Book XIII describes the physical world, atoms, classical elements, the sky, clouds, thunder and lightning, rainbows, winds, and waters including the sea, the Mediterranean, bays, tides, lakes, rivers and floods. The sky is called "caelum" as it has stars stamped on to it, like a decorated pot ("caelatus"). Clouds are called "nubes" as they veil ("obnubere") the sky, just as brides ("nupta") wear veils for their weddings. The wind is called "ventus" in Latin as it is angry and violent ("vehemens", "violentus"). There are many kinds of water: some water "is salty, some alkaline, some with alum, some sulfuric, some tarry, and some containing a cure for illnesses." There are waters that cure eye injuries, or make voices melodious, or cause madness, or cure infertility. The water of the Styx causes immediate death.