Gunpowder is the first physical explosive and propellant. Before its invention, many incendiary and burning devices had been used, including Greek fire. The invention of gunpowder is usually attributed to experimentation in Chinese alchemy by Taoists in the pursuit of immortality, and is popularly listed as one of the "Four Great Inventions" of China. It was invented during the Tang Dynasty (9th century) but the earliest record of a written formula appeared in the Song Dynasty (11th century). Knowledge of gunpowder spread rapidly throughout the Old World possibly as a result of the Mongol conquests of the 13th century, with the earliest written formula for it outside of China contained within the Opus Majus, a 1267 treatise by the English friar Roger Bacon. It was employed in warfare to so