ferrocyanide : Idioms & Phrases

• noun a cyanide compound that is prepared by the reaction of potassium cyanide with ferrous salts; commonly used by gardeners
  yellow prussiate of potash.
  

WordNet

• (Chem.), yellow prussiate of potash; a tough, yellow, crystalline salt, K4(CN)6Fe, the starting point in the manufacture of almost all cyanogen compounds, and the basis of the ferric ferrocyanate, prussian blue. It is obtained by strongly heating together potash, scrap iron, and animal matter containing nitrogen, as horn, leather, blood, etc., in iron pots.

Webster 1913