17. thus—as follows.
of fire—the fiery color of the breastplates answering to the fire which issued out of their mouths.
of jacinth—literally, "of hyacinth color," the hyacinth of the ancients answering to our dark blue iris: thus, their dark, dull-colored breastplates correspond to the smoke out of their mouths.
brimstone—sulphur-colored: answering to the brimstone or sulphur out of their mouths.