Dio's Rome, Volume 5, Books 61-76 (A.D. 54-211‪)‬

"Lucius Lucullus at about this period worsted the lords of Asia, —Mithridates and Tigranes the Armenian, —in the war, and having compelled them, to avoid a pitched battle proceeded to besiege Tigranocerta. The barbarians did him serious injury by means of their archery as well as by the naphtha which they poured over his engines. This chemical is full of bitumen and is so fiery that whatever it touches it is sure to burn to a cinder, and it can not be extinguished by any liquid. As a consequence Tigranes recovered courage and marched forth with an army of such huge proportions that he actually laughed heartily at the appearance of the Romans present there.