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The Plane of Matter (A) comprises the forms of Matter in its form of solids, liquids, and gases, as generally recognized by the text-books on physics. The Plane of Matter (B) comprises certain higher and more subtle forms of Matter of the existence of which modern science is but now recognizing, the phenomena of Radiant Matter, in its phases of radium, etc., belonging to the lower sub-division of this Minor Plane. The Plane of Matter (C) comprises forms of the most subtle and tenuous Matter, the existence of which is not suspected by ordinary scientists. The Plane of Ethereal Substance comprises that which science speaks of as "The Ether," a substance of extreme tenuity and elasticity, pervading all Universal Space, and acting as a medium for the transmission of waves of energy, such as light, heat, electricity, etc. This Ethereal Substance forms a connecting link between Matter (so-called) and Energy, and partakes of the nature of each. The Hermetic Teachings, however, instruct that this plane has seven sub-divisions (as have all of the Minor Planes), and that in fact there are seven ethers, instead of but one.
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Three Initiates (Kybalion: A Study of the Hermetic Philosophy of Ancient Egypt and Greece)