Phlogiston

Phlogiston refers to one of two things: the space between crystal spheres upon the Prime, and the inexplicable fluid that fills that space. As much as the crystal of the spheres, this substance seems impossible to analyze, as it fails to react with any other known substance but for exploding violently upon exposure to any level of flame; a very dangerous phenomenon, as phlogiston slowly leaks into the air envelopes of spelljamming vessels as they travel through the region, making even the slightest spark a potentially fatal danger. Further, the substance seems to only be able to exist within the space between crystal spheres, as when taken into Wildspace proper, it simply vanishes without explanation, providing if nothing else a natural means of clearing a saturated air envelope.

The phlogiston itself seems to be beyond the reach of everything but mortal influence. Neither the Astral nor the Ethereal contact the phlogiston, and all known planar transit or extradimensional spells or effects fail to function outside the bounds of a crystal sphere. Even deities seem to be subject to this, as their powers seem to be impossible to extend beyond a sphere into the phlogiston; even divine spellcasters are unable to refresh their granted powers while traveling between the spheres.

Far more than a still fluid, the phlogiston is filled with bidirectional currents of varied strength, rivers winding through space that stretch from one crystal sphere to another. While the source of these currents is unknown, this doesn't keep spelljamming vessels from taking advantage of them to better speed their travel from one sphere to another. Perhaps the best-known example of these currents is the triangle running from Toril to Oerth to Krynn, a major trade route connecting Realmspace, Greyspace, and Krynnspace to one another.

Reference

• Adventures in Space - The Concordance of Arcane Space, pp.10-11