Aither / R-O-R' Light realm of air, from which heaven came: Neither hot nor cold, neither wet nor dry. Quintessentially, it was either the pneuma divine Holding in place the classical celestial spheres… Or an unseen substance of science for the propagation And transmission of light and gravity. But now it is just a word used by chalky chemists Denoting R-O-R’ with C-O-C linkages.