Here was a most important clew. It was not necessary to fight the germs themselves in every part of the body, but merely to introduce some ferment or chemical substance which would have the power of neutralizing their poison. Instantly attention was turned in this direction, and it was quickly found that if a guinea-pig were injected with a very small dose of the diphtheria toxin and allowed to recover, he would then be able to throw off a still larger dose, until finally, after a number of weeks, he could be given a dose which would have promptly killed him in the beginning of the experiments, but which he now readily resisted and recovered from. Evidently some substance was produced in his blood which was a natural antidote for the toxin, and a little further search quickly resulted in discovering and filtering out of his body the now famous antitoxin. A dose of this injected into another guinea-pig suffering from diphtheria would promptly save its life. Could this antitoxin be obtained in sufficient amounts to protect the body of a human being? The guinea-pig was so tiny and the process of antitoxin-forming so slow, that we naturally turned to larger animals as a possible source, and here it was quickly found that not only would the goat and the horse develop this antidote substance very quickly and in large amounts, but that a certain amount of it, or a substance acting as an antitoxin, was present in their blood to begin with. Of the two, the horse was found to give both the stronger antitoxin and the larger amounts of it, so that he is now exclusively used for its production. After his resisting power had been raised to the highest possible pitch by successive injections of increasing doses of the toxin, and his serum (the watery part of the blood which contains the healing body) had been used hundreds and hundreds of times to save the lives of diphtheria-stricken guinea-pigs, and had been shown over and over again to be not merely magically curative but absolutely harmless, it was tried with fear and trembling upon a gasping, struggling, suffocating child, as a last possible resort to save a life otherwise hopelessly doomed. Who could tell whether the "heal-serum," as the Germans call it, would act in a human being as it had upon all the other animals? In agonies of suspense, vibrating between hope and dread, doctors and parents hung over the couch. What was their delight, within a few hours, to see the muscles of the little one begin to relax, the fatal blueness of its lips to diminish, and its breathing become easier. In a few hours more the color had returned to the ashen face and it was breathing quietly. Then it began to cough and to bring up pieces of the loosened membrane that had been strangling it. Another dose was eagerly injected, and within twenty-four hours the child was sleeping peacefully--out of danger. And the most priceless and marvelous life-saving weapon of the century had been placed in the hand of the physician.