Albertus Magnus is often credited with isolating this element around 1250; it's since been used as a poison
Arsenic 
occurs in many ... Arsenic and its compounds, especially the trioxide, are used in 
the production of ..... Albertus Magnus (Albert the Great, 11931280) is believed 
to have been the first to isolate the element from a compound in 1250, by heating
... [DOC] [TLE] Alchemy - The Mystica [PAR] But, to fully understand the alchemical process and its purpose one must first 
comprehend ... and he indicated that it was similar to ones used by alchemists.