Apocalypse 9:17

And thus I saw the horses in the vision. The horsemen appeared to St. John with breastplates of fire, and of hyacinth, and of brimstone. By this expression is indicated the firing of carabines, or such firearms as cavalry use, which are applied to the breast when shot off. St. John took the fire that issued out of the muskets to come from the horsemen's breast, on which the muskets rested, and so thought the horsemen had breastplates of fire. The prophet here even describes to us the composition of gunpowder, with its three ingredients, viz. brimstone or sulphur, fire or charcoal, and hyacinth or saltpetre; because saltpetre, when set on fire, emits a flame of a fine purple colour, similar to the colour of the hyacinth stone. Here then we see revealed to St. John both the composition and use of gunpowder, to which he and all mankind at that time were strangers. Then it is said: And the heads of the horses, &c. Here is pointed out the artillery of the army, or cannon. He saw in this vision the whole army drawn up at a distance, and the artillery placed upon a line with the cavalry. He seemed to confound the cannon with the horses, and the cannons' mouths with the mouths of the horses, as the height of both from the ground is nearly the same. He describes the appearances as he saw in the vision, not the reality. When therefore he says, the heads of the horses were as the heads of lions, it is the same as if he said, the mouths of the cannon were as to the noise they made, like the mouths of roaring lions. Hence it appears that St. John, in this vision, both saw the fire of the cannon, and heard the explosion.