Nine has been a mystical number since ancient times, when it was known as the "trinity of trinities" (3 times 3). Some word experts speculate that cloud nine - that blissful feeling - refers to high-soaring, thunder-bearing cumulonimbus clouds. According to this theory, the National Weather Service classified clouds by layers. Clouds that reached the ninth layer (30,000 to 40,000 feet) clearly put one in a position to soar.
The purest form of a perfume is sometimes called quintessence, as though the fragrance was five ("quinta") times distilled. Figuratively, quintessence means the most essential part of an idea. Ancient philosophers thought that all matter took one of four forms - earth, air, fire, water. Pythagoreans and medieval alchemists proposed a fifth element, or "quinta essentia." They believed it permeated all things and was the substance of the heavenly bodies. No alchemist succeeded in extracting this fifth element, however, so since 1570, "quintessence" has meant the quality that most characterizes a substance or notion.
SOURCES: 'Why You Say It,' by Webb Garrison; 'Morris Dictionary of Word and Phrase Origins,' by W. and M. Morris; 'Who Put the Butter in Butterfly?' by David Feldman; The Facts on File Dictionary of Clichés; The Encyclopedia of Word and Phrase Origins; The Barnhart Dictionary of Etymology.