Prussian blue
Hydrogen cyanide was first isolated from a blue pigment (Prussian blue) which had been known since 1706, but whose structure was unknown. It is now known to be a coordination polymer with a complex structure and an empirical formula of hydrated ferric ferrocyanide. In 1752, the French chemist Pierre Macquer made the important step of showing that Prussian blue could be converted to an iron oxide plus a volatile component and that these could be used to reconstitute it. The new component was what is now known as hydrogen cyanide. Following Macquer's lead, it was first prepared from Prussian blue by the Swedish chemist Carl Wilhelm Scheele in 1782, and was eventually given the German name "Blausäure" ("lit". "Blue acid") because of its acidic nature in water and its derivation from Prussian blue. In English, it became known popularly as "prussic acid."