what does santiago say when he explains to the sun why alchemy exists and what does the alchemist do?

Santiago says;

"This is why alchemy exists," the boy said. "So that everyone will search for his treasure, find it, and then want to be better than he was in his former life. Lead will play its role until the world has no further need for lead; and then lead will have to turn itself into gold.

"That's what alchemists do. They show that, when we strive to become better than we are, everything around us becomes better, too."

The Alchemist smiled.

"The men were terrified at his sorcery. But there were two people who were smiling: the alchemist, because he had found his perfect disciple, and the chief, because that disciple had understood the glory of God.

The following day, the general bade the boy and the alchemist farewell, and provided them with an escort party to accompany them as far as they chose."