The first and, for a long time, most popular theory regarding the composition of Greek fire held that its chief ingredient was saltpeter, making it an early form of gunpowder. This argument was based on the "thunder and smoke" description, as well as on the distance the flame could be projected from the "siphōn", which suggested an explosive discharge. From the times of Isaac Vossius, several scholars adhered to this position, most notably the so-called "French school" during the 19th century, which included chemist Marcellin Berthelot.