Although the bark of willows (Salix alba) had been used for centuries to treat pain, it was Reverend Edward Stone of
Chipping Norton, Oxfordshire who reported its analgesic properties in 1763.  The active ingredient of the bark is
a glycoside called salicin. Acetylsalicylic acid was first produced in 1853 by the French chemist Charles von Gerhardt 
by mixing sodium salicylate with acetyl chloride.

Reference:
A History of Anaesthesia through Postage Stamps by Alistair McKenzie.