The Last Days of Pompeii by Edward Bulwer-lytton

THE cloud, which had scattered so deep a murkiness over the day, had now / settled into a solid and impenetrable mass. It resembled less even the / thickest gloom of a night in the open air than the close and blind / darkness of some narrow room. But in proportion as the blackness / gathered, did the lightnings around Vesuvius increase in their vivid and / scorching glare. Nor was their horrible beauty confined to the usual / hues of fire; no rainbow ever rivalled their varying and prodigal dyes. / Now brightly blue as the most azure depth of a southern sky -- now of a / livid and snakelike green, darting restlessly to and fro as the folds of / an enormous serpent -- now of a lurid and intolerable crimson, gushing / forth through the columns of smoke, far and wide, and lighting up the / whole city from arch to arch -- then suddenly dying into a sickly / paleness, like the ghost of their own life!

In the pauses of the showers, you heard the rumbling of the earth / beneath, and the groaning waves of the tortured sea; or, lower still, / and audible but to the watch of intensest fear, the grinding and hissing / murmur of the escaping gases through the chasms of the distant mountain. / Sometimes the cloud appeared to break from its solid mass, and, by the / lightning, to assume quaint and vast mimicries of human or of monster / shapes, striding across the gloom, hurtling one upon the other, and / vanishing swiftly into the turbulent abyss of shade; so that, to the / eyes and fancies of the affrighted wanderers, the unsubstantial vapors / were as the bodily forms of gigantic foes -- the agents of terror and of / death.