1. Ethers are a family of organic compounds with the general formula R–O–R', where R and R' are hydrocarbon radicals. Examples are dimethyl ether (CH3OCH3) and diethyl ether (C2H5OC2H5), the latter being known simply as "ether" (see below).
Ethers are volatile, highly flammable substances made by catalytic dehydration of alcohols, catalytic hydration of alkenes, or by reacting an alkyl halide with a sodium alkoxide. They are chemically fairly inert, though they are split by hydrogen halides, and form explosive peroxides on standing in air. See also thioether.
2. Ether is a volatile, highly flammable liquid, (C2H5)2O, with a sickly smell, partially miscible with water. It is derived from distillation of ethanol with sulfuric acid, and widely used in industry (as a solvent) and as an anesthetic. Also known as diethyl ether, ethyl ether, and ethoxyethane. It is made from ethanol by dehydration using concentrated sulfuric acid, or by reacting it with a chloroethane (ethyl chloride) and sodium metal (Williamson's synthesis). Molecular weight 74.1, melting point -116.2°C, boiling point 34.5°C.
3. The ether, or aether, is an all-pervading, infinitely elastic, massless medium formerly postulated by late 19th-century physicists in order to explain how light and other forms of electromagnetic waves could be propagated through otherwise empty space. Light was thus thought of as a mechanical wave motion in the ether. The whole theory was discredited following the failure of the Michelson-Morley experiment to detect any motion of the Earth relative to the supposed stationary ether.