Common Belief (From The Spelljammer Wikipedia Article )
The phlogiston (also known as the Flow) is a bright, extremely combustible gas-like medium that exists between the Crystal Spheres. A signature property of the substance is that it does not exist within the boundaries of a crystal sphere, to the degree that it cannot be brought into a crystal sphere by any known means up to and including the direct will of deities. Every crystal sphere floats in the phlogiston, very slowly bobbing up and down over time. Travel between Crystal Spheres is facilitated by the formation of Flow rivers — sections of the phlogiston which have a current and greatly reduce travel time. Travel through the slow flow (i.e. off the Flow rivers) is possible, but very dangerous.