The Fires of Vesta
Outside and between the Crystal Spheres is a turbulent, rainbow ocean of flammable ether called the phlogiston. The phlogiston is a multicolored sea upon which float the various systems within their crystal shells. The term “phlogiston” is applied equally to both the multicolored medium and to the entire region surrounding the Crystal spheres.
Phlogiston has varying thicknesses in space and forms dense rivers between Crystal Spheres. Voyagers moving along these paths of concentrated phlogiston discover that the greater the density of the flow, the faster a ship can move. A ship can speed up and slow down by penetrating deeper into or raising itself out of these phlogiston rivers. Stellar distances can be covered quickly in such areas. Further, the speed of the ship is at least partially dependent on the surface area it presents to the flow, so many ships carry sails to increase their speed in the interstellar ocean.
Gravity works in phlogiston the same way it does in wildspace. Down is directed toward the major axis of the ship. While there aren’t known to be any celestial bodies in the phlogiston, it could be assumed that gravity would function the same on them as well.
Phlogiston is none of the recognized four elemental matters. It is neither air nor earth, fire nor water. It cannot be reproduced or brought inside the bounds of a crystal sphere. If an attempt is made, whether physically or with magic containment, the phlogiston inexplicably dissipates, leaving no trace. Phlogiston simply cannot exist within wildspace or on the surface of a planet.
Notably, phlogiston is dangerously flammable. Any flame exposed to phlogiston causes the surrounding area (including the phlogiston-populated air envelop of the ship) to burst into flame. Flames started in this way immediately expand to three times their diameter and inflict three times their normal damage. Phlogiston ignites immediately when flames are lit or created. The fire resulting from a fireball spell or similar magic is centered on the magic-user casting the spell.
Because of the nature of phlogiston, all flaming light sources are extinguished by crews before entering the flow. Phlogiston is fortunately radiant, so no other lights are needed on the exposed decks. Below decks, or in the rare Dark Regions of space, magical lighting is usually used.
Travelers can move through the rivers of phlogiston at random, or according to a planned course. Many rivers flow in both directions, forming a road between two Crystal Spheres. In other cases, a river flows in one direction only, forcing voyagers to find different paths to return home.
The Crystal Spheres are not stationary, and bob in the phlogiston like corks in an ocean. As a result, over time the spheres actually shift relative positions to each other. This rarely changes the course of the rivers however, so navigation is possible. Fortunately, when Crystal Spheres drift too close, the phlogiston between the two thickens to the point where they are moved before collisions happen.