It is apparent that being the top medical practitioner, master Xu didn't think there could be any Elixir for Immortality, so he fled with thousands of youngsters to start anew in a new place overseas. He was kind of smart!
No so for other external alchemists. In the prosperous Tang Dynasty (唐朝), there were altogether 21 Emperors, and out of them 5 died from taking Elixir of Immortality (外丹)! You might wonder: why were the Emperors so stupid, after one was dead, his descendants shouldn't follow suit!
That brings us to the hidden logic of Immortality of the external alchemists. Chinese mentality believed that the everything on Earth was made up of five elements (Gold, Water, Wood, Fire, Earth) in different compositions. And some of these compositions, like all living things, will perish, and some, like gold, will stay forever. In order that a person (or any living thing for that matter) can live forever, according to the logic of the Fangshi, his body must be transformed (through changing his internal composition) into that of the compositions, like gold, that can stay forever.
What did the Fangshi or alchemist recommend? He recommended taking Elixir that could react with the human body and change the (perishable) human body into indestructible substance like gold! Clever in theory! How could he convince an Emperor to take his Elixir? Through demonstration. What kind of demonstration? A demonstration in which his Elixir was burnt with some perishable substance. He would need to show, after such burning, the perishable substance would be transformed into indestructible substance. You got it! In essence, a physical demonstration of transforming base metal into silver or gold (check my previous post "The hidden Secret of External Alchemy" on the theory of such transformation).
|Xu Fu statue in Japan (Jofuku Park)By 663highland (663highland) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0/) or CC-BY-2.5 (www.creativecommons.org/licenses/by/2.5)], via Wikimedia Commons|