The 19th century saw mankind's greatest advances in the understanding of electricity and magnetism thanks to pure research carried out by the likes of Faraday, Ampère and Maxwell. Indeed, according to Feynman, the most significant event of the 19th century was Maxwell's four equations for electromagnetic fields published between 1855 and 1865. These four equations described the whole of electricity and magnetism and, for the first time, unified the electric and magnetic forces into one theory of electromagnetism. Maxwell also used these equations to show that light was an electromagnetic wave and accurately predicted the velocity of light. His equations also showed that electromagnetic waves were Lorentz invariant some forty years before Einstein.