A chemical element, in the periodic table Arsenic has the symbol As and atom number 33. Arsenic is chemically very similar to its predecessor Phosphorus, so much so that it will partly substitute for it in biochemical reactions and is thus poisonous. When heated it rapidly oxidises to arsenous oxide, which has a garlic odour. Arsenic and some arsenic compounds can also sublime upon heating, converting to gaseous form directly. Its name comes from the Greek word "arsenikon," meaning "yellow orpiment."
The element has been known and used since ancient times. It has been frequently used for murder, the symptoms of arsenic poisoning being somewhat ill-defined, before the advent of sensitive chemical tests for its presence. There is a theory that Napoleon Bonaparte suffered from arsenic poisoning, samples of his hair being claimed to show high levels of the element. If true, this may not have been deliberate poisoning by Napoleon's enemies; copper arsenate has been used as a pigment in some wallpapers, and microbiological liberation of the arsenic into the immediate environmemnt would be possible. The case is equivocal, in the absence of clearly authenticated samples of the wallpaper.
A later case of arsenic poisoning is that of Claire Booth Luce, the American ambassador to Italy in the years just following World War II; she suffered an increasing variety of physical and psychological symptoms until arsenic poisoning was diagnosed, and its source traced to the old, arsenic-laden flaking paint on the ceiling of her bedroom.
Military use not included yet - Lewisite & WW I, specifically.