperature of a man’s body and the temperature of fever. He cleverly used the sick lungs of guinea-pigs that teemed with bacilli, lungs that held no other stray microbes which might over-grow and choke out those delicate germs which he was sure must be the authors of consumption. The stuff from these lungs he planted dangerously into hundreds of tubes and bottles, but all this work ended in--nothing. In brief, those slim bacilli that grew like weeds in tropic gardens in the bodies of his sick animals, those microbes that swarmed in millions in sick men, those bacilli turned up their noses--that is, they would have if they had been equipped with noses--at the good soups and jellies that Koch cooked for them. It was no go! But one day a reason for his failures popped into Koch’s head: “The trouble is that these tubercle bacilli will only grow in the bodies of living creatures--they are maybe almost _complete_ parasites--I must fix a food for them that is as near as possible like the stuff a living animal’s body is made of!” So it was that Koch invented his famous food--blood-serum jelly--for microbes that are too finicky to grow on common provender. He went to string-butchers and got the clear straw-colored serum from the clotted blood of freshly slaughtered healthy cattle and carefully heated this fluid to kill all the stray microbes that might have fallen into it. Delicately he poured this serum into each one of dozens of narrow test-tubes, and placed these on a slant so that there would be a long flat surface on which to smear the sick consumptive tissues. Then ingeniously he heated each tube just hot enough to make the serum set, on a slant, into a clear beautiful jelly.