The Phlogiston
The Phlogiston refers to the space between the crystal spheres of the Prime Material Plane. The Phlogiston contains flammable fluid that permeates it, a rainbow-colored chaotic material surrounding the spheres. Bizarre creatures inhabited it and preyed on spelljammers traveling between spheres. Spacefarers sometimes called the phlogiston the flow or the Rainbow Ocean. The Phlogiston itself is highly flammable, and the space outside of the spheres is also outside of the influence of the gods (cleric's be warned!).
The phlogiston was far from homogeneous. The fluid itself varied in its density from place to place and also displayed river-like flows, sometimes connecting objects such as the crystal spheres themselves within them. Spelljammers could make use of this flow to greatly increase their speeds through the vast distances within the phlogiston. Increasing the contact area with the flow to acquire greater speed was also the reason some spelljamming ships had sails.
The crystal spheres floated freely in the phlogiston, sometimes enough to significantly change their positions with respect to each other over time. The flow, however, remained unaltered, so navigation between the spheres remained possible. Many of these rivers flowed in both directions, but some flowed in only one way, forcing a return trip to take a different route
