Arsenic: historical information
Arsenic compounds were mined by the early Chinese, Greek and Egyptian civilisations. No doubt they discovered its toxic properties early on.
It is believed that Albertus Magnus obtained the element in 1250 A.D. who obtained it by heating soap together with orpiment (arsenic trisulphide, As2S3).
Arsenic is one of the elements which has an alchemical symbol, shown below (alchemy is an ancient pursuit concerned with, for instance, the transformation of other metals into gold).