Phlogiston (Spelljammer Supplement)
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The Phlogiston, also known as the Flow, is made up of a gaseous substance that is also called phlogiston. This unique substance is unlike any of the four elements (air, earth, fire and water). It is similar in appearance to a radiant gas, emitting brilliant, chaotic and ever changing colors. It is odorless, and non-toxic, but does not provide oxygen and is non-breathable. This substance apparently cannot exist in Wildspace (ie, inside a Crystal Sphere), so even bottled phlogiston will fade away as soon as it passes through the wall of a Sphere.
Groups of the Phlogiston