By itself this weird experiment of the gigantic dose of feebly poisonous soup would only have made microbe hunters laugh. It was scandalous. “What!--if a great flask of diphtheria microbes can make so little poison that it takes a good part of a bottle of it to kill a small guinea-pig--how can a few microbes in a child’s throat make enough to do that child to death? It is idiotic!” But Roux had got his start. With this silly experiment as an uncertain flashlight, he went tripping and stumbling through the thickets, he bent his sallow bearded face (sometimes it was like the face of some unearthly bird of prey) over a precise long series of tests. Then suddenly he was out in the open. Presently, it was not more than two months later, he hit on the reason his poison had been so weak before--he simply hadn’t left his germ-filled bottles in the incubator for long enough; there hadn’t been time enough for them really to get down to work to make their deadly stuff. So, instead of four days, he left the microbes stewing at body temperature in their soup for forty-two days, and when he ran that brew through the filter--presto! With bright eyes he watched unbelievably tiny amounts of it do dreadful things to his animals--he couldn’t seem to cut down the dose to an amount small enough to keep it from doing sad damage to his guinea-pigs. Exultant he watched feeble drops of it do away with rabbits, murder sheep, lay large dogs low. He played with this fatal fluid; he dried it; he tried to get at the chemistry of it (but failed); he got out a very concentrated essence of it though, and weighed it, and made long calculations. One ounce of that purified stuff was enough to kill six hundred thousand guinea-pigs--or seventy-five thousand large dogs! And the bodies of those guinea-pigs who had got a six hundred thousandth of an ounce of this pure toxin--the tissues of those bodies looked like the sad tissues of a baby dead of diphtheria.... So it was Roux made Loeffler’s prophecy come true; it was that way he discovered the fluid messenger of death which trickles from the insignificant bodies of diphtheria bacilli. But he stuck here; he had explained how a diphtheria germ murders babies but he had found no way to stop its maraudings. There was that letter from the mother--but Roux’s researches petered out into various directions to doctors how to grow germs pure out of children’s throats at the bedside, and into suggestions for useful gargles.... He hadn’t Pasteur’s tremendous grim stick-to-itiveness, nor his resourceful brain. III