"A certain use," the other replied cautiously. "Or, rather, it was not he, but an associate, called by him the Physician of Aleppo, who discovered it. This man was the pupil of the learned Rhazes, and the tutor of the equally learned Avicenna, the link, in fact, between them; but his name, for some reason, perhaps because he mixed with his practice a greater degree of mysticism than was approved by the Arabian schools of the next generation, has not come down to us. This man identified the product which had defied Ibn Jasher's tests with a substance even then considered by most to be fabulous, or to be extracted only from the horn of the unicorn if that animal existed. That it had some of the properties of the fabled substance, he proceeded to prove to the satisfaction of Ibn Jasher by curing of a certain incurable disease five persons." "No more than five?" "No." "Why?" "The substance was exhausted." Blondel gasped. "Why did he not make more?" he cried. His voice was querulous, almost savage. "The experiment," Basterga answered, "of which it was the product was costly." Blondel's face turned purple. "Costly?" he cried. "Costly? When the lives of men hung in the balance." "True," Basterga replied with a smile; "but I was about to say that, costly as it was, it was not its price which hindered the production of a further supply. The reason was more simple. He could not extract it." "Could not? But he had made it once?" "Precisely." "Then why could he not make it again?" the Syndic asked. He was genuinely, honestly angry. It was strange how much he took the matter to heart. "He could not," Basterga answered. "He repeated the process again and again, but the peculiar product, which at the first trial had resulted from the precipitation, was not obtained." "There was something lacking!" "There was something lacking," Basterga answered. "But what that was which was lacking, or how it had entered into the alembic in the first instance, could not be discovered. The sage tried the experiment under all known conditions, and particularly when the moon was in the same quarter and when the sun was in the same house. He tried it, indeed, thrice on the corresponding day of the year, but--the product did not issue." "How do you account for that?" "Probably, in the first instance, an impurity in one of the drugs introduced a foreign substance into the alembic. That chance never occurred again, as far as I can learn, until, amusing myself with the same precipitation, I--I, Caesar Basterga of Padua," the scholar continued, not boastfully but in a tone thoughtful and almost absent, "in the last year of the last century, hit at length upon the same result." The Syndic leaned forward; his hands gripped his knees more tightly. "And you," he said, "can repeat it?"