Then one day he ran on to a curious sight under his microscope--a strange transformation of his microbes that gave him a clew to his question; and Koch sat down on his stool in his eight-by-ten laboratory in East Prussia and solved the mystery of the cursed fields and mountains of France. He had kept a hanging-drop, in its closed glass well, at the temperature of a mouse’s body for twenty-four hours. “Ah, this ought to be full of nice long threads of bacilli,” he muttered, and looked down the tube of his microscope--“What’s this?” he cried. The outlines of the threads had grown dim, and each thread was speckled, through its whole length, with little ovals that shone brightly like infinitely tiny glass beads, and these beads were arranged along the threads as perfectly as a string of pearls. To himself Koch muttered guttural curses. “Other microbes have doubtless gotten into my hanging-drop,” he grumbled, but when he looked very carefully he saw that wasn’t true, for the shiny little beads were _inside_ the threads--the bacilli that make up the threads have turned into these beads! He dried this hanging-drop, and put it away carefully, for a month or so, and then as luck would have it, looked at it once more through his lens. The strange strings of beads were still there, shining as brightly as ever. Then an idea for an experiment got hold of him--he took a drop of pure fresh watery fluid from the eye of an ox. He placed it on the dried-up smear with its months-old bacilli that had turned into beads. His head swam with confused surprise as he looked, and watched the beads grow back into the ordinary bacilli, and then into long threads once more. It was outlandish! “Those queer shiny beads have turned back into ordinary anthrax bacilli again,” cried Koch, “the beads must be the _spores_ of the microbe--the tough form of them that can stand great heat, and cold, and drying.... That must be the way the anthrax microbe can keep itself alive in the fields for so long--the bacilli must turn into spores....” Then Koch launched himself into thorough, ingenious tests to see if his quick guess was right. Expertly now he took spleens out of mice which had perished of anthrax--he lifted this deadly stuff out carefully with heated knives and forceps. Protected from all chance of contamination by stray microbes of the air, he kept the spleens for a day at the temperature of a mouse’s body, and, sure enough, the microbes, every thread of them, turned into glassy spores. Then in experiments that kept him incessantly in his dirty little room he found that the spores remained alive for months, ready to hatch out into deadly bacilli the moment he put them into a fresh drop of the watery fluid of ox-eyes, or the instant he stuck them, on one of his thin slivers, into the root of a mouse’s tail.